The present application relates to solar cells, and specifically to improving absorption of incident light in solar cells.
This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
A solar cell is a photoelectric conversion device that transforms solar energy into electrical energy. Solar cells are generally divided into two categories: 1) bulk solar cells and 2) thin film solar cells. Typically bulk solar cells are based on thickness of 200 μm or greater. On the other hand, thickness of thin film solar cells typically ranges between a few nm to tens of μm. A thin film solar cell is typically silicon-based including amorphous silicons, microcrystalline silicons, and polycrystalline thin-film silicons, as well as thin-film solar cells based on compounds including Cu(InGa)Se2, CdTe, and CuInSe2.
A solar cell produces electrical energy by transferring electrons and holes to n-type and p-type semiconductors, respectively, and then collecting electrons and holes in each corresponding electrode, when an electron-hole pair is produced by solar light energy absorbed in a photoactive layer inside the semiconductors. A thin film solar cell has a high light absorption rate in the visible light region compared to the crystalline solar cell. Therefore, for solar cells, it is important to effectively absorb solar energy emitted from the sun, and to increase efficiency of the solar cell absorbing and using the solar energy from the sun.
Referring to one exemplary embodiment of the solar cells found in the related art,
Waves which are incident upon an interface can experience basic phenomenon such as reflection and refraction. The direct outcome of these effects depends upon the basic properties of the materials at the interface. Researchers have long used these properties of reflection and refraction to develop extraordinary technologies such as waveguides and polarization filters, but in many cases these, effects limit the performance of a system. For solar cells formed from various semiconductor materials, there is an inherent and large mismatch between the basic material properties of the structure and the surrounding air. This impedance mismatch causes a significant reflection to occur at this interface, significantly reducing the amount of light which can be coupled into the device.
Referring to
Methods for increasing the efficiency of the thin film solar cell include mounting a rear reflection structure on a solar cell. Rear reflection structure may increase the efficiency by reducing or effectively preventing light entering through a front surface of a solar cell from going out of the solar cell through the rear surface, and using the remaining light reflected by the rear reflection structure in the photoactive layer of the cell.
The other contributor is unwanted reflection. The large difference in impedance between the semiconductor solar cell and air causes a substantial amount of light to be reflected (i.e., 11rl as shown in
On way to increase the in-coupling of light includes utilization of an impedance matching arrangement with air. This method has been demonstrated in several ways including gratings, antireflection coatings, graded index layers, or the use of small metallic structures to gradually increase the impedance from air to that of the solar cell. Anti-reflection coatings use destructive interference to effectively cancel any reflected energy, consequently improving the transmitted energy. Anti-reflective coatings are very sensitive to the wavelength but have been shown to be quite effective. Face reflections as low as 0.01% have been achieved for antireflection (AR) coatings on semiconductor lasers. Gratings reduce reflections by attempting to match the impedance between the materials at the interface with structures that are similar to the size of the wavelength. They can be made more tolerant to the changes in wavelength and may be polarization dependent. In general, gratings also have a limitation in that they consider only a change in the effective permittivity of the medium since the permeability of the constituent materials in the optical regime is that of air. Gradient index layers provide more flexibility, allowing researchers to engineer the conditions at an interface by controlling the composition of constituent materials. Reflections are reduced by forming a smooth index transition between two materials over several wavelengths. These coatings can provide a significant suppression of reflections over a broad range. However, because solid materials have refractive indices greater than that of air, a small but abrupt change in the refractive index with air limits the total efficiency of the structure. Small metallic structures have been used to generate an impedance match. However, the use of metallic spheres or squares only allows for an approximate impedance match using permittivity.
There is, therefore, an unmet need for a solar cell construction that reduces reflection and one which further improves absorption of light.
A solar-energy module is disclosed. The module includes a first electrode configured to receive incident visible light with a different refractive index than the medium through which light travels prior to becoming incident on the first electrode, the first electrode having a first metasurface arrangement formed through the first electrode, and configured to selectively i) match the optical impedances of the first electrode and the medium, and ii) cause light to be refracted substantially away from normal refraction angle, a photon-absorbing material coupled to the first electrode on a first surface of the photon-absorbing material and configured to receive refracted light through the first electrode and adapted to produce an electrical current in response to the refracted light, length of the photon absorbing material substantially larger than thickness of the photon-absorbing material, and a second electrode coupled to the photon-absorbing material on a second surface of the photon-absorbing material.
The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein:
a, 10b, 11a and 11b depict process steps and a flow chart describing an exemplary manufacturing process for making the solar cell according to the present disclosure.
The attached drawings are for purposes of illustration and are not necessarily to scale.
For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended.
A novel approach for avoidance of unwanted reflection and improvement in absorption in solar cells is disclosed. The approaches provided in the present disclosure ensure that substantially all light which does enter the solar cell is absorbed and converted to electrical energy. As a result, light which enters is absorbed within twice the thickness of the cell. For thin-film solar cells, this length-feature can be quite small, on the order of a few microns. In addition, the present disclosure describes use of metasurface to achieve superior impedance matching and light directing to further prevent light from escaping the cell. These metasurfaces differ from the previously utilized roughened back surface which used grating or photonic crystals, thereby the metasurfaces do not rely on scattering or on an abrupt change in only the permittivity in order to reduce unwanted reflection.
Instead Metasurfaces improve the efficiency of the solar cells by tailoring both the permittivity and permeability of an effective layer to achieve an impedance match between the materials at an interface. Metasurfaces can also be designed to provide a polarization independent design as well as structures which are deeply sub-wavelength and require relatively simple scalable chemical or lithographic nanofabrication processes such as etching or nano-imprint lithography, adhesion lithography, or another soft lithography method. The application of a metasurface to a semiconductor-air interface will result in essentially no light being reflected, facilitating and increasing in the in-coupled light efficiency. Metasurfaces could even be used in conjunction with current methods (i.e., grating or photonic crystals) to further reduce the reflection for a narrow band, or generate effective wideband antireflection surfaces.
Consequently, through the use of metasurfaces, the properties of an effective material can be designed to generate an impedance match between the low impedance surrounding and the high impedance solar cell. The application of this metasurface can greatly reduce the reflection at the interface, increasing the amount of light coupled into the solar cell.
Metasurfaces have gained significant attention due to their relative ease in fabrication when compared to their 3-D counterparts. They are traditionally fabricated using patterned metallic structures on a dielectric, or voids in metallic films. However, they are not limited to metals only. Semiconductors can, in principle, also be used as a base material for nano structures. For many applications these structures are more desirable because they do not suffer from the large ohmic losses which plague metallic metasurfaces.
An exemplary metasurface arrangement in form of a supercell 100 according to the present disclosure is provided in
At a metasurface, the generalized reflection and refraction conditions are given by Eq. 1 and Eq. 2:
where dφ/dy is the gradient of phase discontinuity along an air-solar cell interface, μ is the permeability of the material, ε is the permittivity of the material, and λ0 is the free space wavelength of the incident wave. The angular relationships in the above equations are depicted in
The depiction of a wave incident upon a metasurface shows that there are two reflected and refracted beams, Θr, Θra, Θt, and Θeta which are the normal and anomalous reflections and the normal and anomalous refractions respectively. Normal reflections and refractions occur due to the periodicity of the structure. Therefore, at certain distances y, the difference in phase between the two points is dφ/dy=0. This is the traditional case of reflection and refraction. At all other points there is a discrete difference in the phase between two incident rays which leads to the anomalous reflection and refraction.
Through the use of metasurfaces, it is suggested that an impedance match can be engineered between the high impedance semiconductor light source and low impedance air. This approach is unique from previous works using bulk materials or patterned structures as metasurfaces allow us to control both the permittivity and permeability of the material at a deeply subwavelength scale.
The metasurface tailors impedance by employing resonance for both the electric and magnetic fields incident on the material. A depiction of an exemplary “C-shaped” structure having two metallic strips of length L=2a separated by a dielectric of thickness d is shown in
where, H0 is the applied magnetic field,
a is half the length L of the nanowire,
d is the separation of the nanowires,
b is the radius of nanowire, and
k is the wave vector ω/c. The factor ga is provided in equation 4.
Consequently, as depicted in
Referring to
The angle of anomalous reflection does depend on incident angle. Resonances are a function of projections of rays on the plane of the surface. Therefore, the reflective film 462 with metasurfaces can be designed to reduce the extent to which rays are reflected out of the cell. For example, the reflective film 462 with metasurfaces can be designed to control electric (E) field. Near normal, the E-field is in the plane of the reflective film 462. Far from normal, the E-field is almost perpendicular to the reflective film 462. The reflective film 462 with metasurfaces can be designed so that, as the incident angle moves away from the normal, less light is reflected anomalously and more is reflected by Snell's law. In this way, the angle of reflection will be far from normal across a wide range of incident angles.
The solar cell and its metasurfaces 458 and 462 can be designed based on the typical sun angle and spectrum that a given solar cell will encounter in the field. Relevant facts can include whether or not the solar cell will be installed on a rooftop, and whether the solar cell will be fixed or mobile.
Various aspects discussed herein increase the efficiency of solar cells through the use of metasurfaces. Two metasurfaces can be used: an impedance matching layer placed on the top surface and a highly reflective metasurface placed below the solar cell. These two materials serve two purposes: 1) to increase the in-coupled light efficiency, and 2) to ensure that substantially all light which enters the solar cell is absorbed.
The impedance matching metasurface is designed to eliminate reflections for the light which is incident upon the output facet.
A highly reflective metasurface can be utilized to produce large anomalous reflections with an angle that is far from the normal.
The impedance matching metasurface 458 (see
Referring to
Referring to
Referring to
According to one embodiment, the photoabsorptive material can be a semiconductor, ceramic, or other weakly/non-conducting material that has an inter-bandgap energy suitable to absorb incident sunlight. Examples include germanium, silicon, gallium arsenide, indium phosphide, gallium nitride, aluminum nitride, or other group IV (silicon, germanium), group III-V (gallium arsenide, aluminum nitride), or group II-VI (zinc oxide) combinations.
The metasurface structures can be made from metal, semiconductor, ceramic, or other material that exhibits metallic properties (i.e., real permittivity <0) in the wavelength range of operation (visible light). Example include aluminum, gold, silver, copper, nickel, tungsten, platinum, titanium, titanium nitride, zirconium nitride, indium tin oxide, gallium doped zinc oxide.
Those skilled in the art will recognize that numerous modifications can be made to the specific implementations described above. The implementations should not be limited to the particular limitations described. Other implementations may be possible.
The present U.S. patent application is related to and claims the priority benefit of U.S. Provisional Patent Application Ser. No. 61/862,995, filed Aug. 7, 2013, the contents of which are hereby incorporated by reference in their entirety into the present disclosure.
Number | Date | Country | |
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61862995 | Aug 2013 | US |