Broad spectrum solar cell

An alloy having a large band gap range is used in a multijunction solar cell to enhance utilization of the solar energy spectrum. In one embodiment, the alloy is In1−xGaxN having an energy bandgap range of approximately 0.7 eV to 3.4 eV, providing a good match to the solar energy spectrum. Multiple junctions having different bandgaps are stacked to form a solar cell. Each junction may have different bandgaps (realized by varying the alloy composition), and therefore be responsive to different parts of the spectrum. The junctions are stacked in such a manner that some bands of light pass through upper junctions to lower junctions that are responsive to such bands.

FIELD OF THE INVENTION

The present invention relates to solar cells, and in particular to a broad spectrum solar cell.

BACKGROUND OF THE INVENTION

Current solar cells based on single semiconductor material have an intrinsic efficiency limit of approximately 31%. A primary reason for this limit is that no one material has been found that can perfectly match the broad ranges of solar radiation, which has a usable energy in the photon range of approximately 0.4 to 4 eV. Light with energy below the bandgap of the semiconductor will not be absorbed and converted to electrical power. Light with energy above the bandgap will be absorbed, but electron-hole pairs that are created quickly lose their excess energy above the bandgap in the form of heat. Thus, this energy is not available for conversion to electrical power.

Higher efficiencies were thought to be achievable by using stacks of semiconductor with different band gaps, forming a series of solar cells. The concept is that the higher gap materials convert higher energy photons, allowing lower energy photons to pass down to lower gap materials in the stack. Stacks of two semiconductors, GaInP/GaAs and three semiconductors GaInP/GaAs/Ge have been developed over the last decade, and have the highest efficiency of any solar cell. Because of the lack of appropriate semiconductor materials, attempts to make solar cell stacks with more junctions have actually resulted in lower efficiencies.

Currently most efficient tandem cells use fixed gap combinations, 1.85/1.43 eV for two junction cells and 1.85/1.43/0.7 eV for the three junction cells. The cells take advantage of the relatively good lattice match of Ga0.5In0.5P, GaAs and Ge. However the cells based on these fixed energy gap combinations do not take full advantage of the solar spectrum. There is a need for a solar cell that converts more of the light spectrum into electrical power.

SUMMARY OF THE INVENTION

An alloy having a large band gap range is used in a multijunction solar cell to enhance utilization of the solar energy spectrum. In one embodiment, the alloy is a single ternary alloy of In1−xGaxN having an energy bandgap range of approximately 0.7 eV to 3.4 eV, providing a good match to the solar energy spectrum.

In one embodiment, multiple junctions based on In1−xGaxN alloys having different bandgaps are stacked to form a solar cell. Each junction may have different bandgaps, and therefore be responsive to different parts of the spectrum. The junctions are stacked in such a manner that some bands of light pass through upper junctions to lower junctions that are responsive to such bands.

One example solar cell comprises two or more stacked junctions based on In1−xGaxN alloys, wherein the junctions having higher bandgaps are stacked on top of the junctions having lower bandgaps. Thus, lower energy light passes through the high bandgap junctions to the lower bandgap junctions where it is absorbed and converted to electrical power. The higher energy light is absorbed by the higher bandgap junctions and converted to electrical power.

In one embodiment, the solar cells comprise multiple stacked junctions formed of alloys with judiciously chosen compositions to cover substantially the entire solar spectrum.

The multijunction solar cells can be prepared as integrated devices consisting of the separate junctions sequentially deposited on substrate (integrated multijunction cell). One can also make separate junctions and stack them on the top of each other with mating conductors between them.

DETAILED DESCRIPTION OF THE INVENTION

A block diagram abstract representation of a multi-junction solar cell is shown generally at100inFIG. 1. Three junctions, top, middle and bottom are shown at110,115, and120. Each junction has a different bandgap, such that they absorb different energies of light. Light in the form of photons is represented by arrows125,130and135, which are generally indicative of the direction of the light. The bandgaps of the junctions generally decrease, such that higher energy photons represented by arrow125are absorbed by the top layer110, lower energy photons130are absorbed by the middle junction115, and still lower energy photons135are absorbed by the bottom junction120. Further junctions may be provided if desired to absorb even a broader spectrum of light.

In one embodiment, the bandgap energies of the junctions, Eg1, Eg2, and Eg3are selected to enable the junctions to absorb light having the highest energy in the spectrum of sunlight. A single ternary alloy having a large band gap range is used in the multijunction solar cell to enhance utilization of the solar energy spectrum. In one embodiment, the alloy is In1−xGaxN having an energy bandgap range of approximately 0.7 eV to 3.4 eV, providing a good match to the solar energy spectrum. The alloy is grown using molecular beam epitaxy, creating crystals with low electron concentrations and high electron mobilities.

A block diagram of a two-junction solar cell is shown generally at200inFIG. 2. This cell has a maximum theoretical efficiency of 59% for example. Actual efficiency will likely be less. In one embodiment, a buffer layer is grown via an epitaxial deposition method on top of a substrate, such as a sapphire or silicon carbide substrate. The buffer layer provides a base for forming the two junction cell shown inFIG. 2. In one embodiment, the buffer layer is formed of GaN or AlN on a sapphire or a silicon carbide substrate. Other substrates may also be used. The substrate and buffer layer may be mechanically/chemically removed later, leaving the solar cell as shown at200.

A low energy gap junction cell210is formed by growing a layer of p-type InN215followed by the layer of n-type InN220. The layers have an energy gap of approximately 0.7 eV. A tunnel junction225comprising a heavily doped, n-type InN layer followed by heavily doped, p-type layer is then formed. In one embodiment, the heavily doped layers are approximately 1018cm−3or higher electron/hole concentrations. The tunnel junction225provides an electrical connection between the low energy gap210and a large energy gap junction230. Large energy gap junction cell230comprises a grown p-type Ga0.39In0.61N (alloy with approximately 39% Ga and 61% In)235followed by n-type layer237of the same composition. The large energy gap junction230has an energy gap of approximately 1.4 eV. Ohmic (electrical) contacts240and245are formed on the bottom p-type layer of InN and the top n-type layer of Ga0.39In0.61N respectively. An optional antireflection coating250is added to increase the amount of light absorbed and passing through the high energy gap junction230. The 2-junction cells have a theoretical optimized maximum efficiency of approximately 59%.

Typical doping levels for n- and p-type layers range from 1017cm−3to 1018cm−3. The actual doping levels depend on other characteristics of the films and can be adjusted to maximize the efficiency. Silicon is commonly used as an n-type dopant and magnesium as a p-type dopant in GaInN. Higher doping may be used if desired. Films of InN may have electron concentrations in the 1018cm−3to 4.5×1019cm−3range and may have room temperature Hall mobilities ranging from several hundred up to 2050 cm2/Vs when formed using molecular beam epitaxy.

In one embodiment, In1−xGanN films are grown on (0001) sapphire with an AlN buffer layer (approximately 240 nm) by molecular beam epitaxy. The growth temperature is approximately between 470° C. to 570° C. High-quality wurtzite-structured In1−xGaxN epitaxial layers are formed with their c-axis perpendicular to the substrate surface. The composition dependence of the room temperature bandgap in the entire concentration range is well fit by the following standard equation:
Eg(x)=3.42x+0.77(1−x)−1.43x(1−x)
with a constant bowing parameter of b=1.43 eV.

The thickness of the buffer layer in one embodiment ranges from 70 nm to 200 nm. The InN layer thickness is between approximately 200 nm and 4 um.

In a further embodiment, the junction cells are mechanically stacked as shown at300inFIG. 3. The numbering of the junctions is consistent with that ofFIG. 2where appropriate. In this embodiment, each junction cell210and230is separately formed on a substrate utilizing the same process steps as above, and then are mechanically stacked. In addition, junction cell210has an optional antireflection coating253to minimize light reflection. Junction cell230has an ohmic contact255coupled to an ohmic contact260formed on junction cell210by a conductor270. Metal (ohmic, low resistance) contacts are formed by evaporation or sputtering of a metal on semiconductor surfaces. Most metals form good ohmic contacts to n-type InN or GaInN although titanium seems to be most frequently used metal. Ohmic contacts to p-type material are more difficult to make. Gold forms the lowest resistivity contacts to p-type GaInN.

The junctions for the junction cells shown stacked at300are formed separately. The low energy gap junction cell210is formed by growing a GaN or AlN buffer layer on a substrate followed by p-type InN layer followed by n-type InN layer. The large energy gap junction cell230is formed by growing a GaN or AlN buffer layer on a substrate followed by p-type Ga0.39In0.61N (alloy with approximately 39% of Ga and 61% of In) followed by n-type layer o the same composition. The layers have the energy gap of approximately 1.4 eV.

The large gap junction230is stacked on the top of the low gap junction cell210. The n-type layer220of the low gap cell is connected to the p-type layer235of the large gap cell via electrical connection270through ohmic contacts (255,260). The electrical contacts can have the form of a transparent wire grid formed of Indium-Tin-Oxide or other suitable conductive material. Transparent adhesive can be used to mechanically hold the layers together. Also antireflective coating can be applied to the top of each junction. Ohmic (electrical) contact is formed between the bottom p-type layer215of InN and the top n-type layer237of Ga0.39In0.61N.

An integrated design of optimized 3-junction cells with the maximum theoretical efficiency of 67% is shown inFIG. 4at400. A buffer layer (not shown) is formed using an epitaxial deposition method to grow a layer of GaN or AlN on a sapphire or a silicon carbide substrate. The substrate and buffer layers may be mechanically/chemically removed later. A low energy gap junction cell410is grown and comprises a layer of p-type InN412followed by the layer of n-type InN414. The layers have a gap of 0.7 eV.

A tunnel junction416is the grown. The tunnel junction416comprises a heavily doped n-type InN layer followed by heavily doped p-type layer. The junction416provides an electrical connection between the low energy gap to an intermediate energy gap cell420.

Intermediate energy gap cell420has a junction of grown p-type Ga0.27In0.73N (alloy with approximately 27% Ga and 73% In) layer422followed by n-type layer424of the same composition. The layers have the energy gap of approximately 1.16 eV. A tunnel junction426is then formed by growing a heavily doped n-type Ga0.27In0.73N layer followed by heavily doped p-type layer. The junction provides an electrical connection between the intermediate energy gap cell and a large energy gap junction cell430. The large energy gap junction cell430comprises a grown p-type Ga0.55In0.45N (alloy with approximately 55% Ga and 45% of In) layer432followed by n-type layer434of the same composition. The layers have an energy gap of approximately 1.84 eV.

Ohmic (electrical) contacts440and445on the bottom p-type layer of InN and the top n-type layer of Ga0.55In045N. An antireflection coating450is formed on top of layer434prior to formation of contact445in one embodiment.

FIG. 5is an alternative three junction cell arrangement500where the individual cells are mechanically coupled. Numbering inFIG. 5is consistent with that inFIG. 4. As in the mechanically coupled two junction cell, cells410,420and430are separately formed, and then positioned in a vertically stacked arrangement to facilitate conversion of light to electricity. Each cell is provided with an antireflection coating450,505and510. The junctions are also electrically coupled by contacts formed between the cells at515and520.

In one embodiment, the contacts are directly coupled to opposing contacts on adjacent cells to provide the mechanical coupling, and provide spacing between the cells. While the contacts are shown formed on one edge of the cells, the contacts may take any form (e.g. a wire grid) and distribution desired to provide a combination of mechanical and electrical coupling without significantly obstructing propagation of the sunlight. It may be desired to minimize the real estate of the cells covered by the contacts to optimize conversion efficiency. In further embodiments, mechanical coupling is accomplished by structures on sides of the cells. Still further mechanical coupling may be provided in a known manner, such as by a side support indicated at530.

Three junction cell arrangement500has a theoretical optimized maximum efficiency of 67%. As indicated, the gap junctions are formed separately in one embodiment. Low energy gap junction cell410is formed on a grown GaN or AlN buffer layer on a substrate followed by p-type InN layer412followed by n-type InN layer414.

Intermediate energy gap junction cell420is formed on a grown GaN or AlN buffer layer on a substrate followed by p-type Ga0.27In0.73N (alloy with approximately 27% Ga and 73% In) layer422, followed by n-type layer424of the same composition. The large energy gap junction cell is formed on a grown GaN or AlN buffer layer on a substrate followed by p-type Ga0.55In0.45N (alloy with approximately 55% Ga and 45% of In) layer432, followed by n-type layer434of the same composition. The layers have the energy gap of approximately 1.84 eV.

The junctions are stacked on top of each other in a sequence where the low energy gap is at the bottom followed by the intermediate energy gap followed by the large energy gap junction on the top. They are stacked in a manner that selected energies of light received at the large energy gap junction may progress through each of the other junctions.

As indicated above, the junctions are coupled mechanically, and then the n-type layer of the low gap junction is electrically coupled to the p-type layer of the intermediate gap junction and the n-type layer of the intermediate gap junction is electrically coupled to the p-type layer of the large gap junction. Ohmic (electrical) contacts are formed on the bottom p-type layer of InN and the top n-type layer of Ga0.55In0.45N. In further embodiments, larger numbers of junctions are use, each have different energy gaps designed to optimize absorption of incident light to more efficiently convert a large portion of energy in the solar spectrum.

CONCLUSION

The band gap range of the In1−xGaxN ternary alloy extends over a very wide energy range from 0.7 eV to 3.4 eV, and thus provides a good match to the solar energy spectrum. This creates the opportunity to synthesize material with any band gap within the solar spectrum and to design and fabricate new multijunction solar cells with any number of component junctions with optimized band gap. Such cells may approach theoretically predicted maximum efficiencies. The alloy may exhibit great thermal stability and radiation hardness that would be useful in harsh environments with radiation, making it suitable for space and military applications.

In one embodiment, multiple junctions having different bandgaps are stacked to form a solar cell. Each junction may have different bandgaps, and therefore be responsive to different parts of the spectrum. The junctions are stacked in such a manner that some bands of light pass through upper junctions to lower junctions that are responsive to such bands.

The alloy provides the ability to form solar cells with more then three junctions. In principle, any number of junctions may be used. For example, cells with four junctions would greatly improve efficiencies especially for outer space applications.

The examples of maximum efficiencies used herein are for typical terrestrial applications i.e. under Air Mass 1.5 direct normal irradiance (maximum light concentration). These are typical conditions commonly used to compare solar cell performance.