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Conventional photovoltaic cells made from a single absorbing semiconductor such as silicon (Si) are limited in efficiency to less than 30%. The fundamental energy loss mechanisms in single-absorber cells that are used in both concentrator and flat plate modules arise from the mismatch between the solar spectrum and the absorption spectrum of the semiconductor, largely determined by the optical band gap, EG, of the semiconductor. Photons with energy greater than EG can be absorbed, and a properly designed solar cell can extract some of the energy delivered by absorbed photons as electricity. The photon wavelength corresponding to EG is termed the absorption edge. Photons with wavelength shorter than the absorption edge can be absorbed by a solar cell to generate electricity. Photons with wavelength longer than the absorption edge cannot be absorbed by a conventional solar cell made from a single absorbing semiconductor. Solar cells made from absorbing materials of lower EG produce more photo-generated current than materials of higher EG, because more of the solar spectrum can be absorbed when the absorption edge occurs at a comparatively long wavelength. Generation of a high solar cell current requires a low value of EG. For example, current generated by a Si solar cell (EG=1.115 eV) is generally higher than current generated by a gallium arsenide (GaAs) solar cell (EG=1.43 eV).
The solar cell operating voltage is generally higher in cells made from materials with higher EG. For example, a GaAs solar cell yields a higher operating voltage than a Si solar cell. The attainment of a high solar cell voltage requires a large value of EG. The power delivered to an external circuit by the solar cell is the product of (i) the voltage at the terminals of the cell, and (ii) the photo-generated current supplied by the cell at the terminals. Thus there is an optimum range of values of EG that delivers the most power. Absorbing materials with EG ranging between 1.0 and 1.5 eV appear to be best suited for single absorber solar cells. In any single absorber solar cell with EG in this optimal range, some incident solar energy will be lost owing to photons with energy less than EG.
Tandem solar cells known in the prior art are made from two or more absorbing semiconductor solar cells and address energy loss by stacking solar cells with different EG in series optically, so that photons not absorbed in the first solar cell can be transmitted to the second cell in the optical series stack. By using two or more different absorbing semiconductors, tandem solar cells can absorb the high energy part of the solar spectrum in a cell operating at a higher voltage. The light that is not absorbed by the first solar cell is passed to the second where it can be absorbed by a material operating at a lower voltage. In this way a tandem cell attains higher peak efficiency than a single junction cell, although it is more complex and expensive to manufacture.
The present invention relates to an energy conversion device that can be used, for example, in a photovoltaic solar cell. The device minimizes losses due to non-absorption and thermalization in solar cells by up converting the energies of incident photons prior to absorption by the semiconductor and improves the optical coupling between the semiconductor and an up conversion material.
In one embodiment, the device includes a layer of a photon-absorbing semiconductor material having a front photon-receiving surface and a back surface. The semiconductor material has an absorption edge, such that photons having wavelengths at or shorter than the absorption edge are absorbed in the semiconductor material to generate electron-hole pairs. An up conversion composite material is disposed within cavities formed in the photon-absorbing semiconductor material extending inwardly from the back surface. In another embodiment, the up conversion composite material is disposed within cavities formed in a heat spreader bonded to the solar cell.
In one embodiment, the up conversion composite material comprises a mixture of at least two different up conversion materials formed as crystal grains dispersed within a dispersion medium comprising an optically transmitting material. Each up conversion material includes a crystal material doped with dopant atoms capable of absorbing photons having wavelengths longer than the absorption edge of the semiconductor material and emitting photons having wavelengths shorter than the absorption edge. The up conversion composite material may include at least two different crystal materials doped with dopant atoms, the dopant atoms of each crystal material being different to absorb photons having different wavelengths.
The device provides better optical coupling between the solar cell and the up converter material than in prior art devices that incorporate an up conversion process. The use of multiple up conversion materials overcomes the use of a single up-converter material that only absorbs over a few narrow bands of the spectrum. In a concentrator solar cell, the device improves electrical and thermal coupling between the solar cell and the heat sink, in contrast to prior art devices in which a single up conversion material applied to the back of a concentrator solar cell interferes with the electrical and thermal contact to the concentrator heat sink.
The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which:
The disclosure of U.S. Provisional Patent Application No. 61/374,050, filed on Aug. 16, 2010, is incorporated by reference herein.
An energy conversion device according to the present invention, for example, a photovoltaic solar cell, can be understood by reference to
The three solar light rays 6, 7, 8 represent all of the solar photons incident on the solar cell. Ray 6 comprises photons each with energy greater than EG. Ray 7 comprises photons with energy approximately equal to EG. Ray 8 comprises photons with energy less than EG. It should be understood that solar photons generally are incident uniformly on the surface of the solar cell independent of energy, and this representation of individual rays is for illustrative purposes.
The absorption of photons in semiconductor 1 depends highly on the energy; thus, it is appropriate to consider photons in three groups as represented by rays 6, 7, and 8. The spectral bands associated with rays 6, 7, and 8 depend on the value of EG of the absorbing semiconductor. For example, for silicon at room temperature, ray 6 corresponds to photons with wavelength shorter than 1110 nm; ray 7 corresponds to photons with wavelength approximately equal to 1110 nm, and ray 8 corresponds to photons with wavelength greater than 1110 nm.
The fundamental photon absorption process in a semiconductor comprises the excitation by a photon of an electron from a state in the valence band to a state in the conduction band. The smallest photon energy for which this process can occur corresponds to an event that raises the energy of an electron at the valence band state of maximum energy 20 to the conduction band state of minimum energy 10, and energy less than this difference is insufficient for absorption. In other words, photons with energy less than EG cannot excite an electron from the valence band to the conduction band, and such photons are not usefully absorbed. In
With reference to
In a single-absorber solar cell, the amount of energy lost to (i) non-absorption and (ii) thermalization depends on the band gap EG of the semiconductor from which the solar cell is made, and the incident solar spectrum. Table 1 provides the result of a calculation of these losses for the case of a silicon solar cell, assuming an incident spectrum with a total incident power of 100 mW/cm2. Together these losses account for 51 mW/cm2 that is therefore unavailable for conversion to electricity by a silicon solar cell. A loss of a similar magnitude would result from the use of any single semiconductor in a solar cell having EG in the range of about 1 to 1.5 eV. (Beyond this range, the total energy loss would be greater.)
The present device reduces these losses by converting the wavelengths of the incident photons prior to absorption by the semiconductor.
As shown in
Materials suitable for wavelength conversion are known. For example, fluorinated crystals such as NaYF4, BaY2F8 and KY3F10 doped with rare earth elements such as Er are known to provide both up and down conversion. For instance, Er-doped NaYF4 is known to absorb infra-red photons and emit visible photons. The physics of up and down conversion involves cooperative processes comprising nonradiative energy exchange between electrons in rare earth elements that occupy crystal lattice sites in close proximity. Up conversion using Er doping converts two photons with wavelength in the range of 1500 to 1600 nm to one photon with wavelength of approximately 980 nm. Since photons in the range 1500 to 1600 nm are not absorbed by Si, by converting these photons to a single photon at 980 nm, the energy is converted to a form that can be absorbed by Si. In this way, up conversion can increase the photo-generated current of a Si solar cell.
The up conversion composite material 130 filling the cavities 125 is formed from grains of up-conversion crystals dispersed in a continuous medium formed from an optical material that permits transmission of photons, at least in the spectrum of interest for operation of the solar cell and the up conversion materials. The grains may be in the form of a single powder or mixture of different powders. The grain sized should be smaller than the width and depth of the cavity; for example, a grain size less than 100 microns would be suitable in a cavity having a width of 200 microns.
The optical material 195 may be an optical adhesive, such as a thermally cured epoxy. The optical dispersion medium has an index of refraction that is different than the index of refraction of the up conversion powders. Preferably the index of refraction of the dispersion medium is intermediate between the index of refraction of semiconductor material 102 and the index of refraction of the powder grains. In one embodiment, the up converter grains 191 are NaYF4 doped with Er, the up converter grains 192 are NaYF4 doped with Ho, and the optical medium 195 is the optical adhesive commercially available as Norland 83H. The crystals have an index of refraction of approximately 1.47, the optical adhesive has an index of refraction of 1.56, and the mixture is embedded in cavities formed in Si which has an index of refraction of approximately 3.62.
While these embodiments have described two crystal powders mixed with optical epoxy to form the upconverter material, any number of different types of crystal powders may be mixed. Other up conversion materials can include, for example, quantum dots made of, for example, PbS, PbSe, CdS, CdSe, as well as various other semiconductors and carbon nanotubes, can provide for wavelength conversion. Any of these materials can be doped with rare earth elements or other elements. Also, some dopants benefit by incorporation of Yb as a second dopant in the same grain; thus, two dopants can be in one grain if one of the dopants is Yb. While we have described embodiments in which the absorbing material 102 is p-type Si, the absorber may also be n-type silicon, in which case the emitter region 103 is formed by p-type Si, and the back surface field region 190 is formed from highly doped n-type silicon. Alternatively, the solar cell absorber material 102 may be any semiconductor and is not limited to Si. The optical dispersion medium can be any material that holds the crystal grains in dispersion and transmits photons in the spectrum of interest, such as the visible and infrared portion. The wider the transmission band of the dispersion medium, the more general the applicability of the device.
The cavities can be filled in any suitable manner. For example, the cavities can be filled with the powders, then back filled with an optical epoxy and cured. In another method, the powders and an optical epoxy can be mixed, then filled in to the cavities and cured. In any method, air entrapment should be avoided. Air can be removed by, for example, the use of a vacuum. Techniques used for filling small cavities in the manufacturing of LCDs can be adapted.
Operation of the energy conversion device is described more fully in conjunction with
In this device, light may be converted to electricity in a number of ways.
Rays 220 and 221 have wavelengths longer than the absorption edge of material 102 and therefore cannot be directly absorbed by material 102. Ray 220 reflects diffusely from the back metallization 140 and enters an upconverter material 130 where it is absorbed by a dopant atom in a crystal grain (for example 191 or 192 in
A similar process is illustrated by photons 225 and 226 in
In this manner, optical coupling between the semiconductor material and the up conversion material is increased over prior art devices. The use of multiple up conversion materials minimizes non-absorption and thermalization losses by increasing the size of the spectral bands than can be absorbed by the solar cell. This is an improvement over the use of a single up conversion material that only absorbs over a few narrow bands of the spectrum. The energy conversion device can be used in a single junction solar cell, providing higher efficiency with more economical manufacturing than a tandem solar cell structure.
Turning now to a further embodiment illustrated in
If the solar cell 330 is GaAs or another very thin solar epitaxial cell material, the substrate must be removed because the minority carrier diffusion length is too short for electron hole pairs, generated at the back of the substrate by absorption of an up conversion photon, to diffuse to the front where they would be collected by the p/n junction. Accordingly, the substrate must be removed so that electron hole generation occurs in an active layer of the solar cell near the p/n junction. Techniques known in the art as epitaxial liftoff can be used to remove active epitaxial layers from the substrate and to apply them to the up converter heat spreader. In one embodiment, epitaxial GaAs is grown on a substrate prepared with an etch stop or release layer (such as AlAs), followed by formation of front side metallization and deposition of the antireflection coating (similar to the front side processing of Si). After front side processing and application of a solar cell cover glass, the substrate is removed. The back side can be metallized using patterning methods known in the art. The spacing of the metallization pattern 335 (
The energy conversion device described herein can be used in combination with other devices. For example, a down conversion device can be applied to the front side of the solar cell described herein. The formation of down converters on the front of solar cells are described in copending U.S. patent application Ser. No. 12/778,365, filed on May 12, 2010, incorporated by reference herein.
Many single semiconductor and multiple semiconductor combinations have been used to create solar cells. While exemplary embodiments of the invention are primarily shown and described as Si solar cells, it is understood that embodiments of the invention are applicable to a wide variety of solar cells, as well as energy conversion devices that are not photovoltaic solar cells. Additionally, it will be appreciated that various features explicitly described in conjunction with a particular embodiment can also be used with other embodiments even if not explicitly described in conjunction therewith.
Having described exemplary embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims.
This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/374,050, filed on Aug. 16, 2010, the disclosure of which is incorporated by reference herein.
Number | Date | Country | |
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61374050 | Aug 2010 | US |