The present disclosure relates to photovoltaic diode devices.
As is known in the art, some solar cells comprise multiple stacked sub-cells, each comprising a photodiode, with the light to be absorbed passing through the sub-cells in turn, each sub-cell absorbing a different range of frequencies (or equivalently a different range of energies) due to each sub-cell having a different bandgap. These solar cells are often called multi-junction photovoltaic (PV) solar cells. Dilute nitride Group III-V semiconductors are of interest for application in high efficiency multi-junction PV devices as sub-cells with about a 1 eV bandgap. The absorption threshold of these semiconductors (i.e. the minimum photon frequency/energy that will excite an electron across the bandgap) can be adjusted by including a few percent of nitrogen (N) in the semiconductor, making them suitable candidates for fabricating sub-cells that absorb light in the near infrared.
Dilute nitride Group III-V semiconductors can be grown lattice matched to GaAs when indium and/or antimony are included in the material. The incorporation of N into GaInAsSb tends to reduce the minority carrier lifetime to less than 1 ns, resulting in diffusion lengths of 200 nm or less. The conventional approach to solving the problem of short diffusion lengths has been to grow depleted n-i-p junctions, exploiting drift transport of the photo-generated carriers in the depletion region. See for example Jenny Nelson, “The Physics of Solar Cells” from the series “Properties of Semiconductor Materials” 1st Edition (Sep. 5, 2003), published by Imperial College Press (ISBN-10:1860943497, ISBN-13:978-1860943492). The n-i-p diode 100 of such a sub-cell is illustrated in
In this device 100, the diode junction is formed by three layers. The top, emitter layer 101 of the junction is n-type dilute nitride GaInNAsSb emitter layer. Here, “top” is used to indicate the layer of the junction which receives the incident light first (in the diagram of
According to the present disclosure there is provided a photovoltaic diode comprising: an emitter layer of doped Group III-V semiconductor material, having a first conductivity type and a first bandgap in at least part of the layer; an intrinsic layer of dilute nitride Group III-V semiconductor material having a composition given by the formula Ga1-zInzNxAsySb1-x-y, where 0<z<0.20, 0.01<x<0.05, and y>0.80 having a second bandgap; a base layer of semiconductor material having a third bandgap and a second conductivity type opposite to the first conductivity type, wherein the emitter layer, intrinsic layer, and base layer form a diode junction, and wherein the first bandgap is greater than the second bandgap.
That difference in bandgap provides a barrier for minority photo-generated carriers.
The base layer may be a layer of dilute nitride Group III-V semiconductor material having a composition given by the formula Ga1-zInzNxAsySb1-x-y, where 0<z<0.20, 0.01<x<0.05, and y>0.80.
The emitter layer may comprise a wide bandgap emitter layer of Group III-V semiconductor material having the first bandgap and a narrow bandgap emitter layer between the wide bandgap emitter layer and the intrinsic layer, the narrow gap emitter layer having the first conductivity type and being of a dilute nitride Group III-V semiconductor material having composition given by the formula Ga1-zInzNxAsySb1-x-y, where 0<z<0.20, 0.01<x<0.05, and y>0.80, wherein the narrow gap emitter layer has a fourth bandgap that is smaller than the first bandgap.
The fourth bandgap may be the same as the second bandgap. The fourth bandgap may be between the first and second bandgaps. The narrow bandgap emitter layer may be lattice matched to the wider bandgap emitter layer. The narrow bandgap emitter layer may be lattice matched to the intrinsic layer. The narrow bandgap emitter layer may be less in thickness than a diffusion length of the minority carriers. The narrow bandgap emitter layer may be less in thickness than 200 nm. The narrow bandgap emitter layer may be 100 nm in thickness.
The emitter layer may comprise a graded dilute nitride Group III-V semiconductor material layer having a composition and bandgap graded through the thickness of the graded layer, the composition through the graded layer being within the formula Ga1-zInzNxAsySb1-x-y, where 0<z<0.20, 0.01<x<0.05, and y>0.80.
The emitter layer may comprise a graded aluminium gallium arsenide semiconductor material layer having a composition and bandgap graded through the thickness of the graded layer.
The bandgap of the graded layer of the emitter may have an interface with the intrinsic layer and at that interface has a bandgap equal to that of the intrinsic layer at that interface. The bandgap of the graded layer of the emitter may have an interface with the intrinsic layer and at that interface may have a same composition to that of the intrinsic layer at that interface. The graded layer of the emitter may have an interface with or continue in a further compositional grade with a layer of gallium arsenide or aluminium gallium arsenide.
The intrinsic and base layers may have the same composition of semiconductor material. The intrinsic and base layers may have the same band gap as each other.
The base layer may comprise a graded dilute nitride Group III-V semiconductor material layer having a composition and bandgap graded through the thickness of the graded layer, the composition through the graded layer being within the formula Ga1-zInzNxAsySb1-x-y, where 0<z<0.20, 0.01<x<0.05, and y>0.80. The bandgap of the graded layer of the base may have an interface with the intrinsic layer and at that interface may have a bandgap equal to that of the intrinsic layer at that interface. The bandgap of the graded layer of the base may have an interface with the intrinsic layer and at that interface may have a same composition to that of the intrinsic layer at that interface.
The emitter layer may comprise a layer of gallium arsenide. The emitter layer may comprise a layer of aluminium gallium arsenide.
The intrinsic layer may have a bandgap in the range 0.7 to 1.4 eV. The base layer may have a bandgap in the range 0.7 to 1.0 eV.
The emitter, intrinsic and base layers may be lattice matched to each other.
The present disclosure also provides a solar cell comprising the photovoltaic diode.
The present disclosure further provides a multijunction photovoltaic device comprising the photovoltaic diode.
The present disclosure also provides a multijunction photovoltaic device comprising a first one of the said photovoltaic diodes as one of its junctions, and a second one of the said photovoltaic diodes as one of its junctions, wherein the base of the first and second photovoltaic diodes have different bandgaps.
The present disclosure further provides a method of generating electricity using the photovoltaic diode, comprising: directing light into the photovoltaic diode in through the emitter layer in the direction of the intrinsic and base layers, absorbing the light in the intrinsic layer to generate photo carriers, and the diode separating the photo-carriers to generate electricity.
Embodiments will now be described, with reference to the accompanying drawings, of which:
The layers may be grown in turn epitaxially on a lattice matched substrate. This may be in the order base layer 203, then intrinsic layer 202, then emitter layer 201. However, as is known in the art, the layers could be grown on a substrate in the other direction, and then removed from that substrate and turned over before being mounted on another substrate.
A preferred range for the composition of dilute nitride GaInNAsSb layers are given by the formula Ga1-zInzNxAsySb1-x-y, where 0<z<0.20, 0.01<x<0.05, and y>0.80. The base layer 203 and the intrinsic layer preferably have the same composition, but may be of different compositions, which is also possible even in cases where they have the same band gap as well as being lattice matched to each other (given the number of different elements from which the material is formed).
Again, as is known in the art, the top layer 201 may be overlaid (i.e. to the left in the diagram) with other sub-cells (see
As with the known example of
In the present example, i.e. that of
Now, with the known n-i-p homojunction described above with respect to
Also, a useful advantage of having GaAs or AlGaAs for the material of the n-layer 201 is that those are compatible with having an overlying tunnel junction and barrier for minority carrier holes, which are typically be used between the sub-cells of a multijunction solar cell. In such one example of compatibility the next layer above the next later above 201 is formed of GaAs or AlGaAs.
It would also be possible to have a heterostructure “p-i-n” diode in accordance with the invention. An example would be similar to that of
The narrow bandgap emitter layer 301b, because it has a narrower bandgap than main emitter layer 301a absorbs photons passing on from the wide bandgap emitter layer that have an energy greater than the bandgap of the narrow bandgap gap emitter layer, to produce electron-hole pairs. Because the thickness cn (typically 10 nm) of the narrow bandgap emitter layer 301b is less than the absorption length for the photons, not all such photons are absorbed in the narrow bandgap emitter layer 301b and the remainder pass on to the intrinsic layer 302 where they are absorbed, as in the previous examples. Although the narrow bandgap emitter layer 301b is doped, its thickness in this example is equal to or thinner than the diffusion length of the photo carriers in the material of that layer, so quite quickly the electrons diffuse or drift into the wide bandgap emitter layer 301a and, importantly, the minority carrier holes diffuse into the depletion region (where they are transported by the electric field of the depletion region across the intrinsic region 302 to the base 303). The step in the valance band edge between the narrow bandgap emitter layer 301b to the wide bandgap emitter later 301a acts as a barrier to those holes diffusing into the wide bandgap emitter layer 301a, where of course they would recombine with the electrons. In this example then, the overall region that absorbs the photons is that of combined lengths cn and w (noting of course that cn and w overlap slightly). In practice this provides extra length for absorption, since as noted above, the practical length of w is limited by the background doping level of the intrinsic region that is achievable (which is the same as for the example of
In the example of
Similarly the compositional grade 403a narrows the bandgap from its interface with the intrinsic later until it equals that of base layer 403b.
In this example, the grade layers also have grades in the dopant levels, increasing away from the respective interfaces with the intrinsic region. As there is active doping in these regions the depletion region terminates a short distance into each.
The compositional graded layers each further extend the collection length in the solar cell by inducing an electric field in the region of cn and cp, resulting in an active collection length of the combination of cn, w, and cp. (Note that there is a small overlap between w and cn, and between w and cp.) Conveniently, the compositional and doping grades 401 and 403a also provide an electric field to drift the minority carriers, leading to a higher photo-carrier collection efficiency.
The design constraints are that (1) that the thickness (cn & cp) of graded layers 401 and 403a should correspond to the combination of the graded semiconductor materials, doping grade and diffusion length of the doped semiconductor and (2) the intrinsic region 402 thickness ti is determined by the background impurity concentration level, to ensure that the intrinsic layer 402 remains depleted at the operating voltage.
The compositional grade of the base layer may also be used, for example, in the embodiments of
Note that in
The above devices may be fabricated by known techniques such as molecular beam epitaxy (MBE) or metal organic vapour phase epitaxy (MOVPE). The International patent application published as No. WO2009/157870 discloses a method of fabrication of the dilute nitride materials, and is incorporated herein by reference in its entirety.
Number | Date | Country | Kind |
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1618024.2 | Oct 2016 | GB | national |
This application is a continuation of PCT International Application No. PCT/GB2017/053200, filed Oct. 24, 2017, which claims priority to GB Application No. 1618024.2, filed Oct. 25, 2016, both of which are incorporated by reference herein in their entirety.
Number | Date | Country | |
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Parent | PCT/GB2017/053200 | Oct 2017 | US |
Child | 16393815 | US |