Patent Application
20130174901
The invention relates to the field of solar cells. In particular, the invention relates to devices and methods for improving the efficiency of solar cells, and a solar cell thereof.
The efficiency of solar cells is currently so low, that solar energy has not been competitive against fossil fuels during low energy prices. Due to this many technologies have been proposed to make solar cells more efficient and thus increase the competitiveness of solar energy in the global marketplace.
EP 1724841 A1 describes a multilayer solar cell, wherein plural solar cell modules are incorporated and integrally laminated, so that different sensitivity wavelength bands are so that the shorter the centre wavelength in the sensitivity wavelength band is, the more near the module is located to the incidental side of sunlight. This document is cited here as reference.
It is known that in order for a photon to be absorbed into the band gap, and thus produce photocurrent, the energy of the photon has to be greater or equal to the band gap. Prior art solutions, EP 1724841 A1 included, have the clear disadvantage that they are unable to provide band gaps that would collect photons efficiently. A great majority of the photon population is simply wasted as heat.
The invention under study is directed towards a system and a method for effectively collecting the photons from solar light to photocurrent. A further object of the invention is to present a method of production with which more and more efficient solar cells can be designed.
In this application a semiconductor layer is construed as a layer of any material or comprising any material capable of experiencing the photoelectric effect.
One aspect of the invention involves a solar cell with a semiconductor layer with a natural band gap NB1. This semiconductor layer also has at least one electrode designed to produce an ambient voltage V1 into the layer. The incoming photons therefore experience a modified NB1−V1=B1 band gap, referred here to as the apparent band gap. Photons with E>B1 will be absorbed into the band gap B1, and the electron in the semiconductor valence band will get excited onto the conduction band thus resulting in photocurrent. In accordance with the invention, the ability to tune the apparent band gap B1 provides an enormous strength to optimise the incoming photon collection.
The photon population that is not absorbed consists of photons with E<B1 that had too little an energy to get absorbed. Additionally the photons that got absorbed with E>B1 will only give out an energy equal to the apparent band gap B1 in the excitation process of the electron to the photocurrent. The remaining energy E−B1 will be emitted as a secondary photon of energy E2=E−B1 or multiple photons among which energy E2=E−B1 is distributed in accordance with the laws of conservation of energy and momentum and quantum mechanics. These two groups, photons with E<B1 and E2=E−B1 belong to the secondary photon population.
It is also true that some of the photons with E>B1 will not get absorbed, because they are simply unable to find the valence electron and interact with it. This fraction is not influenced by the band gap, however. The number of missed E>B1 is a function of the concentration of the ion/atom/molecule species with the valence electron N1 and the scattering cross section of this electron. Also lattice packing density of the material, temperature etc. may have some effect. In one aspect of the invention the fraction of missed E>B1 in the semiconductor layer is minimised. This group of unabsorbed photons with E>B1 is further added to the secondary photon population.
According to a further aspect of the invention, the secondary photon population and some missed E>B1 photons will pass through the first semiconductor layer and enter a second semiconductor layer with natural band gap NB2. This semiconductor layer also may have at least one electrode designed to produce an ambient voltage V2 into the layer, producing the apparent band gap B2=NB2−V2. The ambient voltage can be used to raise or lower the energy state of the electron or electrons at the valence band. In accordance with the invention, the apparent band gap B2 is optimised to produce as much photocurrent as possible and as desirable a second secondary photon population as possible.
In further embodiments of the invention, multiple semiconductor layers with different band gaps are used. Some of the layers may have an ambient voltage in them. The ambient voltage may be produced by a voltage generator attached to at least one electrode. Some of the collected photocurrent can be fed back in to power the ambient voltages V1, V2 in some embodiments. In some embodiments the semiconductor layers with ambient voltages or without may have electrically insulating films or materials in between them, which are typically transparent to the photons.
With reference to what is explained before, multiple active (with ambient voltage) and passive (no ambient voltage) semiconductor layers can be laid over each other so, that each layer collects a certain part of the solar spectrum, or secondary photon population spectrum. As the use of ambient voltages allows some liberty in tuning to a desired part of the solar spectrum or secondary photon population spectrum, the entire solar spectrum photons can be carefully collected, by collecting the maximum number of photocurrent and producing the minimum number of secondary photons with E>minimum band gap. The secondary photons with E>minimum band gap are typically dissipated as heat, because the photovoltaic system cannot absorb these because none of the photons can reach a band gap.
A further aspect of the invention involves the manufacture of a solar cell system based on the previous principles in accordance with the invention. First, the solar spectrum is measured or known in accordance with the invention. The sunlight is then incident on the first semiconductor layer with band gap NB1, and the first semiconductor layer may be tuned with an ambient voltage to an apparent band gap B1. Also other factors such as dopant concentration, donor concentration, acceptor concentration, lattice constant etc. may be tuned. Typically the first semiconductor layer is transparent to all or some of the secondary photons. Then the resulting sunlight that emerges through the first semiconductor layer, i.e. the secondary photon population is recorded with a second spectrometer. The difference in spectra between the first and the second spectrometer gives the effect of the first semiconductor layer on the solar spectrum. This difference can also be compared with the derived photocurrent from first semiconductor layer, to deduce the efficiency of the first semiconductor layer.
By tuning the parameters of first semiconductor layer, different difference and secondary spectra and efficiencies may be explored in accordance with the invention. In some embodiments, the maximum overall efficiency is derived by maximising the photocurrent collection and maximising the fit of the secondary photon population spectra to the band gap B2 and other parameters of the second semiconductor layer, which is behind the first semiconductor layer. It is clear that multiple layers can be designed in this way, preferably always optimising photocurrent collection at the layer and the fit of the secondary photon population spectra with the response of each subsequent layer.
By the response of the semiconductor layer we mean the way in which the semiconductor responds to an incoming photon spectrum, i.e. how many photons converted to photocurrent at a specific energy, how many photons pass through without interaction at a specific energy, how many photons with E>B1 pass through at a specific energy, how many photons with E<B1 pass through at a specific energy, what is the shape of the secondary photon spectra at a specific energy etc. All or some of these variables also define the bolometric response of the semiconductor layer, i.e. how many photons converted to photocurrent across all energies, how many photons pass through without interaction across all energies, how many photons with E>B1 pass through across all energies, how many photons with E<B1 pass through across all energies, what is the shape of the secondary photon spectra across all energies etc.
It is the objective of the invention to produce a semiconductor layer “sandwich” of multiple layers, each with a response that maximises the collected photocurrent and produces a secondary photon population spectra that has the maximum fit with the response of the subsequent semiconductor layer. Both the collected photocurrent and the secondary photon population spectra can be tuned by the material characteristics, and beyond the material characteristics by tuning the natural band gap with an ambient voltage to an apparent band gap that optimises the collected photocurrent and the secondary photon population spectra. When this optimisation is done for several semiconductor layers, each assigned to different small bands in the solar spectrum, the maximum number of photons can be collected throughout the whole solar spectrum that can be harnessed by photoelectric effect, thereby boosting efficiency of the solar cell system.
A solar cell in accordance with the invention comprises at least one first semiconductor layer with a natural band gap NB arranged to convert incoming photons to electric current and is characterised in that,
A method for operating a solar cell in accordance with the invention, comprises the following steps,
A method for producing a solar cell in accordance with the invention comprises the following steps,
A solar cell with at least two semiconductor layers in accordance with the invention is characterised in that,
In addition and with reference to the aforementioned advantage accruing embodiments, the best mode of the invention is considered to be a multilayer solar cell where some layers have an ambient voltage tuning the natural band gap and some layers are at their natural band gap, and where the overall photocurrent collection of the multilayer solar cell is maximised by optimising the response of each semiconductor layer with respect to the incoming spectra and the secondary photon population spectra and the response of the following layer to this secondary photon population spectra.
In the following the invention will be described in greater detail with reference to exemplary embodiments in accordance with the accompanying drawings, in which
Some of the embodiments are described in the dependent claims.
The semiconductor layer 11 or any subsequent layer mentioned in this application (12, 13, 14, 15, 16, 17, layer 1, layer 2) typically manufactured and/or grown by lithography, molecular beam epitaxy (MBE) metalorganic vapour phase epitaxy (MOVPE), Czochralski (CZ) silicon crystal growth method, Edge-define film-fed growth (EFG) method, Float-zone silicon crystal growth method, Ingot growth method and/or Liquid phase epitaxy, (LPE). Any fabrication method described in the references EP 1724 841 A1, Josuke Nakata, “Multilayer Solar Cell”, U.S. Pat. No. 6,320,117, James P. Campbell et al., “Transparent solar cell and method of fabrication”, Solar Electricity, Thomas Markvart, 2nd Edition, ISBN 0-471-98852-9 and “An unexpected discovery could yield a full spectrum solar cell, Paul Preuss, Research News, Lawrence Berkeley National Laboratory, can be applied to produce a solar cell in accordance with the invention. Any other fabrication method can also be applied to produce a solar cell in accordance with the invention.
The semiconductor layer 11 also contains electrodes 100 and 101 that provide an ambient voltage V1 inside the semiconductor layer, thereby producing an apparent band gap of B1=NB1−V1 between the valence and conduction bands of the atom/ion/molecule species. The electrodes 100 and 101 are typically connected to the voltage generator 200 that generates the ambient voltage V1 into the first layer 11.
The electrodes and electrical contacts are typically manufactured and/or grown into the semiconductor layer 11 by screen printing, as explained in Solar Electricity, Thomas Markvart, 2nd Edition, ISBN 0-471-98852-9 or by any other method in accordance with the invention.
In some embodiments the solar cell also has an antireflection coating on top of semiconductor 11, which antireflection coating can be for example of titanium oxide (TiO2) and/or Silicon Nitride Si3N4 or of any other mentioned in the references and/or any material in accordance with the invention.
The ambient voltage can be generated from electrodes that face each other in a direction opposite to the line of incident sunlight as shown here, or in fact any direction. The important thing is that they provide an ambient voltage, which should preferably be quite homogeneous across the entire first semiconductor layer 11. Some photons of the incident sunlight with E>B1 will be absorbed and converted to photocurrent, whereas some photons with E>B1 may fail to interact with electrons in the valence band, and photons with E<B1 will also pass through. The unabsorbed photons, i.e. the secondary photon population, or some of them, will pass through the electrically insulating layer and enter the second semiconductor layer 12. The insulating layer is typically a transparent material to the secondary photon population, and is made for example from, plastic film, rubber or any other material.
In some embodiments there is no insulating layer. The purpose of the insulating layer is to electrically insulate the two semiconductor layers 11 and 12 so that the ambient voltages V1 and V2 provided by the electrodes 100, 101 and 110, 111 can be controlled accurately in each layer 11 and 12, without them interfering with each other. If there is no need to prevent interference, for example in the case where V1=V2, or V1=V2=0, or the ambient voltage is allowed to distribute freely in the solar cell system 10, then there is no need for an electrically insulating layer between the two layers 11, 12 in some embodiments of the invention. The semiconductor layers 11, 12 are mounted on a substrate which can be of any material, for example a semiconductor material, glass, plastic, rubber, plastic film or the like in accordance with the invention in some embodiments.
The solar cell system 10 can be realised as a stiff solar panel, or it can also be realised as a flexible thin film solar cell, that is easily shaped on various surfaces. The electrodes 100, 101, 110, 111 can be arranged to also collect the photocurrent from the semiconductor layers 11 and 12 in some embodiments of the invention, or other dedicated electrodes may be arranged to handle the photocurrent collection.
In some embodiments the voltage generator 200 is powered with the energy that is derived from the collected photocurrent. Thus the solar cell system 10 is capable of feeding back a portion of its collected solar energy to improve the efficiency to produce more solar energy further still in this embodiment of the invention.
Quite obviously an embodiment where either or both of the semiconductor layers 11 and 12 do not have an ambient voltage or associated electrodes is also in accordance with the invention. For example in some embodiments the semiconductor layer 11 may be at its natural band gap, but the apparent band gap of B2 is tuned from NB2 by V2 to collect the secondary photon population entering semiconductor layer 12 better than without the tuning, i.e. at natural band gap NB2.
Let's assume that the natural band gap NB3=0.935 eV, equaling roughly 1300 nm in wavelength. For the purposes of illustrating the invention, further assume ambient voltage V3=0.16 eV, amounting to apparent band gap B3=0.775 eV. Things do not look very good for the 1.55 eV photon that did not get absorbed at the second semiconductor layer. It can get absorbed, but it will either leave one 0.775 eV photon to hunt for another absorption in third semiconductor layer, which it might not get and which is less probable, or two 0.388 eV photons that cannot get absorbed and will be dissipated as heat.
Things look better for the two secondary 0.775 eV photons that could have been left at the first semiconductor layer 11. They pass through the second semiconductor layer 12 and have only to get one absorption process from the third semiconductor layer, and they have converted their energy to photocurrent and solar energy with 100% efficiency. To explain, it is comparatively more probable for a photon to get absorbed once rather than to get absorbed once AND the secondary photon to also get absorbed.
As is above clearly explained the semiconductor layers can be tuned by the ambient voltages to maximise the overall photocurrent, by optimising the photocurrent and the most desirable secondary photon population spectra at each stage in accordance with the invention.
The next layer comprises the electrical contacts 50, or a electrical conductor layer 50 needed to transport the collected photocurrent. The electrodes providing ambient voltage 100, 101, 110, 111, 120, 121 and electrical contacts 50 are typically manufactured and/or grown into the semiconductor layers 11, 12, 13, by screen printing, as explained in Solar Electricity, Thomas Markvart, 2nd Edition, ISBN 0-471-98852-9 or by any other method in accordance with the invention. Alternatively, they could be implemented as a separate layer on top the semiconductor layers 11, 12, 13 in some embodiments. In this embodiment the conductor layer is typically transparent in accordance with the invention. The electrical contacts and/or the electrodes preferably occupy the minimum area when meshed with the semiconductor layers 11, 12 and/or 13. Semiconductor layer 11 is typically InGaP-layer at approximately band gap 1.93 eV in this embodiment. Alternatively, in some embodiments the semiconductor layer could be realised with a GaN-layer, preferably with a band gap of 3.4 eV in accordance with the invention.
The next semiconductor layer 12 is typically of polycrystalline silicon at band gap of 1.1 eV, and the third semiconductor layer is typically of InSb at a band gap of 0.17 eV. The three layers 11, 12, 13 provide an impressive dynamic range of 0.17-3.4 eV by their natural band gaps, which dynamic range can still be further enhanced by providing at least one ambient voltages V1, V2 and/or V3 to the layers 11, 12 and/or 13. The photon statistics work as explained in
In some embodiments the need for insulating layers is entirely optional, in these embodiments some or all of the insulating layers can be omitted in accordance with the invention. The ambient voltage can also be arranged to vary within the semiconductor layer 11, 12, 13 in some embodiments, for example from one edge of the layer to another edge of the layer, there by causing a distribution of band gaps in the layer. For example, if the ambient voltage varies by +/−V, then there would be a distribution of band gaps in the material, broadened by 2V from the natural band gap.
The apparent band gaps B1. B2 B3, B4, B5, B6. B7 could be tuned into a sequence that optimises photon collection. Not all layers need not have ambient voltages induced by electrodes, some of the layers may be at their natural band gap, for example NB5=B5 in some embodiments. The ambient voltage is arranged to be used to adjust the band gap B1, B2, B3, B4, B5, B6, B7 of the semiconductor layer so that the band gap B1, B2, B3, B4, B5, B6, B7 is optimised with respect to the collected photocurrent, secondary photon population, the response of the subsequent semiconductor layer to the said secondary photon population, quantum efficiency of another possibly subsequent semiconductor layer and/or the energy consumed in providing the ambient voltage. The concentration of the atom/molecule/ion species and/or the thickness of the semiconductor layer is arranged to be optimised in this way also in some embodiments of the invention.
The layers are typically very thin, such as few nanometers at the slimmest or centimetres at their thickest in accordance with the invention. The layer thickness is typically in proportion to the photon population at that energy. If the photon population is a lot higher at E3=B3 than at E1=B1, then the thickness of the first semiconductor layer 11 can be slimmer than that of the third semiconductor layer 13. By similar argument, as more photons need more valence electrons to interact with, the concentration or total number of the atom/molecule/ion species could be higher for third semiconductor layer.
The apparent band gaps (B1-B7) could be set at for example 4.35, 3.73, 3.1, 2.48, 1.86, 1.24, 0.6 eV. One can calculate the photon populations layer (11-17) through layer (11-17) in the same way as shown for the embodiments 30 and 40. It is also in accordance with the invention to set the apparent band gaps so, that they guide a maximum population of the photons to the band where the layers or some layer has the best quantum efficiency. For example, if it is known that the quantum efficiency at 1.86 eV is great for layer 13, it is preferable to set the band gaps in preceding layers so that they form a maximum number of photons at 1.86 eV which can be used with great efficiency in accordance with the invention in some embodiments.
In phase 620 photons with energy E<B1 pass through the first semiconductor layer. Some of the photons with energy E>B1 get absorbed and are converted to photocurrent in phase 630, secondary photons left with E−B1 of energy are left from the absorbed photons to conserve energy in phase 630. Photons with E<B1 and secondary photons with E=E−B1 are incident on second semiconductor layer with natural band gap NB2 in phase 640. Also those photons that had E>B1 but which did not get absorbed belong to this photon population, the secondary photon population consisting of at least the photons of these three groups is incident on the second semiconductor layer in phase 640 in some embodiments of the invention. In phase 650 the tuning of the apparent band gap B2 from the natural band gap is done by applying an ambient voltage V2.
As more semiconductor layers with the same natural and/or apparent band gaps are added to the system, the steps of 610, 620, 630, 640 are repeated for some or all of the subsequent semiconductor layers and natural band gaps in accordance with the invention.
In phase 720 NB1 the natural band gap is adjusted by tuning the concentration N or total number of the atom/molecule/ion species in the semiconductor layer 1 of
In phase 750 NB2 the natural band gap is adjusted by tuning the concentration N or total number of the atom/molecule/ion species in the semiconductor layer 2 of
In phases 720 and/or 750 the ambient voltage and other variables are tuned to maximise the captured photocurrent from the incident sunlight and the fit of the resulting unabsorbed sunlight spectrum with the response of the next subsequent semiconductor layer. Typically NB1, NB2, B1 and/or B2 is optimised with respect to the collected photocurrent, secondary photon population, the response of the subsequent semiconductor layer to the said secondary photon population, quantum efficiency of another possibly subsequent semiconductor layer and/or the energy consumed in providing the ambient voltage V1 and/or V2.
It is within the scope of the invention that any embodiments 10, 20, 30, 40, 50, 60, 70 and/or 80 may be readily combined and or permuted. Any features explained in association with one embodiment 10, 20, 30, 40, 50, 60, 70 and/or 80 can be used with another embodiment 10, 20, 30, 40, 50, 60, 70 and/or 80 in accordance with the invention.
In some embodiments of the invention, the aim is simply to optimise the detector response of each layer to the spectrum emerging from the previous layer. In this embodiment there is not always a need for an ambient voltage V1. The invention may be practiced without an active cell or an ambient voltage by optimising the detector response of the next layer to the spectrum emerging from the previous layer which is composed of uninfluenced photons, scattered photons, recombined photons and photons from photon-phonon interactions. It is in accordance with the invention to also use an ambient voltage.
This embodiment is described in detail in the following. It has been described earlier that some photons simply pass through the first layer without interaction. It has also been explained before that there are some photons that scatter, but still emerge to the next layer. It has further been explained before that some photons that get absorbed produce more secondary photons to abide to conservation of energy and laws of quantum mechanics (recombined photons). What was not explained before is that not only do the recombined photons turn directly to IR-photons which are synonymous with heat radiation, they also heat the solar cell itself by causing thermal vibration in the material itself. The quanta of this vibration is the phonon. Because the solar cell cannot heat to infinite temperatures, i.e. it must be in thermodynamic equilibrium with its surroundings it must radiate some of the heat. Therefore the vibrational phonon quanta turn into new recombinant photon quanta, that may again be photoelectrically collected in accordance with the invention. It is in accordance with the invention to also optimise the band gaps of the materials with respect to these four photon populations, without necessarily using an ambient voltage.
The dopant concentration, acceptor concentration, donor concentration, lattice structure, temperature and/or relative concentrations of the semiconductor materials can all be optimised to deliver the best response to the spectrum emerging through the first semiconductor layer. It is in accordance with the invention to run tests with different thermal environments for the cell materials to measure the photon-phonon-photon spectra at different semiconductor layers, and optimise the detector response to these spectra, i.e. choosing the best thermal environment-detector response couple. Overall, the combined fit of the detector responses to the incoming solar spectrum and the emerging spectra through each semiconductor layer should be optimised to maximise collected photocurrent. This is achieved by measuring the emerging spectrum behind each semiconductor layer and by adjusting the detector response of the next semiconductor layer to match with this spectrum as well as possible.
Now in the invention, a further ambient voltage V is provided, which may vary as a function of position shown as V(r) in
It is clear the V(r) might introduce charge migration in the vertical direction of the page, which may be a significant advantage of the invention. The photocurrent is collected in the horizontal direction of the page in
It is also clear that the embodiment of
The invention has been explained above with reference to the aforementioned embodiments and several commercial and industrial advantages have been demonstrated. The methods and arrangements of the invention increase the efficiency of solar cells. The methods and arrangements of the invention therefore improve the competitiveness of solar energy, and make it more available to people and communities globally.
The invention has been explained above with reference to the aforementioned embodiments. However, it is clear that the invention is not only restricted to these embodiments, but comprises all possible embodiments within the spirit and scope of the inventive thought and the following patent claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20070264 | Apr 2007 | FI | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13632351 | Oct 2012 | US |
| Child | 13693445 | US | |
| Parent | 12594295 | Oct 2009 | US |
| Child | 13632351 | US |
