The present invention relates to photovoltaic devices having more than one subcell for absorbing different parts of the spectrum of the incident light.
Multijunction photovoltaic devices comprise a series of subcells each having a light absorbing semiconductor material and a p-n junction therein to separate the photocarriers, to produce the photocurrent. They work by having a top subcell, i.e. that first exposed to the incident light, that has a large bandgap and so absorbs the shorter wavelengths in the incident light only and passes the longer wavelengths, with the next subcell having a smaller bandgap so that it can absorb part of the light passed by the subcell above, and so on. Solar cells are, of course, one kind of photovoltaic device and are ones used to convert sunlight into electricity for the purpose of generating power.
In this document “top” and “bottom” are to be understood in that sense, i.e. the top subcell is that which receives the incident light first in normal use, rather than the actual spatial orientation of the device. “Above” and “below” are also to be understood similarly, unless the context demands otherwise. Further light is not to be understood as visible light only. For example 1.0 eV and 0.7 eV bandgaps discussed in this document absorb light in the infra-red region.
Multijunction photovoltaic devices often use lattice matched material, which is to say that when one material is grown on another the lattice parameters of the two materials match to an extent that the crystal structure of the material being grown is maintained and strain relieving dislocations are not introduced. When the lattice parameters of the two materials, in the bulk form, are not quite equal the layer being grown becomes strained, i.e. its lattice parameter changes to match that of the layer on which it grown. Where the strain is quite small the layer being grown can be grown to an arbitrary thickness without the introduction of dislocations. The thickness at which strain relieving dislocations first appear is defined as the critical thickness for a material. In this context, the term lattice matched would also refer to strained layers grown to thicknesses below the critical thickness. Some examples of known multijunction solar cells are as follows.
US2009/00140161 (Harris) describes a triple junction solar cell as shown in
US2011/0232730 (Jones) proposes a lattice matched triple junction solar cell having a similar set of subcells to that of Harris.
US2011/0083729 (Lee) also discloses a triple junction solar cell. The structure of Lee is shown in
GB2467934, also owned by the present applicant company, discloses a multijunction solar subcell, shown in
WO2009/157870 (Yoon) discloses an advantageous method for fabricating GaNAsSb, which method, as is explained below, may be used in the preferred examples of the invention. The disclosure of this document is incorporated herein by reference.
According to the present invention there is provided a multijunction photovoltaic device comprising:
The multijunction photovoltaic device may further comprise a light absorbing layer of gallium arsenide material including a photocarrier separating p-n junction.
The multijunction photovoltaic device may further comprise a light absorbing layer of indium gallium phosphide including a photocarrier separating p-n junction and being lattice matched to gallium arsenide.
The multijunction photovoltaic device may further comprise a light absorbing layer of aluminium gallium arsenide including a photocarrier separating p-n junction and being lattice matched to gallium arsenide.
The multijunction photovoltaic device may further comprise a light absorbing layer of aluminium indium gallium phosphide including a photocarrier separating p-n junction and being lattice matched to gallium arsenide.
These additional layers absorb shorter wavelengths than the silicon germanium, or silicon germanium tin, and gallium nitride arsenide antimonide layers.
The multijunction photovoltaic device may comprise a gallium arsenide substrate, the set of layers being on and lattice matched to the substrate. Alternatively the multijunction photovoltaic may comprise a substrate that is lattice matched to gallium arsenide, the set of layers being on and lattice matched to the substrate.
The multijunction photovoltaic device may be a solar cell.
The present invention also provides a method of making a multijunction photovoltaic device comprising:
The method may comprise growing a light absorbing layer of gallium arsenide material, including a photocarrier separating p-n junction.
The method may comprise growing a light absorbing layer of indium gallium phosphide, including a photocarrier separating p-n junction, lattice matched to gallium arsenide.
The method may comprise growing a light absorbing layer of aluminium gallium arsenide, including a photocarrier separating p-n junction, lattice matched to gallium arsenide.
The method may comprise growing a light absorbing layer of aluminium indium gallium phosphide, including a photocarrier separating p-n junction, lattice matched to gallium arsenide.
The method may comprise providing at least one further layer between two neighbouring ones of the said light absorbing layers, the at least one further layer being lattice matched to gallium arsenide.
The method may comprise removing the substrate.
Preferably the light absorbing layers are arranged in order of bandgap. This may often mean that they are grown in order of bandgap but it would be possible to grow only some of the light absorbing layers, remove the substrate and continue growth in the other direction (a substrate usually being provided on the other side).
The light absorbing layers with their p-n junctions of the invention may, as is known in the art, each be comprised in a respective region of a multijunction photovoltaic device conventionally known as a subcell, which may have further layers.
The gallium nitride arsenide antimonide subcell and silicon germanium, or silicon germanium tin, subcell of the invention are able between them to provide good absorption coverage of the spectral wavelengths longer than those absorbed by, for example, a gallium arsenide subcell, offering high absorption efficiency. This is in contrast of the approach of US2009/00140161 (
As noted above, subcells, both in the invention and as is known generally in the art, may comprise additional layers. For example, as is known in the art, tunnel contacts may be inserted between the light absorbing subcells of multijunction photovoltaic devices, and these may be so used in the present invention, to provide good electrical connection between the subcells, and to allow the p-n junction in the neighbouring subcells to have the same polarity so that current may flow through the device. Window layers and back surface field (BSF) layers, as also known in the art, are preferably also incorporated into the structure of the device of the invention. A window is usually provided at the top of each cell and a back surface field at the bottom of each subcell and these are preferably provided in the invention. However, as noted later the invention does provide an advantage in relation to these.
Examples of the invention will now be described, with reference to the accompanying drawings of which:
In this example, additional layers, which are tunnel contacts (which are provided between the subcells to provide good electrical contact), windows and back surface fields, are incorporated into the structure. These are between the subcells and above and below the light absorbing layers as appropriate. These additional layers are also lattice matched to the GaAs substrate, but as their use is well known in the art and for simplicity of illustration these layers not shown in
The composition of the light absorbing GaNAsSb layer of subcell 52 is such that its bandgap is preferably in the range 0.8<Eg<1.2 eV. A bandgap of around 1.0 eV is particularly advantageous since it fills the gap between the parts of the spectrum absorbed by the SiGe and GaAs layers.
The precise proportions of N and Sb in the GaNAsSb for a particular bandgap and lattice matching can of course be experimentally determined easily for any particular case. However for lattice matching to GaAs the ratio of N to Sb is about 1:2.6. For bandgaps equal to GaAs (1.4 eV) down to 0.9 eV the respective proportion of N and Sb, x and y, in the material lie in the range 0<x<6% and 0<y<16%.
To make the SiGe light absorbing layer in subcell 51 lattice matched to GaAs the proportion of silicon is around 0.018 and this material then has a bandgap of around 0.7 eV. As can be seen in
However, if desired a larger bandgap can be obtained for subcell 51 by using SiGeSn instead of SiGe. This material is lattice matched to GaAs where the ratio of Si to Sn is approximately 4:1. Where for example the proportion of Si is 2% and that of Sn is 0.5% this provides a larger bandgap than SiGe lattice matched to GaAs, where the proportion of Si is 8% and that of Sn is 2% the bandgap is wider, and where the proportions are much larger the bandgap can extend further.
In another similar example the light absorbing material layer of subcell 55 is AlInGaP lattice matched to GaAs. Again the bandgap of is preferably about 2.0 eV but since the material is a quaternary some extra flexibility is obtained.
The subcells examples of
Note also that while the substrate of the examples is GaAs the invention also extends to cases where the substrate on which the subcells are grown is another material that is lattice matched to GaAs.
Note further that if not needed for certain applications any one or more of subcells 53, 54 or 55 can be omitted.
The materials of the subcells may be grown using epitaxial techniques including MBE and MOCVD.
For example, GB2467934, also owned by the present applicant company and mentioned above, discloses examples of SiGe materials manufactured on GaAs substrates. The document is incorporated herein by reference. These materials can be grown using an epitaxy process, using a germanium containing precursor (e.g GeH4, GeCl4, etc.) and a silicon containing precursor (e.g. SiH4, SiH2Cl2, SiHCl3, disilane etc.) with a carrier gas (e.g. H2). The p-n junction used to separate the photo carriers may be formed in the SiGe material by various methods. These include doping during the epitaxial growth, or by diffusion in of dopant into a layer of the material when it is grown or is partially grown. An alternative method is given below.
The SiGe or SiGeSn material used in subcell 51 will in many examples have III-V material directly grown on it. There may then be diffusion of Group V atoms from the III-V material into the SiGe or SiGeSn. Arsenic atoms, for example, will do this. Arsenic in SiGe or SiGeSn is an n-type dopant.
So if the SiGe or SiGeSn neighbouring the III-V material grown on it is p-type then the Group V atoms diffusing into the SiGe or SiGeSn may well dope the SiGe or SiGeSn forming a p-n junction below the surface of the SiGe or SiGeSn. (If this junction were to be parasitic in a particular example it can be prevented or controlled by forming a thin Si diffusion barrier between the SiGe or SiGeSn. Three atomic layers is sufficient.) However, as foreshadowed, the junction so formed can be utilised as the photocarrier separating junction in the SiGe or SiGeSn subcell. Alternatively it may be used as a tunnel diode between the SiGe or SiGeSn subcell and the subcell grown on that. A thinner Si barrier can be used to control the amount of diffusion if that is desired.
On the other hand if the SiGe or SiGeSn neighbouring III-V material grown on it is n-type (i.e. an epitaxially grown SiGe or SiGeSn p-n junction) the diffusion in of Group V atoms will not form an extra p-n junction.
The GaNAsSb of the invention layer is also formed by epitaxy. The p-n junction for separating the photocarriers therein is formed either by doping during the epitaxial growth, or, alternatively, after the layer, or part thereof has been grown and diffusing the dopant in. The above mentioned WO2009/157870 described a particularly advantageous method for growing this material.
GB2467934, and also GB2467935 also owned by the present applicant, describe techniques that may be employed in the present invention to remove the GaAs substrate from the rest of the device. Those documents are therefore incorporated by reference. This is useful, for example, to reduce weight of the device (useful for space applications), to allow a heat sink to be bonded to the device or to allow the substrate to be re-used to reduce cost.
The GaAs substrate may be removed when all the subcells have been grown in order on it (which may be in either order). Alternatively the GaAs substrate can be removed at any point during the growth and that may be part way through a subcell including partway through the just one cell. In these methods a new substrate is mounted on the just grown surface.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB2012/051981 | 8/14/2012 | WO | 00 | 8/13/2014 |
Number | Date | Country | |
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61528650 | Aug 2011 | US |