BORON NITRIDE CARBON ALLOY SOLAR CELLS

Abstract

Solar cells fabricated from p-n junctions of boron nitride nanotubes alloyed with carbon are described. Band gaps of boron nitride carbon alloys are tailored by controlling carbon content in the boron nitride nanotubes. High efficiency solar cells can be fabricated by tailoring the band gap of boron nitride carbon alloy nanotubes, and using these nanotubes for fabricating solar cells u. Because boron nitride carbon alloy nanotubes are transparent to most wavelengths of light, the wavelengths not converted to electrons (i.e., absorbed) at a first p-n junction in a solar cell will pass through the stack to another p-n junction in the stack having a different band gap. At each successive p-n junction, each of which has a different band gap from the other p-n junctions in the stack, more wavelengths of light will be converted into electricity. This dramatically increases the efficiency of solar cells.

Claims

1-20. (canceled)

21. A solar cell having at least three layers, the solar cell comprising: a first layer of boron nitride carbon alloy nanotubes having a first carbon concentration; a second layer of boron nitride carbon alloy nanotubes having a second carbon concentration greater than the first; and a third layer of boron nitride carbon alloy nanotubes having a third carbon concentration greater than the second, wherein each of the layers exhibits a band gap different than that of the other layers.

22. The solar cell of claim 21 wherein each of the at least three layers has a density of less than 2 g/cc.

23. The solar cell of claim 21 wherein each layer comprises nanotube felts.

24. The solar cell of claim 21 wherein each layer comprises nanotubes oriented perpendicular to a plane of the layers.

25. The solar cell of claim 21 wherein each layer comprises nanotubes oriented horizontally to a plane of the layers.

26. The solar cell of claim 21 wherein one of the layers has a first wavelength of light for which it is most efficient at converting to electricity and at least one of the remaining layers is substantially transparent to that first wavelength.

27. The solar cell of claim 21 wherein each of the at least three layers transmits more than 50% of incident light.

28. The solar cell of claim 21 wherein each of the at least three layers exhibits a band gap of 0.7 eV to 2 eV.

29. The solar cell of claim 21 wherein the carbon atoms reside at a lattice site and not at an interstitial site or defect.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1A is a schematic illustration of a structure of boron nitride in plan view, in an embodiment.

[0010] FIG. 1B is a schematic illustration of a structure of a boron nitride carbon alloy in plan view, in an embodiment.

[0011] FIG. 1C is a schematic illustration of a structure of a boron nitride carbon alloy nanotube, in an embodiment.

[0012] FIG. 2 is a graph of illumination power versus sunlight wavelength as measured at the Earth's surface, in an embodiment.

[0013] FIG. 3 is a graph of band gap (E.sub.g) versus composition for a boron nitride carbon alloy, in an embodiment.

[0014] FIG. 4 is a schematic illustration of a solar cell of the present disclosure including multiple p-n junctions, each of which is designed to convert a different wavelength of sunlight to electricity, in an embodiment.

[0015] The figures depict various embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein.

DETAILED DESCRIPTION

Overview

[0016] Embodiments of the present disclosure include solar cells fabricated from p-n junctions of boron nitride (BN) nanotubes that have been alloyed with carbon. Band gaps of boron nitride carbon (BNC) alloys are tailored by controlling carbon content in the BN nanotubes. By tailoring the band gap of BNC alloy nanotubes and fabricating solar cells using appropriately tailored BNC alloys, high efficiency solar cells are fabricated. The high efficiency of BNC alloy solar cells has at least two sources. First, BNC alloy nanotubes have, in some embodiments, a band gap (E.sub.g) tailored to match wavelengths of light most prevalent at the surface of the Earth. Second, in some embodiments, layers of BNC nanotube alloys are used to fabricate a solar cell stack. Each BNC nanotube alloy layer can have a different band gap configured to convert a particular wavelength (or range of wavelengths) into electricity. Because BNC alloy nanotubes are substantially transparent to most wavelengths of light, the wavelengths not converted to electrons (i.e., absorbed) at a first p-n junction will pass through the stack to another p-n junction in the stack having a different E.sub.g. At each successive p-n junction, each of which can have a different band gap from the other p-n junctions in the stack, more wavelengths of light will be converted into electricity. This dramatically increases the efficiency of BNC nanotube alloy solar cells.

Boron Nitride and Boron Nitride Carbon Alloy Structure

[0017] FIG. 1A illustrates the structure and composition of BN. BN and BNC alloys can occur in both nanotube and planar sheet allotropes. While embodiments of the present disclosure are generally directed to BN nanotubes alloyed with carbon, the structure in FIG. 1A (and FIG. 1B) are presented in a planar sheet configuration for convenience of explanation.

[0018] As shown, BN (whether occurring in a nanotube configuration or a planar sheet) is generally structured as adjacent hexagons. A single hexagon within a sheet is referred to herein as a unit cell when convenient. At each of the vertices of each hexagon is disposed one of either a boron atom or a nitrogen atom. Generally the boron and nitrogen are disposed at alternating vertices. The boron and nitrogen atoms are bonded together by sp.sup.2 covalent bonds, with a slight ionic character to the bond caused by the relative electronegativities of the boron and nitrogen atoms.

[0019] A plurality of these hexagons, such as the ones shown in FIG. 1A, are bonded together to form a BN sheet or nanotube. BN sheets are, in some examples, formed macroscopically as one or more of (1) a monolayer (2) abi-layer, (3) a tri-layer. Structurally, BN sheets are comparable to the carbon allotrope graphene because of the hexagonal symmetry within a sheet and the formation of the sheets into monolayers, bilayers, trilayers, and combinations thereof. The inter-sheet spacing of bilayers or trilayers of BN is smaller than that of graphene. This is due to the partial ionic character of the sp.sup.2 bonds between the boron and nitrogen atoms, which provides more inter-sheet attraction than is present between carbon graphene sheets. BN nanotubes are generally formed by a single layer of atoms configured into a tubular allotrope but can include more layers extending over some or all of a tube. For example, tubes may be singled walled, double walled, triple walled or multiwalled.

[0020] FIG. 1B illustrates an example of a BNC alloy, in an embodiment. As in FIG. 1A, FIG. 1B is shown in a planar configuration for convenience of explanation even though some embodiments of the present disclosure described below are presented in the context of BNC alloy nanotubes used to fabricate solar cells. In FIG. 1B, some of the boron atoms and some of the nitrogen atoms in the BN structure have been substituted by carbon atoms. Because of the similarity in atomic size of boron (atomic number 10), carbon (atomic number 12), and nitrogen (atomic number 14), carbon atoms can substitute for either of boron atoms or nitrogen atoms within a single unit cell or within an array of unit cells of BN. This substitution forms a BN-carbon alloy in either one of a sheet or a nanotube and does not change the symmetry of the unit cells and does not change the dimensions of a unit cell (i.e., lattice parameters) by more than 5%.

[0021] Alloying BN with carbon is unlike doping intrinsic semiconductors with a dopant. In a semiconductor doping process, a semiconductor host is implanted with a dopant that is either a donor of electrons or a donor of holes. The donor, often implanted using a high energy ion beam, is inserted into the crystal structure of the host at an interstitial site or a crystallographic defect, such as a vacancy. Donor concentrations of dopants are in a range of parts per million with respect to the host composition. An example is implantation of arsenic (a hole donor) into silicon. The atomic size of arsenic is substantially smaller than silicon and therefore can conveniently fit into interstitial sites in the crystal structure of silicon.

[0022] Alloying BN with carbon is unlike this doping process in that carbon atoms substitute directly on lattice sites in the molecular structure for either boron atoms, nitrogen atoms, or both, without disturbing the BN unit cell. That is, carbon alloying atoms reside at a lattice site and not an interstitial site or defect location. In the boron nitride alloy, carbon atoms take the place of boron or nitrogen atoms in the lattice structure. Thus, carbon alloying atoms are present in the BN crystal structure in stoichiometric amounts.

[0023] While the symmetry of BN is essentially unchanged upon alloying with carbon, the electronic structure does change. This will be explained in more detail in the context of FIG. 2

[0024] FIG. 1C schematically shows a BNC alloy nanotube of the present disclosure. The hexagonal unit cell symmetry and composition of BN and BNC, as described above, is unchanged in nanotubes.

Boron Nitride Carbon Alloy Synthesis and Properties

[0025] Alloying of BN nanotubes with carbon is accomplished in any one of several possible fabrication methods. In one example method, the alloyed nanotubes can be made by forming the BNC alloyed nanotubes in situ, by substituting boron and carbon into a carbon nanotube (CNT), or by substituting carbon into a BN nanotube. In one example method, a plasma assisted chemical vapor deposition (CVD) is used to react methyl chloride gas with nitrogen to produce BN nanotubes. In another example method, borazine gas is reacted in a plasma of a plasma CVD process to produce BN nanotubes. The BN nanotubes, regardless of the method used to produce them, are then alloyed with carbon by heating the nanotubes to approximately 800 C. in the presence of flowing ethylene gas or other carbon containing gas. In another example method, BN nanotubes are alloyed with carbon by dissolving Tetracyanoquinodimethane (TCNQ) in acetone. BN nanotubes are then dipped into the TCNQ acetone solution, with carbon substituting for boron and nitrogen atoms in the BN nanotube hexagonal structure, as described above. TCNQ doped BNC alloy nanotubes are n-doped. In different embodiments, a carbon alloyed BN nanotube can comprise greater than 1%, greater than 3%, greater than 5% or greater than 10% carbon, on a molar basis. Alloyed BNC nanotubes may be void of any other elements or may be essentially void of any other elements. For example, BNC nanotubes may consist of or consist essentially of carbon, boron and nitrogen.

[0026] BN monolayers and nanotubes have an E.sub.g, prior to doping with carbon, of about 5.6 eV. Materials having a value of E.sub.g this high are typically considered insulators. However, upon alloying BN monolayers and/or nanotubes with carbon, the E.sub.g of BN monolayers and nanotubes changes as a function of carbon content. This enables BNC alloys to be synthesized into solar cells tailored to convert a range of wavelengths of sunlight into electricity.

[0027] FIG. 3 is a graph showing band gap (E.sub.g) versus composition for a boron nitride carbon alloy, in an embodiment. In embodiments, depending on the amount of carbon used to alloy BN nanotubes, E.sub.g of the resulting BNC alloy ranges from 0.7 eV to 2 eV. In some examples, a band gap of a BNC alloy can be adjusted by application of a transverse electric field. This graph can be found in Journal of Physics, volume 76, number 6, published June 2011.

[0028] Once fabricated, a BNC alloy is doped to p-type by simple exposure to oxygen. A BNC alloy, whether undoped or p-doped, can be converted to an n-type material by exposure to borazine, monovalent gold, TCNQ, and combinations thereof.

Boron Nitride Carbon Alloy Solar Cells

[0029] One feature of embodiments of the present disclosure, described above, is the high efficiency of solar cells comprising BN nanotubes. There are at least two reasons for this: (1) BNC alloys are mostly transparent (i.e., transmit greater than 50%, 60%, 70%, 80%, 90% or 95% of incident intensity) to many wavelengths of light (barring occasional scattering from crystal defects) including those from 400 nm to 700 nm and (2) the band gap of BNC nanotube alloys can be tailored over a very wide range of eV values. An example solar cell of the present disclosure that utilizes these two advantages is illustrated in FIG. 4.

[0030] As shown in FIG. 4, an example solar cell 400 of the present disclosure includes a plurality of stacked layers, 404, 408, 412, and 416, each of which is fabricated from BNC alloy nanotubes that have been tuned with different carbon contents to have different band gaps. While four layers are shown in the example solar cell 400, other embodiments are not limited to four layers and may have more than four or fewer than four layers in a stack.

[0031] The layers 404, 408, 412, and 416 in the example solar cell 400 are alternately doped n-type and p-type. When configured in a stack, the alternating n-type and p-type layers form p-n junctions at each interface. Furthermore, as described above, the band gaps of the layers at each interface are configured to convert a particular range of wavelengths of light to electricity, as described above. Because the layers 404, 408, 412, and 416 are substantially transparent to most wavelengths of light, the solar cell 400 has an extremely high efficiency because any wavelength of light not converted to electricity at one interface travels deeper into the stack, for possible conversion to electricity at a different interface exhibiting a different band gap.

[0032] For convenience of explanation, light incident onto the solar cell 400 includes three wavelengths of light, .sub.1, .sub.2, .sub.3. The layer 404 is doped p-type and the layer 408 is doped n-type. The BNC nanotube alloys in each of 404 and 408 are alloyed with a carbon content so that the corresponding band gap converts light of wavelength .sub.1 to electricity. As with most BNC nanotube alloys, layers 404 and 408 are transparent to many wavelengths of light, permitting the transmission of light having wavelengths .sub.2, .sub.3 deeper into the stack of layers of the solar cell 400. As shown, the transmitted wavelength .sub.2 travels to the interface between n-type layer 408 and p-type layer 412. Analogous to the p-n junction between layers 404 and 408, the p-n junction formed at the interface between layers 408 and 412 has a band gap designed (via carbon content) to absorb light having a wavelength .sub.2. The light having a wavelength .sub.3 is transmitted through the layers 408 and 412 to a p-n junction formed at the interface between layers 412 and 416. As shown, the layer 412 is doped p-type and the layer 416 is doped n-type. The band gap of the p-n junction formed by the interface of the layers 412 and 416 is configured (via carbon content) to convert light of wavelength .sub.3 to electricity. In some embodiments, higher energy wavelengths are absorbed at deeper levels in the stack. In additional embodiments, some layers having the same or similar band gap may be repeated in a stack to capture light that was not absorbed at a higher level in the stack. In other embodiments, the bottom surface of the stack may include a mirror to reflect any transmitted light back upwards through the stack of BNC nanotube layers.

[0033] Consistent with the above example, the number of layers (and corresponding p-n junctions formed at the interfaces of the layers) and the associated band gaps are configurable to convert any desired range of light wavelengths to electricity. In some examples, the band gaps of the various p-n junctions are configured to convert some or all of the wavelengths of light available at the Earth's surface. In other examples, the p-n junctions are configured to maximize electricity production of black body radiation for space applications.

[0034] In some embodiments, the various layers of an example solar cell (such as the example solar cell 400) are fabricated from BNC alloy nanotube felts in which the constituent nanotubes are not oriented within a layer. In other examples, the layers are fabricated from BNC alloy nanotube fabrics in which the nanotubes are oriented either perpendicular to a horizontal plane of the layers (i.e., parallel to a z-axis) or parallel to the horizontal plane of the layers (i.e., parallel to an x-axis). In still other embodiments, the layers are fabricated from woven threads and/or fabrics of BNC alloys which are alloyed and doped according to embodiments of the present disclosure and placed into intimate contact to form p-n junctions. In some cases, the solar cell can be flexible. For example, the layers may be curved repeatedly to a radius of less than 1 meter, less than 10 cm or less than 1 cm. Single BNC alloy nanotubes can also be used in some embodiments to form p-n junctions, as described above.

Benefits and Advantages

[0035] Benefits of solar cells fabricated using BNC alloys according to the present disclosure include (1) a lighter weight than conventional silicon or III-V semiconductor solar cells owing to a BNC density of approximately 0.8 g/cc compared to densities of silicon and III-V semiconductors of approximately 2 to 4 g/cc. BNC alloys are also more flexible that conventional semiconductors, with BNC alloys able to be bent into a curved or arcuate conformation to a radius of curvature of several nanometers whereas conventional semiconductors are inflexible and brittle. This enables solar cells to be fabricated that conform to an underlying surface (even a non-planar underlying surface), have at least one arcuate portion, or are even cylindrical. BNC alloys are also thermally stable (with little or no change in physical or electrical properties) up to about 900 C. in air. This benefit is particularly advantageous in space applications (e.g., powering satellites) or other applications in which the service environment is demanding (e.g., military applications).

[0036] Another advantage of BNC alloys is their transparency to most of the visible, near IR, and near UV wavelengths of light (e.g., from approximately 100 nm to approximately 1000 nm). This transparency enables the layered configuration, and resulting high efficiency, described above.

[0037] BNC alloys are resistant to radiation, with little or no change in crystallographic structure, p-n junction structure, and mechanical properties even when exposed to radiation of space that is unfiltered by the Earth's atmosphere. This benefit is advantageous for space applications and is in distinct contrast to multi junction solar cells fabricated from conventional semiconductor materials that do degrade when exposed to space radiation.

[0038] BNC allows are mechanically stronger than conventional solar cell materials and can withstand the forces involved in a space launch.

[0039] In other examples, embodiments of the present disclosure are applied to produce a high fidelity display.

FURTHER CONSIDERATIONS

[0040] The foregoing description of the embodiments of the disclosure has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the claims to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure.

[0041] The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the disclosure be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.