Polarized light based solar cell

Abstract

A solar cell is provided wherein a circular polarizer is positioned proximate an absorptive semiconductor layer which itself is separated from an electrode, such as a loop of conductive metal, by an electrically insulative layer. Upon exposure to non-polarized light, a portion of the incident light is polarized and transmitted to the semiconductor layer. Under the influence of this polarized light, photo excited electrons in the semiconductor layer are induced to move in a circular motion, thereby generating magnetic fields. These magnetic fields drive the flow of current within the electrode.

Description

CROSS REFERENCE TO RELATED CASES

This Continuation-in-Part Application claims priority to U.S. Non-Provisional patent application Ser. No. 14/545,022 filed Mar. 26, 2015, entitled Polarized Light Based Solar Cell, which in turn claims priority to U.S. Provisional Application 61/967,868, filed Mar. 27, 2014, of the same title.

STATEMENT OF GOVERNMENT SUPPORT

None

FIELD OF INVENTION

This invention relates to a solar cell device and method for using same wherein a film of a light polarizing substance is coated onto a light absorptive semiconductor material, absorbed light used to generate a magnetic field. The magnetic field in turn drives a current in an electrode disposed below the semiconductor material for the generation of electricity. Thus, using readily available, inexpensive materials, an inexpensive solar cell alternative is provided.

BACKGROUND OF THE INVENTION

Presently the most ubiquitous candidate for supplying solar energy is still the silicon-based module using a p-n junction. The p-n junction refers to the interface between two segments of silicon that have been doped with impurities, such as boron or phosphorus so that there is a preponderance of holes and electrons in these two segments, respectively. The majority charge carrier in the boron-doped segment is the hole and this segment is denoted as p-type. The majority charge carrier in the phosphorus-doped segment is the electron and this segment is denoted as n-type. As shown in FIG. 1A, upon light absorption (the arrow) an electron (black circle) is promoted from the valence band to the conduction band leaving a hole (white circle) in the valence band. These carriers are free to drift under the influence of the energetic gradient established by the p-n junction to their respective electrodes, as illustrated in FIG. 1B, for electricity production.

The environmental abundance of silicon coupled with accumulated knowledge on the operation of the device is what makes it commercially attractive. Furthermore, for every doubling of cumulative production there has been a 20% cost reduction over the past 30 years. Industry leaders such as Sunpower are selling modules that are 20% efficient and are achieving huge cost savings as they scale up production.

A common trend among solar panel manufacturers is to increase module efficiency in hopes that fewer solar panels will be required to generate a given amount of power. Reducing the total number of panels also helps reduce the balance of systems (BOS) cost, typically roughly $1.30/Watt for utility-scale power generation. Even for state-of-the-art silicon based cells sold by companies like Sunpower, the high BOS cost is not necessarily because of the module itself, but because of all the additional components that must be installed with it. The BOS includes the land on which the module is mounted, the mounting, monitoring systems, and labor. While the module alone may cost about $0.70/Watt, the BOS increases costs by roughly $1.30/Watt, making the entire installation nearly twice as expensive as the current $1/Watt threshold for market competitiveness. Even the installation of a free module would be prohibited because the BOS cost of $1.30/W is excessive.

Any high performing solar panel must absorb light, generate electric current, and conduct electric current very well. Silicon based solar panels clearly have many merits, but also have significant shortcomings. One of the most prominent limitations of these modules is thermalization after light absorption, which reduces the energy available to do electrical work. When an electron-hole pair is excited optically with energy greater than the band gap of silicon, 1.1 eV, the electron reaches a state higher than the conduction band edge, then relaxes back to the band edge by rapidly emitting thermal energy. In essence, any photon energy greater than 1.1 eV is thus wasted in this process of thermalization. For this reason, light energy is not utilized efficiently with this mechanism.

Due to such inherently low efficiency, other approaches have been sought for converting solar energy into electricity. In one alternative approach an organic based solar cell has been provided (G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger. Science. 1995, 270, 1789.), a representative organic solar cell made from a polymer, poly(3-hexylthiophene), or P3HT, and phenyl-C61-butyric acid methyl ester, or PCBM. In a P3HT:PCBM solar cell, P3HT serves as an electron donor and hole conductor and the PCBM serves as an electron acceptor. The functionality of the donor-acceptor junction under light absorption strongly resembles that of the p-n junction (as illustrated in FIG. 2). When light (represented by the arrow in FIG. 2A) is absorbed, bound electron-hole pairs (the electron represented by the black circle, the hole by the white circle), or excitons, are generated primarily in P3HT as it is the more highly absorbing compared to PCBM. After this exciton is split at the donor-acceptor interface, the electron drifts to the cathode and the hole to the anode, as depicted in FIG. 2B to provide electricity.

This technology suffers in the phases of light absorption and in charge transport of photo-generated carriers (electrons and holes). The organic absorber tends to have a narrow absorption range so it does not harvest the full solar spectrum. Charge transport of photo-generated carriers is hampered because of the disorder of the material, whether it is a polymer or a small molecule. Charge transport in organic materials is typically poor in comparison to crystalline inorganic materials and, as a result, the electrons may recombine with holes before they reach the electrodes. Rather than providing electricity, these charges recombine and release light which is counter-productive. Thus, organic solar cells are limited both in terms of the amount of light absorbed and in the extraction of electricity.

Given these drawbacks of the prior art, a most viable strategy is one which aims to produce a module at low cost and reduce the BOS cost, and that requires an ultra-efficient module with the performance exceeding the Shockley-Queisser limit of 30%.

SUMMARY OF THE INVENTION

According to this invention, markedly different from either the inorganic p-n junction or the organic donor-acceptor varieties of solar cells, a circular polarizer is employed. When light strikes the circular polarizer, a portion of the incident light is transmitted as circularly polarized before entering an absorptive semiconductor layer. In one embodiment, this absorptive layer is composed of iron doped lithium niobate, Fe:LiNbO₃. The polarized light absorbed in this layer causes the photo excited electrons in the layer to move in a circular or elliptical motion, thereby generating magnetic moments. The magnetic field generated by this electron motion induces an electric current in an electrode situated in proximity with the semiconductor. In one embodiment, this electrode is composed of copper coated with (or otherwise separated by) an insulating layer. In some embodiments the insulating layer may be a polyimide, such as Kapton. In still another embodiment, the electrode can be wound into a coil.

Not only does this device use wholly different materials from a traditional inorganic or organic solar cell, but the functionality is revamped from the ground up. There are no p-n or donor-acceptor junctions and electrical current does not flow into or out of the semiconductor. Moreover, this device represents the first opportunity to utilize a circular polarizer and magnetism to drive solar energy conversion. Also of note is that the electrode is arranged in one embodiment into a coil rather than the traditional strip. The size of each cell is determined by the area outlined by each electrode. The best performing solar panel would theoretically contain a single electrode that outlines the entire panel, with an area of 40″-60″ in line with conventional silicon solar panels. However, the panel is rendered more robust if there are several smaller cells in parallel. In this way, a failure in one cell does not cause the entire panel to fail, but just an isolated region.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with respect to particular exemplary embodiments thereof and reference is accordingly made to the drawings in which:

FIG. 1A is a schematic of the energy level diagram of a p-n junction showing the generation of an electron (black circle)-hole (white circle) pair upon light absorption (arrow). FIG. 1B is a schematic showing the p-n junction after the electron has drifted to the cathode and the hole to the anode to provide electricity. (Note: bottom black segment represents the valence band of silicon and the top segment represents the conduction band).

FIG. 2A is a schematic of the energy level diagram of a donor-acceptor junction showing the generation of a bound electron (black circle)-hole (white circle) upon light absorption (arrow). FIG. 2B is a schematic showing the donor-acceptor junction after the electron has drifted to the cathode and the hole to the anode to provide electricity. (Note: bottom left and right segments represent the highest occupied molecular orbitals, or HOMOs, of P3HT and PCBM, respectively. The top left and right segments represent the lowest unoccupied molecular orbitals, or LUMOs, of P3HT and PCBM, respectively.)

FIG. 3 is a schematic cross section of the most basic solar cell structure according to one embodiment of the invention.

FIG. 4A is a cross sectional view of the wire loop disposed below the circular polarizer/semiconductor material stack according to an embodiment of the invention.

FIG. 4B is a schematic top down view of the wire loop of FIG. 4A.

FIG. 5A is a three dimensional depiction illustrating how unpolarized incident light becomes circularly polarized (both right- and left-handed) upon striking a circular polarizer. FIG. 5B is a cross sectional view of the device of FIG. 3 illustrating the device physics of the solar cell, which utilizes the light transmitted through the polarizer.

FIG. 6 is a schematic illustrating the solenoid-like current produced by an electron under the influence of circularly polarized light.

FIG. 7 is an exemplary plot of current density vs. voltage in both dark and in light.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing detailed description of the present invention is provided for purposes of illustration and is not intended to be exhaustive or to limit the invention to the embodiments disclosed.

In the first instance, the structure and operation of the solar cell will be described, and a method for manufacturing the cell described thereafter.

FIG. 3 is a schematic cross section of one embodiment of the invention. With reference to FIG. 3, a solar cell structure 100 is illustrated comprising a first, circular polarizer layer 102. In one embodiment, the circular polarizer may be monochromatic. A suitable polarizer material manufactured by American Polarizers (http://www.apioptics.com/) may be employed. It consists of a linear polarizer on a monochromatic wave retarder affixed to a suitable substrate, such as either a glass or acrylic substrate. This material can accept unpolarized light and transmit close to 50% of this light as either left-hand or right-hand circularly polarized light and reflects the rest, at a slightly diminished rate of approximately 40%. Moreover, this effect is observed across a broad wavelength range of 400 nm-700 nm.

Alternatively, a broadband circular polarizer material can be used. This variant is capable of polarizing the full visible spectrum. In one embodiment, it consists of a linear polarizer on an achromatic wave retarder affixed to a substrate, which in some embodiments can either be a glass or acrylic substrate. In another embodiment, the circular polarizer layer utilizes a cholesteric liquid crystal with varying pitch deposited upon a polyethylene terephthalate substrate. In general, any material that converts unpolarized light to circularly polarized light can be incorporated into the polarized light based solar cell. However, the system works best with broadband functionality and when the substrate is transparent.

Below circular polarizer layer 102 is a absorptive semiconductor layer 104, which may, in some embodiments, be formed of Fe:LiNbO₃. In one embodiment, Fe:LiNbO₃, is preferred due to its relative, abundance and low cost. Other suitable semiconductor materials for layer 104 include Te, Bi₁₂SiO₂₀, Pb₅Ge₃O₁₁, and InN. In addition, other dopants such as Co, Ni, Cr, and Cu can be utilized to enhance light absorption.

Finally, an electrode 106 is provided on the opposite side of semiconductor layer 104 in such fashion in an embodiment as to create a coiled electrode, as illustrated in FIGS. 4A and 4B. This electrode, in various embodiments, can comprise metals such as copper and metallic semiconductors such as graphene, Al:ZnO, ITO and FTO.

In FIG. 4A, a cross sectional view, the electrode is shown in one embodiment coated in an insulator, such as polyimide, so that the incoming lead and outgoing lead are separated spatially and electronically. In another embodiment, the electrode comprises a coil with several turns. In FIG. 4B, a plan view, the looped nature of the electrode is shown. Also shown is a magnetic field, B, which is generated when the polarized light strikes the semiconductor material, the magnetic field inducing an electrical current indicated with arrows, as explained more fully hereinafter.

The shape of the electrode itself is not critical. In fact, the electrode can be one formed in several configurations, such as in a flat circular pattern, a flat square pattern as illustrated in FIG. 4, a spiral pattern, a flat coil arranged in a snake-like grid pattern, and the like. In some embodiments, the electrode can be formed from a wire wrapped multiple times around an insulating core, such as a polyimide core.

In operation, light entering the device passes through circular polarizer layer 102, in which light transmitted through the layer is circularly polarized as shown in FIG. 5, transmitted as left-hand or right-hand circularly polarized light. This circularly polarized light is then used to drive a current. The handedness of the polarized light can be changed depending on the circular polarizer used. The solar cell will function no matter which handedness is used; the only difference is that the resulting magnetic field and, therefore, the current produced will point in the opposite direction.

In the usual case it is not necessary to provide a protective layer to circular polarizer layer 102. However in an embodiment, if desired, a protective layer (not shown), such as one made of glass, may be provided, with light passing first through the protective layer before encountering the circular polarizer layer. In still another embodiment, while not necessary to the functioning of the device, a separate insulating layer, such as a from glass, quartz, Kapton, and the like, may be interspersed between the semiconductor layer and electrode 106 to provide additional insulation or rigidity to the structure.

The theory of operation will now be discussed. The inverse Faraday effect, predicted by Pitaevskii in 1961 (“Electric Forces in a Transparent Dispersive Medium”, Soviet Physics Jetp-USSR 12(5): 1008-1013), holds that a magnetic field is generated by circulating electrons driven by circularly polarized light. The mechanism resembles a microscopic solenoid, as illustrated at FIG. 6 in which a single moving electron represents the electric current,

$\begin{array}{cc}I=\frac{\mathrm{\phi \omega }}{2\pi },& \left(1\right)\end{array}$

which establishes a magnetic moment of

$\begin{array}{cc}\mu =\frac{{\phi }^sE_o^{2i}}{2m^2{\omega }^s}\cos \theta .& \left(2\right)\end{array}$

As for the device of the invention, unpolarized light (black curved arrow in FIG. 5B) strikes the circular polarizer and the light emerging from the polarizer will generate magnetic moments in the semiconductor on the underside. From the point of view of the polarizer, the polarized light (white curved arrows) below will drive the current (white dots) in a left-handed fashion. The magnetic moments established by each electron will uniformly have the north pole pointing down. As these charges generate magnetic moments and this flux penetrates the electrode, the energy absorbed by the light is used to drive an electric current. That is, the generated magnetic flux permeates the electrode wrapped into a coil on the bottom surface of the device. This current is driven by induction according to Lenz's Law. Under the influence of increasing magnetic flux (indicated by B of FIG. 4B) into the plane of the diagram, current will be driven counter-clockwise within the square loop from left to right in the incoming and outgoing leads.

It is fairly straightforward to predict an efficiency of energy conversion. Following an approach of Haines (Haines, M. G. (2001). “Generation of an Axial Magnetic Field from Photon Spin.” Physical Review Letters 87(13): 135005-135009), Maxwell's equations when applied to circularly polarized light lead to the determination that

$\begin{array}{cc}\frac{\partial B_z}{\partial t}=-\frac{1}{r}\frac{\partial }{\partial r}\left({\mathrm{rE}}_{\theta }\right),& \left(3\right)\\ \mathrm{where}{\mathrm{rE}}_{\theta }=-\frac{{\alpha }_{\mathrm{ao}}r}{n_ee\omega L}\frac{\partial I}{\partial r}.& \left(4\right)\end{array}$

In the preceding equation, α_ab is the fraction of light absorbed over a distance L, r is the distance at which the magnetic flux is detected by the electrode, I is the intensity of light, n_e is the electron number density, e is the electron charge, and ω is the frequency of light. The magnetic flux,

ϕ_B=B·A,  (5)

which permeates the electrode wrapped into a coil on the bottom surface of the device is what establishes an electromotive force, c, to drive a current within the coil by induction according to Lenz's law:

$\begin{array}{cc}\varepsilon =-\frac{d{\phi }_B}{\mathrm{dt}}.& \left(6\right)\end{array}$

If reasonable values are substituted (α_ab=0.5, I=1000 W/m², ω=10¹⁵ s⁻¹, L=10⁻⁶ m, e=1.6*10⁻¹⁹, r=10⁻⁷ m, n_e=1.6*10¹⁶ m⁻³),

$\frac{\partial B_z}{\partial t}=-12500m^{-2}.$

The values above are chosen for the following reasons: α_ab=0.5 because the transmitted intensity is 50% of the incident intensity, I is the intensity of solar illumination, w is approximately 10¹⁵ s⁻¹ for visible light, L=10⁻⁶ m is the distance over which light interacts with the polarizer, r=10⁻⁷ m is the thickness of the electrode that responds to the magnetic field, and n_e=1.6*10¹⁶ m⁻³ is an achievable carrier concentration in semiconductors. Correspondingly,

$-\frac{\partial {\phi }_B}{\partial t}=-\frac{\partial B_z}{\partial t}A=\varepsilon =-\mathrm{.39}V$

for an electrode spanning an area of 1 cm². For a copper electrode with a resistivity of 1.68*10⁻⁸ Ωm, a length of 4 cm (1 cm on each side of a square), and thickness of 100 nm, the resistance is 3.36Ω. A voltage of 0.36V across a resistance of 3.36Ω generates a current of 0.12Ω. A and a power of 45 mW. Considering that the solar flux is 100 mW/cm², this represents an efficiency of

$\frac{45\mathrm{mW}}{100\mathrm{mW}}=45\%.$

This efficiency represents the energy conversion of light absorbed by the cell.

It is important to note that in this calculation, an incident intensity of 1000 W/m² has been assumed, which is much lower than that typically used to study the Inverse Faraday Effect. In contrast, Haines routinely uses laser intensities on the order of 10²¹ W/m² with the understanding that the Inverse Faraday Effect is a weak phenomenon. A very large light intensity was required because the plasma that Haines studied had a photo excited charge concentration of 2.1*10²⁵ m⁻³. Whereas here, in sacrifice of light intensity, the photo excited charge concentration is also substantially reduced so that an appreciable magnetic flux is still observable, according to equations 3 and 4. The photo excited charge concentration can be tuned from ˜0 m⁻³ in an insulator to ˜10²⁶ m⁻³ in a doped semiconductor.

The fabrication of the novel solar cell of this invention will now be described. The fabrication of the solar cell is quite straightforward, and by example will be described in connection with the embodiment of FIG. 3. First, the semiconductor layer 104 is fashioned using commercially available techniques such as the Czochralski process, in the case of Fe:LiNbO₃.

Next, a version of the circular polarizer, 102, can be formed when a linear polarizer is affixed to a wave retarder. In various embodiments, the linear polarizer consists of either a birefringent crystal, a reflective non-metallic surface, or a dichroic absorber. The retarder is, in one embodiment, a birefringent material with a fast (extraordinary) and slow (ordinary) axis. Alternatively, a circular polarizer may be fashioned from a cholesteric liquid crystal film sandwiched between two glass wafers by vacuum filling. The bottom section of the circular polarizer is then joined to the semiconductor surface using any adhesive such as a glue.

Finally, electrode 106 may in one embodiment be formed by evaporation onto the opposite sides of an electrical insulator layer using commercially available techniques to create the coiled electrode. This can be achieved using 2 evaporations. The first evaporation is performed to deposit the outgoing lead before a thin insulating film, shown in FIG. 4A, is affixed on top the lead. The second evaporation is then performed to complete the coiled electrode. The insulating film of a material such as polyimide is necessary to ensure that the wire as formed electrically is a loop. Finally, this electrode assembly is capped on its outer surfaces with an insulating sheath. Alternatively, a wire pre-coated with an insulator can be used, particularly if the electrode has multiple turns.

As for the dimensions of the solar cell, the relative thickness of the various layers depicted in FIG. 3 are not to scale, and are only representative. The size of each cell is determined by the area of each electrode coil. The area enclosed is the active area as it is this area wherein the magnetic field is generated to produce a current. The area directly above the electrode is effectively “dead space” because it cannot be used to produce electricity. For this reason, this dead space must be minimized by reducing the total space occupied by an electrode to maximize performance. In practice, however, it is risky to construct one large electrode because a failure, such as a crack, at any one part would cause the entire panel to fail. For this reason, several smaller cells will be arranged in parallel so that cell failures can be isolated and not affect the entire panel. Generally, and by way of example only, the cells will be 130 mm×130 mm, but the manufacturer has a great deal of flexibility as to optimizing these dimensions on the basis of performance and robustness. In one embodiment of the polarized light based solar cell of the invention, a linear polarizer, such as an achromatic wave retarder, a crystal of Fe:LiNbO₃, and a pickup coil made of polyimide insulated copper can be employed.

FIG. 7 is an exemplary plot of current density for a solar cell made according to an embodiment of the invention. Both the upper and lower curves are current density vs. voltage plots with the upper curve measured in the dark while the lower curve is measured under solar intensity. The lower curve is shifted down relative to the upper curve to indicate the presence of photogenerated current. The lower curve intersects the vertical axis at the value of the short circuit current density (J_sc), the photocurrent generated at zero applied bias. When a critical applied bias is provided, the open circuit voltage (V_oc), the current can be reduced to zero, which is where the lower curve intersects the horizontal axis. As noted previously, very high efficiencies of approximately 45% may be achievable according to the novel solar cell design of the invention. The cell uses materials that are abundant and safe to use, making it commercially compatible and economically attractive.

This invention has been described herein in considerable detail to provide those skilled in the art with information relevant to apply the novel principles and to construct and use 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 modifications, both as to the equipment and operating procedures, can be accomplished without departing from the scope of the invention itself.

Claims

1. A multi-layer solar cell device comprising;

a circular polarizer layer having a top face and a bottom face, said top face situated for exposure to incoming light;

a light absorptive semiconductor layer proximate to the bottom face of said circular polarizer layer;

an electrode disposed below said semiconductor layer.

2. The multi-layer solar cell device of claim 1 wherein the electrode is separated from said semiconductor layer by an insulating material.

3. The multi-layer solar cell device of claim 4 wherein said electrode is separated from said semiconductor layer by an insulating material coated over said electrode.

4. The multi-layer solar cell device of claim 3 wherein the electrode is further separated from said semiconductor layer by an insulating layer interposed between said semiconductor layer and said electrode.

5. The multi-layer solar cell device of claim 1 wherein said electrode is a coiled electrode

6. The multi-layer solar cell device of claim 5 wherein said electrode is wound around an insulating material.

7. The multi-layer solar cell device of claim 1 wherein the circular polarizer is a broadband circular polarizer.

8. The multi-layer solar cell device of claim 1 wherein the circular polarizer is a monochromatic circular polarizer.

9. The multi-layer solar cell device of claim 1 wherein the circular polarizer layer comprises a linear polarizer on a monochromatic wave retarder affixed to a substrate, an achromic wave retarder affixed to a substrate, or a cholesteric liquid crystal affixed to a substrate.

10. The multi-layer solar cell device of claim 1 wherein the electrode comprises a conductive wire.

11. The multi-layer solar cell device of claim 10 wherein the material of the wire is selected from the group comprising copper, graphene, Al:ZnO, ITO and FTO.

12. The multi-layer solar cell device of claim 1 wherein the material used to form the light absorptive semiconductor layer is selected from the group comprising LiNbO3, Te, Bi12SiO20, Pb5Ge3O11, and InN.

13. The multi-layer solar cell device of claim 1 wherein the material used to form the light absorptive semiconductor layer is LiNbO3.

14. The multi-layer solar cell device of claim 13 wherein said light absorptive semiconductor layer is doped with at least one metal selected from the group comprising Fe, Co, Ni, Cr, and Cu.

15. The multi-layer solar cell device of claim 14 wherein said dopant comprising at least one metal is iron.

16. The device of claim 1 further including a top protective layer adjacent the top face of said circular polarizer layer.

17. A method of generating electricity including the steps of:

providing a solar cell incorporating a circular polarizer,

exposing said solar cell to non-polarized light,

transmitting a portion of said incident light through said circular polarizer, whereby the portion of incident light transmitted is circularly polarized;

exposing an light absorptive semiconductor material to said transmitted circularly polarized light, whereby a magnetic field is generated; and,

providing an electrode proximate to said absorptive semiconductor material and situated within said generated magnetic field, whereby a current is induced in said electrode.

18. The method of claim 17 wherein the electrode is a conductive coil.