GB2277198A - A differential voltage solar cell - Google Patents

Abstract

The invention provides a differential voltage cell, such as a solar cell, in which both the photo-voltaic and Peltier effects are combined. The cell includes a series of interconnected cell units each comprising two spaced bodies 32, 33 including p-type and n-type semiconductor material respectively, a lens 30, an upper transparent electrical conductor 31 bridging said semiconductor bodies, and a lower electrical conductor 34 connected to each of said bodies at a location spaced from the upper conductor, the arrangement being such that the upper and lower conductors never bridge the same pair of semiconductor bodies. In another arrangement, the semiconductor bodies comprise PNP heterostructures (Fig 9, not shown). <IMAGE>

Description

"A differential voltage cell" The invention relates to a differential voltage cell, such as a solar cell and provides such a cell having a novel internal construction.

Solar cells are well-known devices which produce electrical energy directly from sunlight using the photo-voltaic effect. The present invention relates to cells of the kind wherein layers of semiconductor material are sandwiched between two conductors. In conventional solar cells of this type light is turned into electrical energy by the creation of hole-electron pairs at the junctions of the photo-voltaic cell.

There are a number of factors which influence the electrical output of a solar cell. One of these is temperature, which affects the efficiency of the cell.

There are also limits on the type of semiconductor material which can be used effectively. For normal terrestrial purposes, only semiconductors with certain bandgap energies can be used and these produce unwanted heat during the photo-voltaic process, which must be conducted away, so that the efficiency of the solar cell is not compromised.

Also well known are Peltier devices which produce electrical energy from thermal radiation. A Peltier device requires a temperature difference between its junctions to produce a voltage differential and hence electrical energy. The present invention provides a semiconductor differential voltage cell in which the photo-voltaic effect is combined with the Peltier effect to produce significant advantages.

A differential voltage cell according to the invention allows the regulation of heat flow through the device giving advantages of operation in a wider range of environments compared with conventional photo-voltaic cells. It may also permit the use of semiconductor materials with energy gaps outside the range normally associated with conventional photo-voltaic cells, and hence allow cheaper materials to be used.

The regulation of heat flow serves another purpose by allowing a greater concentration of light, using focusing, to be directed on to a cell, therefore allowing less semiconductor material to be used and hence the possibility of cheaper cells.

The bandwidth of the device may also be increased, so that more of the available spectrum of light absorbed by the cell is turned into useful electrical energy.

According to the invention there is provided a differential voltage cell including at least one cell unit comprising two spaced bodies including p-type and n-type semiconductor material respectively, an upper electrical conductor bridging said semiconductor bodies, and a lower electrical conductor connected to each of said bodies at a location spaced from the upper conductor, said upper conductor being at least partly transparent to light, whereby the cell unit operates both as a photo-voltaic device and as a Peltier device when said upper conductor is exposed to light and a temperature difference is applied between said upper and lower conductors.

The cell preferably comprises a plurality of said cell units connected in series or parallel, adjacent units being connected by a common lower conductor, so that the upper and lower conductors never bridge the same pair of semiconductor bodies.

At least one of the semiconductor bodies of each cell unit may comprise more than one type of semiconductor material. The upper conductor and/or the lower conductor may be formed from semiconductor material.

Each lower conductor is preferably formed with cooling fins. Preferably a focusing lens is disposed to concentrate radiation to said upper conductor. An exposed surface of said upper conductor may be disposed within a chamber between said focusing lens and the cell unit.

Each said lower conductor may be coated with a dark pigment. At least one of said semiconductor bodies may be at least partly transparent to light.

In any of the above arrangements said body including n-type semiconductor material is replaced by a metal electrical conductor.

The following is a description of embodiments of the invention, by way of example, reference being made to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a prior art photo-voltaic cell, Figures 2 and 3 are diagrammatic representations of Peltier devices operating according to prior art principles, Figure 4 is a diagrammatic representation of a single cell unit of a combined Peltier and photo-voltaic device according to the present invention, Figures 5 and 6 are diagrammatic perspective views of differential voltage cells in accordance with the present invention, Figure 7 is a diagrammatic view of part of the cell shown in Figure 6, Figures 8 and 9 are similar views to Figure 7 showing modified forms of construction, and Figure 10 is a diagrammatic representation of a further form of differential voltage cell in accordance with the invention.

Figure 1 shows diagrammatically a simple conventional solar cell in accordance with the prior art. It shows the creation of hole-electron pairs at a junction by optical wave lengths of radiation.

The solar cell comprises two abutting layers of semiconductor material, an n-type upper layer 10 and a p-type lower layer 11. The labels "n" and "p", as is well known, refer to the type of doped semiconductor material used.

On the upper side of the n-type layer 10 is a metal ohmic contact grid 12 and on the underside of the p-type layer 11 is a metal ohmic contact layer 13.

The voltages, as indicated by the vertical arrows, are determined by the doping concentration of the semiconductor layers and by the number of free electrons in the metal contacts 12 and 13. The incidence of sunlight on the semiconductor layers, through the grid 12, changes the voltage mainly at the junctions between the layers 10 and 11, such that charge carriers and hence current is produced.

Some of these voltages are in opposition to the net generated output voltage and current. As will be described, in a differential voltage cell in accordance with the present invention, as a result of the construction and by careful selection of doping concentrations the solar radiation can be used to diminish, negate or change the junction voltages of the differential voltage cell to produce improved electrical output.

This is achieved by so constructing the differential voltage cell as to give rise to the Peltier effect in addition to the usual photo-voltaic effect.

Figure 2 illustrates diagrammatically a prior art Peltier device. This comprises alternating n-type and p-type semiconductors 14, 15 alternately connected by upper metal conductors 16 and lower metal conductors 17. As is well known, a temperature difference between the conductors 16 and 17 will induce a flow of current in the device.

In the arrangement shown the lower metal conductors 17 are provided with cooling fins 18 and a lens system 19 is mounted above the device so as partly to focus the suns rays 20 on the upper conductors 16.

Each component of the lens system comprises a partcylindrical lens 21, each located above a conductor 16 and arranged to concentrate the suns rays 20 on the conductor.

Depending on the characteristics of the semiconductor material 14 and 15, which is sandwiched between the upper conductor 16 and the lower conductor 17, a certain quanta of radiation is absorbed by the materials 14 and 15.

The p-type and n-type material is so called for reference only. They may contain, or may be partly made up of, many other constituents, e.g. intrinsic material, sintered oxides. Constructional details of such Peltier devices are well known.

For reference purposes only one example of cell dimensions will be given, since it will be appreciated that the design is dependent on the environment in which the cell is used. The parameters which determine the optimum output performance for semiconductor materials are well known in the literature.

Typical dimensions are as follows: Cross-sectional area of 14 and 15: 2 cm2 Thickness of conductor 16: lmm Height between upper conductor 16 and lower conductor 17: 1-2cm Dopants P=l x 1016 cm'3 N=1 x 1015 cm'3 Concentration of lenses (area) is 10:1 (or lm2 focussed onto an area of 10cm2) The absorption of heat radiation from the sun causes a change in voltage near the surface 16 compared with the lower surface 17. The net effect, depending on doping concentrations, produces a current flow from one end of the device to the other. Thermal energy is absorbed by the conductor 16 and emitted by the conductor 17 using some form of convection via the cooling fins 18.

Depending on doping concentrations, if the thermal energy absorbed at 16 increases and/or the energy emitted at 17 increases, so the voltage differences between the layers increases.

Figure 3 shows another prior art device which is generally similar in principle to the device of Figure 2, and similar components have similar reference numerals. Again, thermal radiation from the sun is concentrated very strongly by lenses 21 on to upper conductors 16. The conductors 16 may be formed from copper coated with a black pigment.

The upper surfaces of the conductors 16 are contained in a sealed chamber which may be partly evacuated, the chamber being formed by the lenses 21 and a polymer (polypropelyene, or PTFE) 22, which fills the gaps between the conductors 16. The chamber between the lenses and the polymer helps trap heat in this area so that it must be conducted away by the conductors 16.

This heat energy is then transferred as described in relation to Figure 2 and a change in potential occurs between the two sets of conductors 16 and 17 and a current flows along the length of the device. The heat energy is given up from the cooling fins 18 by a convection process.

This device does not require its upper surface to be kept perpendicular to the direction of the sun's radiation. Concentrated radiation can fall anywhere on the surfaces 16 which are closer to the lenses 21 than their focal length.

The temperature T/OK of a black body is related to its rate of absorption, E, over a unit area by E = oT4, where o = 5.67 x 10-8W/M2 (or)4. K = 2730 + C where C is Celsius.

It can be seen from this equation that (as a guide) if the intensity of the radiation falling on a black body is increased, using focusing, its temperature will increase so increasing the potential between surfaces 16 and 17.

Bandgap materials outside conventional values could be used. This Peltier device thus allows the use of materials which are unsuitable for conventional photo-voltaic solar cells, such as cadmium sulphide and possibly allows the use of a carbon based substance.

The lack of need for as much purity of the semiconductor and the abundance of other suitable materials may render the Peltier device cheaper than a conventional photovoltaic cell.

Figure 4 illustrates diagrammatically the basic unit of a differential voltage cell in accordance with the present invention where the Peltier effect is combined with the photo-voltaic effect of a conventional solar cell.

The differential voltage cell unit comprises an n-type semiconductor 23 and a p-type semiconductor 24 coupled by an upper conductor 25 and each individually connected to a different lower conductor 26. The upper conductor 25 is transparent, or partly transparent, to light and the semiconductors 23 and 24 may or may not or wholly transparent. The conductors 25 and 26 are normally metallic but may in certain embodiments comprise doped semiconductors.

The arrangement of Figure 4 combines the Peltier effect and the photo-voltaic effect in one cell so that with careful direction of the radiation 27 from the sun and corresponding designed doping concentrations around the cell an electrical output is produced.

Depending on construction airflow, as indicated at 28 and 29, can be used to cool either the upper surface 25, the lower surface 26, both surfaces or neither.

By careful selection of the doping concentrations of the semiconductors 23 and 24 the solar radiation is used to diminish, negate or change the junction voltages of the cell to produce the required electrical output.

The junction voltage (Vj) is given by the following equation relating to doping densities: Vj = KT ln NA ND q n2 Where q/K = 11600 T = temperature in degrees Kelvin NA = concentration of acceptor ions ND = concentration of donor atoms ni = intrinsic carrier concentration NA and ND are effectively the doping densities and ni the hole-election concentration due to thermal radiation.

Current I is determined by Ij = Isc - ITH Where Ij = total current or resultant current Isc = short circuit current generated by optically induced charge carriers ITH = current produced by thermal energy which opposes Isc and must be kept low for Isc to remain effective.

Typical characteristics of semiconductor materials such as depth and widths for optimum performance are known in the prior art as mentioned, for example, in specifications GB 2116775, and US 4342044.

For example direct bandgap materials require about 1 im of thickness to absorb sunlight effectively and about 100 tm for indirect bandgap materials.

Gallium arsenide (GaAs) is a direct bandgap material and crystalline silicon (Si) is an indirect bandgap of material.

In a typical construction according to the invention, the surface 20 could be 1 mm thick. The narm 18 could have 1 mm2 cross sectional area with ND < 1 x 1022 atoms/cm3 (i.e. metal). The p-arm may have 2-3 cm2 cross sectional area with NA = 1 x 1017 atoms/cm3.

Figures 5-10 illustrate, by way of example, specific devices according to the invention in which the Peltier effect is combined with the photo-voltaic effect.

The device according to the invention shown in Figure 5 is somewhat similar in construction to the Peltier device of Figure 3, but is so designed as to combine the photo-voltaic effect with the Peltier effect.

Lenses 30 concentrate the sun's radiation on to the upper surfaces of conductors 31. Each conductor 31 bridges the upper surfaces of an n-type semiconductor 32 and a p-type semiconductor 33. The undersides of adjacent pairs of semiconductors are connected by lower conductors 34 formed with cooling fins 35. The arrangement is such that each lower conductor 35 never bridges the same pair of semiconductors as is bridged by an upper conductor 31.

The lenses 30 serve to concentrate the radiation from the sun on the upper surfaces of the conductors 31. The amount of concentration is dependent on the thickness and curvature of the lenses 30. The upper surfaces of the conductors 31 lie at a distance from the lenses 30 which is within their focal length so as to avoid the danger of focusing all the sun's radiation on one spot.

The conductors 31 are formed from a highly doped semiconductor or metal which is transparent to light, such as tin oxide, alloys of tin and indium oxides. The concentrated radiation from the sun then penetrates the conductors 31. Since the conductors 31 are transparent the semiconductors 32, 33 are subjected to sunlight and the device therefore operates as a photo-voltaic cell generating electrical energy. At the same time, the device operates as a Peltier device and generates electrical energy as a result of the thermal radiation from the sun, as in the arrangements of Figures 2 and 3.

Electrical output is influenced by a great number of factors. For example, the current direction, performance of the cell with temperature, voltage output, depends greatly on the material of the n-type and p-type slices 32, 33 and how they are doped with respect to the other conducting surfaces. From the vast range of possible constructions, a specific embodiment, the simplest of this type, will be described.

The n-type material 32 is more heavily doped, i.e. has greater concentration of dopant in terms of atoms used per cubic centimetre, than the p-type material 33 and has the same number of electrons per cubic centimetre as the upper and lower conducting layers 31 and 34 respectively. This serves two purposes. The effective resistance of the p-type slice 33 is reduced (compared to being the same size as the ntype slice 32) and there is a greater area for the creation of hole-electron pairs in the depletion region which extends a small distance into the p-type material.

(The n-type material 32 could be omitted).

As radiation is absorbed by the material, hole-electron pairs are created and a current is set up which, using standard convention, flows from 36-37.

In conventional photo-voltaic cells efficiencies range from around 5%-30% (e.g. amorphous silicon Ga As cells respectively). The majority of the rest of the energy is turned into unwanted heat, which must be conducted away by some means. The heat energy in the semiconductor contributes to the inefficiency of the cell by reducing the lifetime of useful charge carriers.

In the embodiment of the invention shown in Figure 5, unwanted heat is transferred as part of the conduction process and is dissipated by the cooling fins 35. In this form of the invention it could be desirable to keep the temperature difference to a minimum between the two conductors 31, 34. This again theoretically depends on doping concentration.

This helps to regulate the temperature and could allow semiconductor materials which have lower energy gaps to be used where previously they could not.

For example germanium has Eg = 0.72eV and has a higher mobility for both holes and electrons over silicon.

Hence, quanta of lower energies can be absorbed, which would contribute to useful current along with the increased mobility. Materials such as germanium or even indium antimonide could be used in extreme cold environments, such as in the Arctic. Indium antimonide has potentially excellent low temperature characteristics.

Figure 6 shows another embodiment of a differential voltage cell in accordance with the invention, which is designed to work in a certain environment.

In this case the device comprises elements 38 of p-type semiconductor material to the upper surfaces of which are applied transparent or partly transparent conductors 39. Applied to the lower surface of each element 38 is a lower conductor 40 and the upper conductor 39 of one element 38 is connected to the lower conductor 40 of an adjacent element 38 through a slab 41 of n-type semiconductor material. The device is embedded in a layer 42 of polymer, for example as in the arrangement of Figure 3.

Lenses 43 are provided to concentrate the sun's radiation on the conductors 39.

The device may be placed on an upper surface of a moving vehicle, such as a car. Airflow 44 would pass over the upper conductors 39, cooling their surface. The upper conductors 39 are separated and partly insulated from the lower conductors 40 which remain uncooled.

Figure 7 shows in greater detail the operation of the device. Sunlight 45 passes through the transparent upper conductor 39. Shorter wavelength radiation with energies higher than the energy gap, Eg, at the junction 46 between the conductor 39 and the ptype material 38 is absorbed and creates hole electron pairs and hence current ISC over and above the thermal current ITH. Total current is then I = ISC - ITH Radiation with energies < Eg of 46 passes through the junction and into the partly transparent ptype material 38 and absorbed by the time it reaches the dark or black lower conductor 40 at junction 47 reducing voltage VL which opposes Vu.

By using an n-type material to join the upper conductor 39 to an adjacent lower conductor 40 it can be seen that, by the Peltier effect, when current I is passed between conductor 39 and semiconductor 38 heat is absorbed at the upper surface of the conductor 39 cooling junction 46 and emitted at junction 47, effectively making that junction hot.

A current travelling in direction 48 is produced by the creation of hole-electron pairs and a voltage. The extent of the current and voltage depends on a number of factors, such as external/internal cooling, doping concentrations and construction.

The embodiment of Figure 6 is suitable for use with moving vehicles, and may be used, for example, to trickle charge batteries in a car, although the device may also be used for other purposes.

Even if absolute current and windflow directions are found experimentally or otherwise to be incorrect (for example due to excessive heat causing the junction voltage direction to change) the device may be configured or constructed to allow for different directions. For example the p-type and n-type positions may be swapped around.

Further embodiments of the invention are shown in Figures 8 and 9 and give examples of how different materials can be used.

In Figure 8 50 is the upper conductor and 51 the lower conductor of a p-arm Peltier configuration.

52 and 53 are semiconductor materials which have different energy gaps, Eg, and are preferably transparent direct bandgap materials. Doping concentrations of 52 are less than 53 and in this example both are p-type material. Let Eg of 52 be greater than that of 53. Let 52 have say 1013 dopants cm-3 and 53 have 1018 dopants cm'3. The polarity of the junction voltages at V1 and V2 will hence be in the same sense, and also the depletion region (where charge carriers are produced by a photo-voltaic process) of which the majority will extend into 52 and 53 and hence make use of the different bandgaps of the material. The radiation 54 of energies greater than, or equal to, the Eg of 52 will be absorbed by the junction between the conductor 50 and the semiconductor 52. The rest of the radiation 55 will then pass on to the semiconductor 53 and energies greater than or equal to Eg of 53 will be absorbed by the junction between 52 and 53.

In this way charge carriers and hence current would be produced at both junctions. The residual longer wavelength radiation 56 would be absorbed by the junction between the material 53 and the lower conductor 51.

In the example of Figure 8 only two materials are used but more can be inserted to "tune" the device to the spectrum.

If only a single bandgap material is used only energy equivalent to its Eg will be absorbed. Radiation with energies in excess will be absorbed but the excess will be dissipated as unwanted heat, which may be detrimental to its operation.

Figure 9 shows another variation on a multijunction device. In this arrangement 50 and 51 are as before. 57 is n-type material with a high energy gap and is thin, transparent and highly doped. For example the doping may approach 1022cm~3 free electrons, i.e.

the material is nearly metallic. Its Eg is high compared with conventional cells, for example 2.44.0eV. 58 and 59 are p-type material, direct or indirect bandgap materials where 58 has a higher Eg than 59. The materials 58 and 59 are transparent or partly transparent and can have the same doping concentrations.

In this cell depletion widths or junction widths are larger than they are in the embodiment of Figure 8.

As explained in relation to Figure 8, charge carriers would be produced at all junctions except that this process would be seriously diminished at the junction between materials 58 and 57. This has an opposite polarity to the junctions producing useful current. It will be diminished because it has the same Eg as the junction above it and hence the majority of that radiation will have been absorbed. So radiation of an Eg's less will pass through, but its voltage will be in opposition. The longer wavelength radiation will be absorbed by the dark surfaces between the material 59 and the lower conductor 51.

Once again the construction can be altered to incorporate more junctions between the conductors 50 and 51 so that material with the highest Eg are at the top and the next highest underneath and so on.

By "tuning" and effectively reducing the amount of unwanted heat, the lifetime of charge carriers is increased and hence the distance between the upper and lower conductors is maintained to keep a more useful temperature differential between the conductors.

The embodiment of Figure 10, which comprises cell units connected in parallel, will now be described.

This composite cell can incorporate some of the features previously described, such as multi-layers.

Top conductors 60, arms 61 and bottom conductors 62 are n-type material with doping densities n3, n2, nl cm-3 respectively. Bottom surfaces 63 and arms 64 have doping densities P1 and P2 cm-3 respectively. The doping is such that the junction voltages indicated by the arrows line up in the polarity shown, at equilibrium, when no radiation is absorbed or radiated by the device.

The upper conductors 60 and the arms 61 and 64 are transparent to allow longer wavelength radiation to be absorbed at the darker surfaces of conductors 62 and 63, and shorter wavelength radiation to create holeelectron pairs at the upper surfaces as described previously. There may be airflow over the device as indicated, for example, at 64.

Alternatively, 61 and 64 could be dark, i.e.

not transparent, and the doping concentration between 63, 64 and 61 such that the voltage at the lower junctions is in the direction indicated by the dashed arrows. Here optical wavelength radiation would only travel as far as the upper junctions and cooling would be provided by the airflow 64.

Many of these cells can be built with more than two unit cells. They can be made up in series or parallel and different combinations of p-type and n-type doped material may be used in other locations to that shown.

For example, to obtain a suitable temperature differential conductors 62 and 63 may be extended to a suitable distance such that one of the bodies shown remains in the dark and becomes the hot cell and the other is allowed to absorb the sunlight to induce the photo-voltaic process. Once again the photo-voltaic process and the Peltier effect are included to give a net output voltage and current, with the bodies at different temperatures.

Claims (11)

1. A differential voltage cell including at least one cell unit comprising two spaced bodies including p-type and n-type semiconductor material respectively, an upper electrical conductor bridging said semiconductor bodies, and a lower electrical conductor connected to each of said bodies at a location spaced from the upper conductor, said upper conductor being at least partly transparent to light, whereby the cell unit operates both as a photo-voltaic device and as a Peltier device when said upper conductor is exposed to light and a temperature difference is applied between said upper and lower conductors.

2. A differential voltage cell according to Claim 1, comprising a plurality of said cell units connected in series, adjacent units being connected by a common lower conductor, so that the upper and lower conductors never bridge the same pair of semiconductor bodies.

3. A differential voltage cell according to Claim 1 or Claim 2, wherein at least one of the semiconductor bodies of each cell unit comprises more than one type of semiconductor material.

4. A differential voltage cell according to any of Claims 1 to 3, wherein the upper conductor and/or the lower conductor is formed from semiconductor material.

5. A differential voltage cell according to any of Claims 1 to 4, wherein each lower conductor is formed with cooling fins.

6. A differential voltage cell according to any of Claims 1 to 5, wherein a focusing lens is disposed to concentrate radiation to said upper conductor.

7. A differential voltage cell according to Claim 1, wherein an exposed surface of said upper conductor is disposed within a chamber between said focusing lens and the cell unit.

8. A differential voltage cell according to any of Claims 1 to 7, wherein each said lower conductor is coated with a dark pigment.

9. A differential voltage cell according to any of Claims 1 to 8, wherein at least one of said semiconductor bodies is at least partly transparent to light.

10. A differential voltage cell according to any of Claims 1 to 9, wherein said body including n-type semiconductor material is replaced by a metal electrical conductor.

11. A differential voltage cell substantially as hereinbefore described with reference to any of Figures 4 to 10 of the accompanying drawings.