The present invention belongs to the field of solar cells.
Single gap solar cells are made of a single bandgap semiconductor. The energy of the bandgap of this semiconductor, EG, is the main parameter limiting the performance of these single gap solar cells, since photons from the sun having energy lower than this bandgap cannot be absorbed. To the inventors' knowledge, the first patent of a single gap solar cell was based on silicon and was issued on Feb. 5, 1957 to Chapin, Fuller, and Pearson as U.S. Pat. No. 2,780,765 entitled “Solar Energy Converting Apparatus.”
The basic structure of a single gap solar cell is that of a p-n junction, including a p-type semiconductor (1) and an n-type semiconductor (2), as illustrated in
A limitation of single gap solar cells is that only photons with energy above the semiconductor bandgap are absorbed by these cells. To overcome this limitation of photons with energy below the bandgap being wasted, multijunction solar cells have been proposed. See, e.g., E. D. Jackson, “Areas for improvement of the solar energy converter,” Trans. Conf. on the Use of Solar Energy, Tucson, 1955, University of Arizona Press, Tucson 5, 122-126 (1958). Under this approach, cells with different gaps are piled one over the other, the one with the highest bandgap being the first facing the sun and the others being located behind this one in decreasing order of their bandgaps.
In this respect, the simplest solution provided for contacting the two cells relies on the use of a semiconductor tunnel junction. A tunnel junction is a p-n junction characterized by an extremely high doping of the p and n layers. The notation usually used to designate a tunnel junction is p++-n++. While a conventional non-illuminated p-n junction prevents electrical current from circulating from the n-side towards the p-side of the junction, a tunnel junction does allow electrical current to circulate from the n++ side to the p++ side of the junction. Hence, as illustrated in
The present invention provides a solar cell according to the claims, as well as a method of generating electric power utilizing at least one solar cell as described herein. The dependent claims define preferred embodiments of the invention.
In certain embodiments according to the present invention, a solar cell comprises
a three layer semiconductor structure comprising a top layer, a bottom layer, and a middle layer, the middle layer being placed between and in contact with the top layer and the bottom layer, the three layer semiconductor structure being a p-n-p structure or an n-p-n structure, and
three electrical contacts, each electrical contact being connected to a different one of the top layer, the bottom layer, and the middle layer,
wherein:
each of the top layer and the middle layer comprises at least one semiconductor having a bandgap higher than the bandgap of a semiconductor of the bottom layer, and
the middle layer has a higher dopant concentration than the top layer.
Advantageously, embodiments of the present invention allow the minimum number of layers of the monolithically integrated multijunction solar cell to be reduced to three layers.
Thus, embodiments of the invention may comprise an n-p-n or a p-n-p semiconductor structure where the top layer and the middle layer are made of a semiconductor of higher bandgap than the bottom layer and where the middle layer has a higher dopant concentration than the top layer. The top layer will be understood as the layer intended to be firstly illuminated by solar light. Three terminals or contacts are provided, with each terminal or contact being connected to a different semiconductor layer.
In certain embodiments the semiconductor structure comprises, or alternatively consists of, an n-p-n structure.
In certain embodiments the semiconductor structure comprises, or alternatively consists of, a p-n-p structure.
In certain embodiments the solar cell comprises a passivating layer provided on the top layer, on the surface intended to be exposed to the solar light, i.e. the surface of the top layer opposed to the surface of the top layer in contact with the middle layer. Advantageously, the passivating layer reduces surface recombination. In certain embodiments, this passivating layer provided on the top layer comprises, or alternatively consists of, a material with a bandgap higher than the bandgap of the top layer in order to avoid photon absorption and, because of this property, the passivating layer may also be called a window layer.
Alternatively or additionally to any previous embodiment, in certain embodiments a solar cell comprises a passivating layer provided on the bottom layer, on a surface of the bottom layer opposed to the surface of the bottom layer in contact with the middle layer. Advantageously, the passivating layer reduces surface recombination.
Some common materials used as passivating and window layers (e.g., when the solar cell is based on III-V semiconductors) are AlGaAs and Al(Ga)InP.
Alternatively or additionally to any previous embodiment, in certain embodiments a solar cell comprises a contact layer provided between at least one of the semiconductor layers (i.e., to top layer, the bottom layer, and the middle layer) and the electrical contact connected to said semiconductor layer. In certain embodiments, any two or all three of the top layer, the bottom layer, and the middle layer has associated therewith a contact layer between (i) the top layer, the bottom layer, and/or the middle layer, and (ii) the electrical contact connected to said layer(s). Advantageously, the contact layer improves the contact between the semiconductor layer and the associated electrical contact. In certain embodiments, contact layers may comprise (or alternatively consist of) the same material of the semiconductor layer to be contacted. When the semiconductor layers comprise III-V semiconductors, the one or more contact layers may comprise GaAs. In preferred embodiments, contact layers are characterized by a high doping concentration (e.g., in the range of 1019 cm−3).
Alternatively or additionally to any previous embodiment, in certain embodiments a solar cell comprises an anti-reflecting coating deposited on the top layer, on a surface of the top layer opposed to the surface of the top layer in contact with the middle layer. If a passivating or window layer has been deposited on the top layer, the anti-reflecting coating is deposited on top of the passivating or window layer on the surface of the passivating layer or window layer opposed to the surface in contact with the top layer. Such anti-reflecting coating preferably embodies a topmost surface of the solar cell. Advantageously, the anti-reflecting layer minimizes the amount of sunlight that is reflected by the cell. In certain embodiments, this anti-reflecting coating may comprise, or alternatively consist of, more than one layer, such as for example MgF2/ZnS or MgF2/Ta2O5 double anti-reflecting coatings.
Alternatively or additionally to any previous embodiment, in certain embodiments of the solar cell, the electrical contact connected to the top layer comprises or alternatively consists of a grid allowing light to pass through the electrical contact and thereby impinge on the top layer.
Alternatively or additionally to any previous embodiment, in certain embodiments of the solar cell the electrical contact connected to the top layer comprises or alternatively consists of a semitransparent conductor allowing light to pass through the electrical contact and thereby impinge on the top layer. An example of a semitransparent conductor useful for forming a contact for connection to a top layer is Indium tin oxide (ITO).
Alternatively or additionally to any previous embodiment, in certain embodiments of the solar cell the electrical contact connected to the bottom layer comprises or alternatively consists of a layer at least partially covering the bottom layer on a surface of the bottom layer opposed to the surface of the bottom layer in contact with the middle layer.
Alternatively or additionally to any previous embodiment, in an embodiment of the solar cell the top and middle layers are made of AlxGa1-xAs and the bottom layer is made of AlyGa1-yAs, with y<x.
Alternatively or additionally to any previous embodiment, in an embodiment of the solar cell the top and middle layer comprise, or alternative consist of, GaAs and the bottom layer comprises, or alternatively consists of, Ge.
Alternatively or additionally to any previous embodiment, in certain embodiments of the solar cell, the top and middle layer comprise, or alternatively consist of, AlGaAsP and the bottom layer comprises, or alternatively consists of, Ge or GaAs or InP or Si.
The invention also provides a method of generating electric power comprising use of at least one solar cell as disclosed herein.
All the features described in this specification (including the claims, description and drawings) and/or all the steps of the described method can be combined in any combination, with the exception of combinations of such mutually exclusive features and/or steps.
The features and advantages of the invention will become clearly understood in view of the detailed description of the invention which becomes apparent from a preferred embodiment of the invention, given just as an example and not being limited thereto, with reference to the drawings.
With continued reference to the solar cell of
In certain embodiments, the top contact (17) does not cover completely the n-type top layer (14) but has the form of (or comprises) a metallic grid allowing light to pass through the top contact (17) and reach the semiconductor structure (e.g., the top semiconductor layer (14)). Alternatively, the top contact (17) may be made of (or comprise) a semitransparent conductor and, in this case, it can cover completely the surface of the top semiconductor layer (14).
The top (17) and middle (18) contacts are connected to a load (20). The bottom (19) and middle (18) contacts are also connected to a load (21). The electrical energy produced by the solar cell is collected at these loads. The motivation for choosing a high bandgap material for the n-type top layer and the p-type middle layer is two-fold: a) collecting the energy of those photons with energy higher than EH, and b) limiting the voltage across the load (20) to EH/e volts (with e being the electron charge) and not to a lower value.
The solar cell of the invention works as follows. When the cell is illuminated (e.g., with the sun or another radiation source (6)), those photons having energy lower than the bandgap EH reach the bottom semiconductor layer (16) and produce electrical current. This current, IC (22), flows through the load (21), where the photon energy can be collected or stored (for example, in a rechargeable battery). For later reference, the value of this load (21) will be designated as RBC. The voltage across this load (21) biases the middle-bottom contacts (18, 19) with a voltage VBC (higher at the p-type middle layer (15) than at the n-type bottom layer (16)). If the contact and semiconductor resistances are low, this voltage will also be approximately the voltage across the junction between the p-type middle layer (15) and the n-type bottom layer (16).
On the other hand, photons with energy higher than EH are absorbed by the top (14) and middle (15) semiconductor layers. Ideally, the thickness of the n-type top layer (14) and p-type middle layer (15) should be made high enough as to absorb all photons with energy higher than EH. The absorption of photons in the n-p junction formed by the top and middle semiconductor layers (14, 15) produce an electrical current IE (23) that flows through the load (20), where energy can also be collected or stored. The electrical current through the load (20) biases the middle-top contacts (18, 17) with a voltage VBE (higher at the p middle contact (18) than at the n top contact (17)). For later reference, the value of this load (20) will be designated as RBE. If the contact and semiconductor resistances are low, this voltage will also be approximately the voltage across the junction between the p-type middle layer (15) and the n-type top layer (14).
Following a one-dimension semiconductor low injection model to explain the performance of the solar cell of the invention and neglecting the carrier recombination at the space charge region appearing at the semiconductor junctions, currents IC and IE can be decomposed as follows:
I
C
=I
LC
−I
hC(VBC)−IeC(VBE,VBC) (1a)
I
E
=I
LE
−I
hE(VBE)−IeE(VBE,VBC) (1b)
where:
ILC, indicated by reference number 24 in
IeC(VBE,VBC), indicated by reference number 25 in
IhC(VBC), indicated by reference number 26 in
ILE, indicated by reference number 27 in
IeE(VBE,VBC), indicated by reference number 28 in
IhE(VBE), indicated by reference number 29 in
Assuming, for simplification purposes, the same area for both the p-type middle layer/n-type top layer and p-type middle layer/n-type bottom layer semiconductor junctions, the electron currents IeC (25) and IeE (28) can be expressed as follows:
where:
The current difference:
I
rb
=I
eE(VBE,VBC)−IeC(VBE,VBC) (4)
represented in
At this point the electrical power, P, delivered by the cell can be calculated. It is given by
P=I
E
V
BE
+I
C
V
BC
=[I
LE
−I
hE(VBE)]VBE+[ILC−IhC(VBC)]VBC−Λ(VBE,VBC,αT) (5)
where
represents a power loss factor that has to be minimized. In this respect, the following should be observed.
The losses decrease as the factor transport, αT, increases. From this point of view, for the best performance of the invention, αT should approach 1, which is the maximum value this parameter can achieve. Given the dependence of this transport factor with the thickness of the p-type middle layer, the thickness of the p-type middle layer should be made as thin as possible. However, making this layer too thin could increase the ohmic losses of the device since all the current (IE+IC) is extracted laterally through the middle contact (18). An alternative solution, explained in the next paragraph, is thus provided.
The losses also decrease if I0 is minimized. Since, as advanced before, this current is proportional to
I0 can be minimized by increasing the doping NB of the middle layer, which results in a top-middle junction characterized by a low injection efficiency. This efficiency is defined, without illumination, as the ratio IeE(VBE,0)/I0.
The solar cell of the invention has been described in terms of the minimum layers it requires to work. In order to implement the solar cell in practice, additional layers can be added to or otherwise incorporated into the solar cell. The functions of these additional layers are well-known to persons skilled in the art of designing solar cells; therefore, detailed description of these layers is not necessary. These layers include basically: layers to decrease the surface recombination of the semiconductors (known as back surface field layers), layers to produce ohmic contacts between metal and semiconductor junctions (contact layers), buffer layers to prepare semiconductor surfaces for ulterior growth, and layers to decrease light reflection (anti-reflecting layers).
In certain embodiments, solar cells according to the invention can be fabricated by molecular beam epitaxy (MBE), as described below.
With this growth, the semiconductor structure of the cell is completed. The steps to create the contacts to the semiconductor structure are described below.