HK1141627A - Low resistance tunnel junctions for high efficiency tandem solar cells - Google Patents

Description

Low resistance tunnel junction for high efficiency tandem solar cells

Statement of government support

The invention described and claimed herein was made in part using funds provided by the U.S. department of energy under contract No. de-AC02-05CH 11231. The government has certain rights in this invention.

Background

The present invention relates to tandem (tandem) photovoltaic cells or solar cells, and more particularly to low resistance tunnel junctions for high efficiency tandem solar cells or photovoltaic cells and methods of making the same.

As used herein, the term "photovoltaic cell" includes any semiconductor p/n junction that can convert photons to electricity. This includes, but is not limited to, the well-known photovoltaic cells that convert visible light into electricity and thermophotovoltaic cells that convert long wavelength or thermal photons into electricity.

These photovoltaic cells are generally characterized by solid state crystalline structures having an energy bandgap between their valence electron bands and their conduction electron bands. When light is absorbed by the material, electrons occupying low energy states are excited to pass through the band gap to higher energy states. For example, when electrons in the valence band of a semiconductor absorb sufficient energy from photons of solar radiation, they can jump the bandgap to a higher energy conduction band. Electrons excited to a higher energy state leave unoccupied low energy sites or holes. Like the free electrons in the conduction band, such holes can move between atoms in the lattice and thus act as charge carriers and contribute to the conductivity of the crystal. Most photons absorbed in the semiconductor generate such electron-hole pairs that generate a photocurrent and, in turn, a photovoltage exhibited by the solar cell. Semiconductors are doped with different materials to create a space charge layer (space charge layer) that separates holes and electrons that act as charge carriers. Once separated, these collected hole and electron charge carriers generate a space charge that induces a voltage across the junction region as a photovoltage. These holes and charge carriers constitute a photocurrent if they are allowed to flow through an external load.

There is a fixed amount of potential energy difference across the bandgap in the semiconductor. For an electron in the lower-energy valence band to be excited to jump the band gap to the higher-energy conduction band, it must generally absorb a sufficient amount of energy from the absorbed photon, at least equal to the potential energy difference across the band gap. Semiconductors are transparent to radiation having a photon energy less than the band gap. An electron may jump the band gap if it absorbs more than a threshold amount of energy, for example from a higher energy photon. The absorbed energy exceeds the threshold amount required for the electron to jump the bandgap, producing an electron with energy higher than most of the other electrons in the conduction band. The excess energy is eventually dissipated as heat. The net result is that the effective photovoltage of a single bandgap semiconductor is limited by the bandgap. Therefore, in a single semiconductor solar cell, in order to capture as many photons as possible from the solar radiation spectrum, the semiconductor must have a small band gap so that even photons with lower energy can excite electrons to jump the band gap. There are limitations because the use of small bandgap materials results in a reduction in the photovoltage and power output of the device. In addition, photons from higher energy radiation produce excess energy that is dissipated as heat.

However, if the semiconductor is designed to have a larger energy band gap to increase the photovoltage and reduce the energy loss caused by thermalization of hot carriers, photons having lower energy cannot be absorbed. Therefore, in designing a single junction solar cell, it is necessary to balance these considerations and optimize the band gap and try to design a semiconductor with an optimal band gap. Much work has been done in recent years to address this problem by fabricating tandem or multi-junction (tandem) solar cell structures in which the top cell has a larger bandgap and absorbs higher energy photons, while lower energy photons pass through the top cell into the lower or bottom cell having a smaller bandgap to absorb lower energy radiation. These band gaps are ordered from high to low, from top to bottom, to achieve an optical cascading effect (cascade). In principle, any number of subcells may be stacked in this manner; however, practical limits are generally considered to be two or three. Multijunction solar cells enable higher conversion efficiencies because each subcell converts solar energy to electrical energy over a small photon wavelength band that can efficiently convert the energy. Techniques for making such tandem cells are described in U.S. patent No. 5,019,177, which is incorporated herein by reference in its entirety.

As the cost of hydrocarbon fuels increases, efforts to improve the efficiency of photovoltaic devices become more acute. Most solar cells on the market today are made of silicon, but in recent years higher efficiency cells made of other materials have been investigated. Gallium arsenide and related alloys have attracted particular attention. As explained herein, significant increases in solar cell efficiency are enabled by utilizing tandem sub-cells of different materials having different energy bandgaps between their valence electron bands and their conduction bands. The lattice constants of the compounds and alloys used to form photovoltaic cells are known. When combining these materials in a device with subcells of different materials, the lattices of the different materials should have the same lattice constant or a small lattice constant difference. This avoids the formation of defects in the crystal structure that can drastically reduce the efficiency of the device.

In any tandem cell arrangement, electrical connections must be made between the sub-cells. Preferably, the ohmic contact between the cells should have a minimum resistance so that the electrical power loss between the cells is extremely low. There are two known methods for fabricating such inter-cell ohmic contacts, metal interconnects and tunnel junctions (or tunnel diodes). Metal interconnects can provide low resistance, but they are difficult to manufacture, they result in complex processing and can result in a significant loss of device efficiency. Therefore, tunnel junctions are generally preferred because monolithically integrated devices having multiple subcells with tunnel junctions between them can be fabricated. However, the tunnel junction must meet a number of requirements such as low resistance, high peak current density, low optical energy loss, and crystallographic compatibility through lattice matching between the top and bottom cells.

Currently, tandem solar cells use tunnel junctions to ensure efficient current flow through 2-4 photovoltaic cells connected in series. The battery operates most efficiently when the currents generated in each sub-cell are matched. In order for current to flow through the cell such that the subcells voltage is stacked in series, a junction that allows electron-hole recombination between the subcells is useful.

To accommodate band offsets in current tandem cells, heavily doped tunnel junctions are used. The tunnel junction connects the top and middle cells of a standard three junction (3J) cell, thereby efficiently annihilating, for example, electrons from the InGaP top cell and holes from the InGaAs middle cell. Reference is made, for example, to tandem solar cells having indium phosphide subcells and indium gallium arsenide phosphide described in U.S. patents 5,407,491 and 5,800,630, which are incorporated herein by reference in their entirety. The tunnel junction is heavily doped to enable tunneling transport due to energy band mismatch (mismatch) between the Valence Band (VB) of InGaP and the Conduction Band (CB) of InGaAs. In this case, the junction is p + + InGaP or p + + AlGaAs and n + + lnGaAs or n + + AlInP. This is undesirable because it additionally increases the manufacturing process steps of the solar cell and increases the design complexity.

Accordingly, it is desirable to provide a low resistance tunnel junction without adding additional manufacturing process steps to the solar cell and without increasing the complexity of the solar cell design.

Disclosure of Invention

The present invention overcomes the above-mentioned shortcomings in the prior art by providing a low resistance tunnel junction for a high efficiency tandem solar cell.

It is therefore an object of the present invention to provide a high efficiency tandem solar cell that does not require heavily doped tunnel junctions to ensure recombination at the cell junction regions.

It is another object of the present invention to provide and manufacture a high efficiency indium nitride based tandem solar cell.

It is a further object of the present invention to provide an indium nitride based tandem solar cell as described above having a low resistance or near zero resistance tunnel junction.

It is another object of the present invention to provide GaSb/InAsSb based tandem solar cells with low or near zero resistance tunnel junctions.

According to one embodiment of the present invention, a semiconductor structure comprises: a first photovoltaic cell comprising a first material; and a second photovoltaic cell comprising a second material and connected in series with the first photovoltaic cell. A conduction band edge (conduction band edge) of a first material adjacent to a second material is at most 0.1eV higher than a valence band edge of the second material adjacent to the material. Preferably, the first material of the first photovoltaic cell comprises In_1-xAl_xN or In_1-yGa_yAnd N, the second material of the second photovoltaic cell comprises silicon or germanium.

Alternatively, the first material of the first photovoltaic cell comprises InAs and the second material of the second photovoltaic cell comprises GaSb. Preferably, the first material of the first photovoltaic cell comprises InAsSb and the second material of the second photovoltaic cell comprises GaAsSb.

According to one embodiment of the present invention, a semiconductor structure comprises: a p-type silicon layer; and an n-type semiconductor nitride layer in contact with the p-type silicon layer. The conduction band edge of the n-type semiconductor nitride layer is at most 0.1eV higher than the valence band edge of the p-type silicon layer. Preferably, the n-type semiconductor nitride is selected from In_1-xAl_xN and In_1-yGa_yN, wherein x is preferably 0.2-0.6, and y is preferably 0.4-0.6. The P-type silicon layer is preferably (111) silicon or Si (111).

According to one aspect of the invention, the current-voltage characteristics of the semiconductor structure are symmetric. Preferably, the junction formed by the p-type silicon layer and the n-type semiconductor nitride layer has a resistance substantially equal to the series resistance of the silicon and nitride.

According to one embodiment of the present invention, the semiconductor structure as described above further comprises: a p-type semiconductor nitride layer in contact with the n-type semiconductor nitride layer and an n-type silicon layer in contact with the p-type silicon layer.

According to one embodiment of the invention, in a semiconductor structure, an n-type semiconductor nitride layer is part of a first photovoltaic cell and a p-type silicon layer is part of a second photovoltaic cell. The first photovoltaic cell and the second photovoltaic cell are connected together in series.

Various other objects, advantages and features of the present invention will become apparent from the following detailed description, and the novel features will be particularly pointed out in the appended claims.

Drawings

The following detailed description, given by way of example and not intended to limit the present invention, will be better understood in conjunction with the accompanying drawings, in which:

figure 1 shows the valence and conduction band positions of InAlN and InGaN alloys.

Fig. 2 is an energy band diagram of an InGaN/Si tandem cell incorporating a near-zero resistance tunnel junction in accordance with an exemplary embodiment of the present invention.

FIG. 3 shows the current-voltage curve of the tunnel junction between n-InGaN and p-Si (111).

Figure 4 is a tandem solar cell design incorporating a low resistance tunnel junction according to one embodiment of the present invention.

Fig. 5 shows calculated efficiency values of a two junction (2J) InGaN/Si tandem solar cell according to an exemplary embodiment of the present invention as a function of the InGaN bandgap.

Fig. 6 shows a low resistance junction between p-type GaSb and n-type InAsSb according to an exemplary embodiment of the invention.

Detailed Description

The band gap tuning range of group III nitrides encompasses almost the entire useful range of the solar spectrum with respect to energy conversion, making these materials attractive for use in photovoltaic cells. In order to increase efficiency and produce more power, it has become increasingly more common to design tandem photovoltaic cells made of thin films and electrically connected in series. But there are difficulties with tandem junctions.

As illustrated herein, tandem solar cells use tunnel junctions to ensure efficient current flow through multiple photovoltaic cells connected in series. The battery operates most efficiently when the currents generated in each sub-cell are matched. In order for current to flow through the cell such that the subcells voltage is stacked in series, a junction between the subcells that allows electron-hole recombination is useful.

To accommodate the band offset in current tandem solar cells, heavily doped tunnel junctions are used. The tunnel junction connects the top and middle cells of a standard three junction (3J) cell to efficiently annihilate, for example, electrons from the InGaP top cell and holes from the InGaAs middle cell. The tunnel junction is heavily doped to enable tunneling due to the energy band disparity between the Valence Band (VB) of InGaP and the Conduction Band (CB) of InGaAs. In this case, the junction is p + + InGaP or p + + AlGaAs and n + + InGaAs or n + + AlInP. This is undesirable because it additionally increases the process steps for manufacturing the battery and increases the complexity of the design.

The experimental work confirms the indium aluminum nitride and indium gallium nitride alloy (In)_1-xAl_xN and In_1-yGa_yN) absolute position of the Conduction Band (CB) and Valence Band (VB). See s.x.li et al, "fermlevel Stabilization Energy In Group III-nitriles" phys.rev.b71, 161201(R) (2005), the entire contents of which are incorporated herein by reference. FIG. 1 shows In_1-xAl_xN and In_1-yGa_yThe energy of the CB and VB edges of N are plotted as a function of x and y. The location of the VB and CB edges of silicon (Si) and germanium (Ge) are also shown in fig. 1. The composition of Si in which the valence band coincides with the conduction band is indicated by a dotted line. For a value of "x" of about 0.3, this corresponds to the composition In_0.7Al_0.3N，In_1-xAl_xCB of N is identical (matched) to VB of Si. For a value of "x" of about 0.5, this corresponds to the composition In_0.5Ga_0.5N，In_1-yGa_yCB for N coincides with VB for Si. According to an exemplary embodiment of the present invention, a junction may be formed between N-type InAlN and p-Si or between N-type InGaN and p-Si with near defect-free band matching, thereby creating a very low (near zero or near zero) resistance tunnel junction. Fig. 2 is a calculation result showing the almost defect-free energy band matching of the nitride-based tunnel junction of the present invention. For p-Ge (corresponding to composition In) with higher Al (or Ga) content at x-0.4 (or y-0.6)_0.6Al_0.4N (or In)_0.4Ga_0.6N)), there is a similar near perfect or excellent band match. In general, band matching is considered excellent when the conduction band edge is no more than about 0.1eV above the valence band edge.

FIG. 2 shows In with a near zero resistance tunnel junction_0.46Ga_0.54N p/n + Si p/n 2J tandem cell band diagram. The acceptor (Na) and donor (Nd) concentrations used for this calculation were 1 × 10, respectively¹⁸cm^-3And 5X 10¹⁹cm^-3. InGaN and Si cells have a p/n junction and are used as normal p/n junction (1J) solar cells, i.e. under illumination, electrons in the nitride material flow into the cell away from the surface and holes in the Si move towards the surface. The tunnel junction is located about 400nm below the surface at the interface between n-InGaN and p-Si. Electrons from the N-InGaN and holes from the p-Si can recombine at the interface. Under such current matching conditions, the voltages of the two cells can be stacked in series. Since the chosen InGaN composition has nearly perfect band matching, there is only a slight amount of "band bending" at the interface. This results in a very low resistance.

According to an exemplary embodiment of the present invention, a layer of n-type nitride material is deposited on p-type Si (111) to form a junction. Electrical tests were performed on the tunnel junction between n-InGaN and p-Si (111). In particular, layer In on p-type Si was measured_0.4Ga_0.6The resistance of a junction of N composition (i.e., composition whose conduction band is approximately matched to the valence band of Si). The resistance of the junction was determined to be ohmic and the resistance value was low. The observed resistance was 12 ohms and the performance was ohmic up to the current limit of the test device. FIG. 3 shows n-In_0.4Ga_0.6Current-voltage curve of the junction between N and p-type Si. The measured composition of the InGaN alloy is close to that predicted to yield a near defect free band composition illustrated in fig. 2. The current-voltage curve of fig. 3 is fully symmetrical, indicating the lack of an electrical barrier at the heterointerface (junction). The junction pair is at least 50mA cm in size^-2Has an ohmic characteristic and a low resistance, and has a current density higher than that of a current in a general solar cell. Thus InGaN andthe junction between Si has no limitation on the photocurrent that can be generated by the solar cell including the indium nitride-based junction according to an exemplary embodiment of the present invention. In general, it is beneficial to make the resistance of the ohmic tunnel junction smaller than the series resistance of the component semiconductors. For an optimized solar cell, the front and back ohmic contacts should be on the order of a few ohms/cm²。

Fig. 4 illustrates a two-junction tandem cell with an indium nitride-based material having an energy band gap of 1.8eV as the top cell and Si (energy band gap ═ 1.1eV) as the bottom cell, according to an exemplary embodiment of the present invention. It will be appreciated that this structure is close to ideal for a top cell matched to Si in terms of maximum power conversion efficiency.

Using acceptable values for the light absorption and charge transport parameters for InGaN and Si, fig. 5 shows a plot of calculated efficiency values for an InGaN/Si tandem cell as a function of InGaN bandgap according to an exemplary embodiment of the present invention. The cell structure comprised 0.1 μm p-InGaN, 0.8 μm n-InGaN, 0.1 μm p-Si and 1000 μm n-Si as substrates. The efficiency of AM (air mass) 1.5 direct solar spectrum (ASTM ground reference spectrum for photovoltaic performance evaluation) was calculated. Specifically, fig. 5 shows the calculated 300KAM 1.5 efficiency of a two junction (2J) InGaN/Si tandem solar cell. For the range of the InGaN top cell bandgap, efficiencies above 30% are predicted. The maximum efficiency of using InGaN (in0.5ga0.5n) having an energy band gap of 1.7eV or less is 35%. The following electrical and transport parameters of InGaN were used for the calculations: electron mobility 300cm²V^-1S^-1(ii) a Hole mobility, 50cm²V^-1S^-1(ii) a Effective mass of electron of 0.07m₀(ii) a Effective mass of holes 0.7m 0; zero surface recombination velocity. For InGaN/Si tandem cells, a maximum of over 30% is good, and for the optimal structure 35% is reached. The low resistance tunnel junction between the cells allows for efficient recombination of carriers at the junction, thereby enabling the present invention to achieve practical efficiencies close to the theoretical limit. In addition, the present invention greatly simplifies the design of 2J cells by eliminating the need for heavily doped tunnel junctions. Namely, the present invention is advantageousThe doping step required in the fabrication of existing tandem solar cells to ensure recombination at the cell junction region is eliminated.

It is understood that there are other semiconductor pairs that may be used to form the low resistance tunnel junctions of the present invention. For example, the conduction band of InAs is well aligned with the valence band of GaSb. While the band gaps of these materials (both less than 1eV) are below what is considered ideal for tandem cells that respond to sunlight, the InAs/GaSb design can be optimized for converting near-infrared and infrared light from a heat source in thermophotovoltaic cells that can generate electricity from heat.

Small amounts (up to a few percent) of Sb are introduced into InAs to form an InAsSb alloy and/or As or P is introduced into GaSb to form a GaAsSb alloy, which may be used for lattice constant matching and varying band offset between the semiconductor compositions forming the tunnel junction according to one exemplary embodiment of the present invention. FIG. 6 shows the data for p-GaSb and n-InAs_0.94Sb_0.06Calculated band diagram of the knot. The low potential barrier at the interface shows a very low resistance junction. The calculations are based on the following cell structure.

Layer composition Doping concentration Layer thickness

n-InAsSb (contact layer) 1X 10¹⁸cm^-3 100nm

n-InAsSb 1×10¹⁷cm^-3 500nm

p-GaSb 1×10¹⁷cm^-3 500nm

2X 10 of p-GaSb (substrate)¹⁷cm^-3 1000nm

The present invention has been described herein in considerable detail in order to provide those skilled in the art with information relevant to the application of the novel principles and constructions and the use of such specialized components as are required. However, it is to be understood that the invention can be carried out by different equipment, materials and devices, and that various changes, both as to the equipment and operating procedures, can be made without departing from the scope of the invention itself.

Claims (12)

1. A semiconductor structure, comprising:

a first photovoltaic cell comprising a first material; and

a second photovoltaic cell comprising a second material and connected in series with the first photovoltaic cell; and is

Wherein a conduction band edge of the first material adjacent to the second material is at most 0.1eV higher than a valence band edge of the second material adjacent to the material.

2. According to claim 1The semiconductor structure of (1), wherein the first material of the first photovoltaic cell comprises In_1-xAl_xN or In_1-yGa_yN, the second material of the second photovoltaic cell comprising silicon or germanium.

3. The semiconductor structure of claim 1, wherein the first material of the first photovoltaic cell comprises InAs and the second material of the second photovoltaic cell comprises GaSb.

4. The semiconductor structure of claim 1, wherein the first material of the first photovoltaic cell comprises InAsSb and the second material of the second photovoltaic cell comprises GaAsSb.

5. A semiconductor structure, comprising:

a p-type silicon layer; and

an n-type semiconductor nitride layer in contact with the p-type silicon layer; and is

Wherein a conduction band edge of the n-type semiconductor nitride layer is at most 0.1eV higher than a valence band edge of the p-type silicon layer.

6. The semiconductor structure of claim 5, wherein the current-voltage characteristics of the semiconductor structure are symmetric.

7. The semiconductor structure of claim 5, wherein a resistance of a junction formed by the p-type silicon layer and the n-type semiconductor nitride layer is substantially equal to a series resistance of the silicon and the nitride.

8. The semiconductor structure of claim 5, wherein the n-type semiconductor nitride is selected from In_1-xAl_xN and In_1-yGa_yN。

9. The semiconductor structure of claim 8, wherein x is 0.2 to 0.6 and y is 0.4 to 0.6.

10. The semiconductor structure of claim 5, wherein the p-type silicon layer is (111) silicon.

11. The semiconductor structure of claim 5, further comprising a p-type semiconductor nitride layer in contact with the n-type semiconductor nitride layer and an n-type silicon layer in contact with the p-type silicon layer.

12. The semiconductor structure of claim 5, wherein the n-type semiconductor nitride layer is part of a first photovoltaic cell and the p-type silicon layer is part of a second photovoltaic cell, wherein the first and second photovoltaic cells are connected together in series.