Thinned solar cell

Abstract

The present invention relates to a solar or photovoltaic cell wherein the thickness of the photovoltaic substrate is reduced such that the efficiency of the photovoltaic cell is about 100% greater than a typical photovoltaic cell. The solar cell can also include a cold plate located adjacent to the solar cell to remove heat from the solar cell.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

FIG. 1 is a graphical depiction of a normalized solar spectrum

FIG. 2 is a general diagram of the thinned solar cell capturing a larger portion of the solar spectrum.

DETAILED DESCRIPTION OF THE INVENTION

The “Thinned Solar Cell” invention described herein makes use of existing Si-based solar cells and increases the efficiency up to 90% or greater for peak absorption. The outer layer of silicon based solar cell can be reduced in thickness in order to bring the depletion region or p-n junction closer to the surface where desired photon absorption occurs. Photon absorption in this region converts incident photons into useable electrons in the form of electric current. The thickness of the outer layer of PV material can be reduced to within the diffusion depth of an incident photon. The thickness of the outer layer can be designed to maximize the spectral range of absorbed photons that contribute to useable electrical power.

FIG. 1 shows the relative amount of energy available from the sun over a broad spectral range from 200 nm to 1,100 nm. The majority of Ultra-Violet (UV) light below approximately 300 nm is absorbed by the earth's atmosphere. Near-Infrared (NIR) light beyond 1,100 nm is not absorbed by silicon based photo-voltaic (PV) material. FIG. 1 shows the measured solar radiation from the earth as well as the calculated (approximated) energy using Planks Blackbody equation, which states that the spectral radiance of an object can be calculated based on the temperature of the object. Therefore, it is possible to estimate the amount of solar energy available within a particular bandwidth or wavelength range by integrating the Plank equation using the desired lower and upper wavelength limits. It can be observed in FIG. 1 that the peak irradiance from the sun (˜6,000 Kelvin) is near the green wavelength.

A large portion of the available solar radiation is not utilized by existing PV solar cell technology. An embodiment of the thinned solar cell invention consists of a solar cell that is reduced in thickness such that a greater portion of the solar spectrum reaches the vicinity of the depletion region within a diffusion length and is not absorbed at an unusable location in the material. The thinned solar cell invention can integrate a temperature controlled cold plate to reduce the operating temperature of the solar cell substrate.

FIG. 2 shows that by reducing the thickness of the outer layer of the PV material, a larger percentage of photons are able to reach the vicinity of the depletion region established by the doped layers within the silicon and contribute to useable electricity. In FIG. 2, A represents an anode, B represents a P doped silicon layer, C represents a depletion region, D represents an N doped silicon layer, E represents a silicon substrate layer and F represents a cathode.

The thinned solar cell can then be coated with an anti-reflection coating to reduce the number of lost photons resulting from reflections at the surface of the material. These surface losses are due to the large Fresnel reflections at the boundary between air (index of refraction n=1.0) and silicon (index n=3.6). An ideal anti-reflection coating would have an index of refraction equal to the square root of the substrate index or n=1.9. An example of an anti-reflection coating applied to silicon can include hafnium dioxide, titanium dioxide or silicon nitride. Existing solar cells are often coated to reduce surface reflections. Here the anti-reflection coating is applied to a thinned solar cell.

Typically PV material used as a solar cell for power generation is used in forward bias configuration. The photo-generated current is linearly proportional to the number of incident photons over a large range. Noise electrons (or noise current) are also generated which do not contribute to useable photo-generated current, therefore limiting the output of a solar panel. Some of the noise current is generated when the material operates at elevated temperatures. The thinned solar cell includes a cooling layer that consists of a semi-conductor Thermal Electric (TE) cooling device of the Peltier type, liquid or any other cooling technique to minimize noise current such as thermally generated electrons typically associated with Johnson noise or resistive heating. The cooling layer or cooling jacket is bonded to (or part of) the electrical backplane of the photo-voltaic substrate. The cooling system will lower the operating temperature of the solar cell using a portion of the generated electrical output, therefore lowering the noise and ultimately increasing the efficiency of the solar cell.

The thickness of the substrate can be reduced using chemical, mechanical or any other means of etching (liquid or dry), grinding or polishing (e.g., chemical, mechanical, chemo-mechanical, ion bombardment). This will increase the efficiency of existing solar cells by 100% or greater.

The process of thinning doped silicon has been successfully applied to Charge Coupled Devices (CCD) imaging detector arrays where greater than 90% peak Quantum Efficiency (QE) has been achieved. The electrical biasing of the silicon material in a CCD is not equivalent to that of a solar cell which is typically “forward biased” for electrical power generation. A solar cell would not produce a low noise imaging sensor. Noise in a solar cell arises from electrons that are generated in the material but do not contribute to useable electricity. Schott and Johnson noise limit amount of photo-generated electrons that contributes to useable current. Therefore, thinning a solar cell may not have the gain or increased efficiency to that experienced in CCD imaging sensors. However by thinning existing solar cells it is possible in increase the percentage of incident photons that are absorbed at the desired depth in the doped silicon material therefore contributing to larger current densities and increasing the efficiency of existing solar cells at a low cost.

As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to be embraced by the appended claims.

Claims

1. A photovoltaic cell comprising a substrate, wherein the thickness of the outer layer of the substrate is reduced such that the efficiency of the photovoltaic cell is about 100% greater than that of a photovoltaic cell in which the outer layer of the photovoltaic cell is not reduced.

2. The photovoltaic cell of claim 1, wherein substrate comprises silicon and doped silicon layers.

3. The photovoltaic cell of claim 1, further comprising a temperature controlled cold plate adjacent to the photovoltaic cell to remove heat therefrom.

4. The photovoltaic cell of claim 3, wherein the cold plate comprises a semi-conductor Thermal Electric cooling device of the Peltier type.

5. The photovoltaic device of claim 1, further comprising an anti-reflective coating.

6. The photovoltaic device of claim 5, wherein the anti-reflective coating comprises hafnium dioxide, titanium dioxide, silicon dioxide, or mixtures thereof.

7. The photovoltaic device of claim 5, wherein the anti-reflective coating has a refractive index of about n=1.9.

8. The photovoltaic device of claim 1, wherein the thickness of the outer layer of photovoltaic substrate is reduced by grinding.

9. The photovoltaic device of claim 1, wherein the thickness of the outer layer is reduced by chemical etching.

10. The photovoltaic device of claim 1, wherein the thickness of the outer layer is reduced by ion bombardment.

11. A method of increasing the efficiency of a photovoltaic cell comprising a substrate by at least 100%, the method comprising reducing the thickness of the outer layer of the substrate.

12. The method of claim 11, wherein the thickness of the outer layer of the substrate is reduced by etching the substrate.

13. The method of claim 11, wherein the thickness of the outer layer of the substrate is reduced by grinding the substrate.

14. The method of claim 11, wherein the thickness of the outer layer of the substrate is reduced by ion bombardment of the substrate.

15. The method of claim 11, further comprising adding a cooling device adjacent to the photovoltaic cell to cool the cell during use.

16. The method of claim 15, wherein the cooling device is a Peltier cooler.

17. The method of claim 11, further comprising adding a reflective coating to the photovoltaic cell after the thickness of the outer layer of substrate is reduced.