CA2658297A1 - Photovoltaic cell - Google Patents

Abstract

A photovoltaic cell having a plurality of stages, each stage being adapted to preferentially absorb photons within a different spectral frequency range. In the preferred embodiment, within each stage are a plurality of sequences of layers, each with an altered composition to preferentially absorb photons of a different frequency.

Description

Attorney Docket: 2278-2/MBE
PHOTOVOLTAIC CELL

FIELD OF THE INVENTION
[00011 This invention relates to photovoltaic cells. In particular, this invention relates to a photovoltaic cell for use in generating electricity for consumption and/or storage.
BACKGROUND OF THE INVENTION
[00021 Photovoltaic cells are used to generate electrical energy from sunlight. A
photoelectric generator typically comprises a plurality of photovoltaic panels, each comprising a plurality of photovoltaic cells connected in parallel. The photovoltaic panels are arranged in a desired array, oriented to maximize the incidence of sunlight striking the panels, and connected in parallel to feed an output to a load or electrical storage device. The photoelectric generator so configured is capable of generating a flow of electrons which can be used to power a load, or stored as potential electrical energy in a battery or other storage device.

[00031 A conventional photovoltaic cell or "wafer" can generate a limited amount of power. For example, a standard one square meter photovoltaic panel can generate 150 to 200 millivolts (75 to 100 millivolts per cycle). As such, because of the broad surface area required to generate a significant voltage, photovoltaic cells are considered to be impractical for many applications.

[00041 Additionally, because a mechanism for moving such a large array of panels would be cumbersome and would itself consume considerable energy, typically a photovoltaic array is constrained to a fixed orientation, which results in the reduction of photon density on the surfaces of the photovoltaic panels as the sun moves away from a position normal to the photon-absorbing surfaces of the photovoltaic cells. In such an arrangement, maximum efficiency is only achieved during a portion of the day, because for much of the day the sun occupies positions in the sky which do not allow it to cast sufficient light on the stationary photovoltaic panels to saturate the photovoltaic cells.

[00051 It would accordingly be advantageous to provide a photovoltaic cell capable of converting a greater proportion of photonic energy into electricity, to thus maximize the energy available from a photovoltaic array.

[00061 It would further be advantageous to provide a system for increasing photonic saturation of the photovoltaic cells when the sun occupies positions in the sky which are displaced from the optimal position normal to the photon-absorbing surfaces of the photovoltaic cells, to maximize the output of the photovoltaic array.

BRIEF DESCRIPTION OF THE DRAWINGS
100071 In drawings which illustrate by way of example only a preferred embodiment of the invention, 100081 Figure 1 is a schematic perspective view of a photovoltaic cell according to the invention.

[00091 Figure 2 is a schematic end elevation of the photovoltaic cell of Figure 1.
100101 Figure 3 is a schematic perspective view showing the layers in a stage in the photovoltaic cell of Figure 1.

100111 Figure 4 is a schematic diagram showing the relative magnitude and direction of the changing electron/electromagnetic field during electron excitation occurring from the passing of photonic energy.

[0012] Figure 5 is a schematic diagram showing the translational velocity developed in Figure 4 as tangential velocity.

[00131 Figure 6 is an enlarged schematic elevation of the top cathode layer showing the sub-layers of concave receptors.

DETAILED DESCRIPTION OF THE INVENTION
100141 Figures I to 3 illustrate an embodiment of the photovoltaic cell 10 of the invention. The photovoltaic cell 10 comprises a plurality of stages encased within an encapsulate 16, each stage being adapted to absorb photons within a different frequency range. The photovoltaic cell 10 is mounted on a conductive substrate 12, for example a conventional 1x3 (1/2 inch) flat block formed from a plurality of 20nm wafers composed of a conventional carbon-based (carbide or carboloid) semiconductor material, which also has the advantage of providing the required density and reflectivity for the mirror array (described below).

100151 In the preferred embodiment the stages are formed from a plurality of carbide-based layers containing a high proportion of a C-60 carboloid for efficient electron absorption. The base elements in each layer include carbon (primarily in the C-form) and trace amounts of nickel, zinc and copper. In addition, each layer is provided with a substantial amount of a photon-absorbing element chosen to most effectively absorb photons within a selected range of the sunlight spectrum, as described in detail below.

[00161 As shown in Figure 3 the stages are arranged in stacked relation, and in the preferred embodiment each stage comprises a plurality of layers including the following layers which repeat, in sequence, throughout the stage:

1. A cathode layer formed from an n-type material providing predominantly electrons (negative charges) comprising a conductive grid embedded within each cathode layer, having a terminal external to the photovoltaic cell, to carry electrons from the conductive substrate 12 into the cathode layer to replace the depletion of electrons from the cathode layer across the p-n junction into the p-type layer.

2. An anode layer formed from a p-type material, providing predominantly holes (positive charges), forming a forward-biased p-n junction between the anode layer and the cathode layer which allows electrons to pass only from the cathode layer to the anode layer, as is conventional.

3. A back contact or "energy sink" for the anode layer, which draws electrons from the anode layer to the semi-conductive substrate 12, as described in greater detail below. The back contact 30 provides a terminal 32 projecting through the encapsulate 16 and thus exposed from the photovoltaic cell for connection to a negative bus leading to the photovoltaic panel output (not shown). In the embodiment shown the back contact 30 also serves as a heat sink, dissipating heat in order to keep the layers in the photovoltaic cell cool and improve efficiency, and optionally allowing heat energy to be drawn off to be used elsewhere. The back contact 30 is thus preferably formed from a thermally and electrically conductive plate. For example, the back contact 30 may be composed of about 60% C-60 carboloid, which has a high heat capacity, plus a combination of the semi-conductive elements used in the various stages (as described below) and preferably including a slight amount of cadmium. The back contact 30 may be pressed transversely into a sinusoidal wave pattern as shown with a wavelength which may be determined by the wavelength of the light spectrum targeted for the particular layer (within the long-wave IR
through UVB portion of the sunlight spectrum), and particularly the differential between the third or eleventh harmonic and the fundamental frequency of the wavelength of the light, in order to reduce impedance, and the increased surface area dissipates heat more effectively. The eleventh harmonic determines the particular portion of the spectrum targeted by a particular layer, and the third harmonic of the average wavelength within that portion of the spectrum is the wavicle harmonic used as the waveguide shape for the back contact 30.

100171 An anti-reflective coating (not shown) may optionally be disposed over the exposed surface of the cathode layer, as is conventional.

100181 The semiconductor layer in each stage is preferably less than 100 nm down to 30nm thick. The layers may be formed using a gas condensation process, also known as a gasification/condensation process in which magnetic flux is used to form the nano-structures making up the layers, in which the base carbon and elements are heated and then cooled to condense in the desired substrate format, at which time the base mole layer is pressed to a high density solid design. It is believed that optimum efficiency is achieved by layers ranging from 30-50 nm; however layers up to and even greater than 100 nm will still be effective.

Different conventional processes of gas condensation can be used. In general, the sinusoidal back contact 30 is formed in a housing between the anode and cathode layers, and then the layers are pressed by a magnetic field resonating at the desired frequency to create the sinusoidal pattern, which may for example be determined as follows: When electron excitation occurs from the passing of the photonic energy, the electron develops translational velocity. Figure 4 shows the relative magnitude and direction of the changing electron/electromagnetic field due to the translational velocity. As the translational velocity occurs, this is defined as tangential velocity VR, with R0, being the center of the electron (at rest) and V being the translational motion (direction) of the electron. The natural sinusoidal wave pattern becomes helical in nature as velocity increases in direction of the positive polymer energized by the passing photon.Therefore:

2rR 2rgR
C _f,,)r And therefore:

Tõ

100191 Thus, it can be seen that the wavicle energy potential will increase with the increase in tangential velocity toward the positively charged polymer in direct relation to the energy absorbed from the passing photon. This is the expression of the time dilation that occurs as the electron excitation occurs from the passing photons and absorption of the photonic energy. These equations can be used to determine the optimum sinusoidal shape of the conductive grid 26 and back contact layer 30 in order to provide the lowest impedance to the path of the electrons through these structures.

[00201 The wave front develops a helical sinusoidal wave pattern about the electron as in develops tangential velocity. This will of course vary dependent upon the energy of the photon, and this is dependent upon the spectrum being attracted, causing electron excitation. The true representation of the sinusoidal wave pattern is not linear (2 dimensional) in nature, but rather a sinusoidal pattern in a rotating spin to match the photonic spectral frequency and the resulting expression of energy (XE) of the wavicle as tangential velocity occurs. This resulting pulse or spike is actually a minor distortion of the helical spin of the electron as it transits to the positive polymer.
100211 With the exception of the bottom stage, which can be opaque, each layers is effectively transparent to all electromagnetic radiation (emr) wavelengths except for those within its particular photon absorption range. In the preferred embodiment of the invention, the stages are arranged so that the highest frequency (most energetic) photons are absorbed near the exposed (light-capturing) surface 14 of the photovoltaic cell and lower frequency (less energetic) photons penetrate into photovoltaic cell to be absorbed by the lower layers. This configuration provides advantages that will be described below.

[00221 Additional residue can be absorbed by other elements from the other layers mixed into the layer in smaller amounts, which gives some absorption outside the preferred frequency for that layer. In the preferred embodiment, the semiconductor layers in each stage are composed predominantly of an element that absorbs primarily photons within the particular absorption range of that particular stage, but preferably mixed with smaller proportions of at least some of the elements used in the other stages so that "spillover" from photons which are within the absorption range of a stage but are not absorbed (due for example to saturation of the semi-conductive layers in that stage) may be absorbed by one of the successive stages.

[00231 Photon absorption is preferably staged so that the most energetic photons are absorbed preferentially by the upper layers 20, 40 of the cell, while the least energetic photons are absorbed preferentially by the lower layers 60, 80 of the cell.
Each layer 20, 40, 60, 80 may composed of a mixture of all base elements, but with proportionally greater amounts of the particular element absorbs the spectral component desired for that layer. The base elements and spectral absorption characteristics are as follows:

Iridium: Short wavelength UVA, UVB
Iridium/Titanium: Higher excitation UVA, UVB
Gallium: Visible Blue and Long Wavelength UV cascade Cadmium: Visible Yellow/Green Beryllium: Short Wavelength IR
Beryllium/Indium: Long Wavelength and Higher excitation IR

[00241 Accordingly, in the illustrated embodiment, the first stage 20 comprises a series of layer sequences 21, each layer sequence 21 comprising a cathode layer 22 and an anode layer 24 each composed primarily of iridium or an iridium/titanium mixture, which absorbs high energy photons within the short wavelength ultraviolet (UV) portion of the emr spectrum. The conductive contact grid 26, which is preferably formed from a combination of the semiconductor elements used in other stages (to avoid interference in absorption frequency), or alternatively may be an alloy, is embedded within the iridium cathode layer 22. The contact grid 26 is preferably provided with at least some cadmium and is slightly magnetically charged, to concentrate the p and n charges and serve as a waveguide for the p and n charges.
The contact grid 26 provides a terminal 28 projecting through the encapsulate 16 and thus exposed from the photovoltaic cell for connection to a positive bus leading to the photovoltaic panel output (not shown).

[0025] The contact grid 26, rather than being linear, may be configured with its longitudinal and transverse conductors formed in a sinusoidal wave shape, with a wavelength based on the midpoint of the spectral range for that particular stage, for example as determined by the equations set out above. This configuration reduces impedance in the contact grid 26 and thus increases efficiency by maximizing electron flow and reducing heat losses.

[00261 Forming the contact grid 26 in a `tree' configuration as shown can be advantageous, because the junction of the contact grid 26 with the cathode semiconductor material 22 (in the case of the top stage primarily iridium) provides a high absorption point for free electrons. The `tree' configuration shown in Figure 1, being entirely embedded within the semiconductor layer 22, thus allows the contact grid 26 to absorb electrons more readily than a conventional contact grid pattern comprising an array of conductor cells.

[0027] In the embodiment shown the anode layer 24 is provided with a back contact 30, which can also serve as an energy sink (including a heat sink). The back contact 30 provides a terminal 32 projecting through the encapsulate 16 and thus exposed from the photovoltaic cell for connection to a negative bus leading to the photovoltaic panel output (not shown).

[00281 The second stage 40 comprises a series of layer sequences, each layer sequence comprising a cathode layer 42 and an anode layer 44 each composed primarily of gallium, which absorbs photons within the visible blue and long wavelength UV portions of the spectrum. A conductive contact grid 46, preferably formed from a combination of the semiconductor elements used in other stages, or alternatively an alloy, is embedded in the cathode layer 42 and provides a terminal 48 projecting through the encapsulate 16 and thus exposed from the photovoltaic cell for connection to the positive bus leading to the photovoltaic panel output (not shown). In the embodiment shown the anode layer 44 is provided with a conductive back contact 50, which may also serve as a heat sink as described above. The back contact provides a terminal 52 projecting through the encapsulate 16 and thus exposed from the photovoltaic cell for connection to the negative bus leading to the photovoltaic panel output (not shown).

100291 The third stage 60 comprises a series of layer sequences, each layer sequence comprising a cathode layer 62 and an anode layer 64 each composed primarily of cadmium-beryllium, which absorbs photons within the visible yellow/green through short wavelength infrared portions of the spectrum. A conductive contact grid 66, preferably formed from a combination of the semiconductor elements used in other stages, or alternatively an alloy, is embedded in the cathode layer 62 and provides a terminal 68 projecting through the encapsulate 16 and thus exposed from the photovoltaic cell for connection to the positive bus leading to the photovoltaic panel output (not shown). In the embodiment shown the anode layer 64 is provided with a conductive back contact 70, which may also serve as a heat sink as described above.
The back contact 70 provides a terminal 72 projecting through the encapsulate 16 and thus exposed from the photovoltaic cell for connection to the negative bus leading to the photovoltaic panel output (not shown).

100301 The fourth stage 80 is preferably composed of a series of layer sequences, each layer sequence comprising a cathode layer 82 and an anode layer 84 each composed primarily of titanium, which absorbs photons within the long-wavelength and higher excitation infrared (IR) portions of the spectrum. A conductive contact grid 86, preferably formed from a combination of the semiconductor elements used in other stages, or alternatively an alloy, is embedded in the cathode layer 82 and provides a terminal 88 projecting through the encapsulate 16 and thus exposed from the photovoltaic cell for connection to the positive bus leading to the photovoltaic panel output (not shown).

100311 In the embodiment shown the anode layer 84 is provided with a back contact 90, which may also serve as a heat sink. The back contact 90 provides a terminal 92 projecting through the encapsulate 16 and thus exposed from the photovoltaic cell for connection to the negative bus leading to the photovoltaic panel output (not shown).
The back contact 90 is preferably composed of a thermally and electrically conductive plate, for example composed of C-60 carboloid with trace copper.

[00321 Each photovoltaic cell is mounted on substrate portions 12a and 12b which are electrically insulated from one another by an insulative jacket 12c, as shown in Figure 1, such that the portions 12a and 12b form the tenninals of the photovoltaic cell 10.
An array of photovoltaic cells 10 may be created by mounting the cells on a B-type circuit board. A plurality of photovoltaic panels thus created may be arranged in any desired configuration, typically in planar alignment, and in a fixed orientation selected to optimize the capture of sunlight over the course of a day.

100331 To compensate for times during the day when the sun occupies positions in the sky which are displaced from the optimal position normal to the exposed surfaces 12 of the photovoltaic cells, and thus maximize the incidence of photons striking the stages of the cell and optimize collection of photonic energy, each cathode layer 22, 42, 62, 82 may optionally be formed from a plurality of nano-scale concave receptors.
The curved structure of the receptors provides a plurality of surfaces normal to the direction of sunlight no matter where the sun is in the sky. As shown schematically in Figure 6, the closer the angle of incidence of the sunlight is to the normal, the greater is the photon density striking the surface and thus the greater is the photon absorption and number of electrons released. The receptors have the composition of the cathode layer in which they reside, with proportionally greater amounts of the particular element or elements that absorb the spectral component desired for that layer, and are thus substantially opaque to light having a wavelength within the desired spectral component range and substantially transparent to other wavelengths of light.

[0034] For example, Figure 6 illustrates a plurality of concave receptors 23 forming the cathode layer 22 in the top layer 20 of the cell. The receptors 23 capture photons within the photon-absorbing matrix (primarily about the surfaces of the contact grid, not shown in Figure 6).

100351 The receptors 23 may be formed by the same gasification/condensation techniques described above, using a magnetic flux press to form the concave configurations. The receptor surfaces are batch-formed at the desired angles by embedding absorptive surfaces, for example in the cathode layer 20 composed of iridium embedded in a transparent substrate, for example a C-60 carboloid aligned for transparency. In this fashion multiple layers of receptors oriented to different angles can be overlaid in a multi-layer receptor array if desired, to direct incident light over as much of the surface of the photon-absorbing cathode layer 20 as possible.

[00361 In this embodiment the cathode layer 20 is preferably composed of a plurality of sub-layers (three in the embodiment shown) disposed in a staggered fashion, such that the receptors 23 are misaligned vertically within the cathode layer 22, increasing the probability that as photons progress through the matrix of sub-layers they are eventually captured by one of the layers of receptors 23 and directed toward the photon-absorbing matrix in that sequence of layers.

[00371 Thus, light rays having a wavelength within the desired spectral component range (shown as arrows with solid lines in Figure 6), in this case composed of iridium or an iridium/titanium mixture which absorbs high energy photons within the short wavelength ultraviolet (UV) portion of the emr spectrum, will strike the receptors 23.
Most of these light rays will be absorbed by one of the three sub-layers of receptors 23, some after reflecting off of receptors 23 in the higher sub-layer(s) and being absorbed by receptors 23 in the lower sub-layer(s). Occasionally a light ray having a wavelength within the desired spectral component range will pass through all sub-layers (for example, the light ray on the far right in Figure 6), in which case the light ray may be absorbed by one of the lower layers or may be dissipated as heat.
Similarly, most of the light rays having a wavelength outside the desired spectral component range (shown as arrows with dotted lines in Figure 6) will pass through the three sub-layers of receptors 23 and be absorbed by one of the lower cathode layers 42, 62 or 82. Occasionally a light ray having a wavelength outside the desired spectral component range will be absorbed by the cathode layer 22 and give off electrons.

100381 In a further embodiment (not shown) the receptors 23 may alternatively be in groups oriented at varying angles such that, over the array of receptor sub-layers, groups of receptors 23 change angles in increments of 3 degrees in a plane through a 180 degree East-West horizon, in order to capture the solar energy in full concentration for as much of the day as possible as the sun moves across the sky. The receptors and/or sub-layers of receptors may also be differentially-spaced if desired.
100391 If desired the sub-layer(s) beneath the contact grid 26 (not shown in Figure 6) may be formed from parabolic reflective receptors 23. The focal point of each receptor may be directed toward the contact grid 26, guiding captured photons directly to the contact grid 26 (which may optionally be surrounded by a particularly high-absorption composition within the cathode layer 22).

[00401 In these embodiments the other cathode layers 42, 62, 82 may be similarly formed from sub-layers of concave receptors. If desired the 3 degree angular change of the concave receptors may be replicated through each of the cathode layers 22, 42, 62, 82 of the over the entire cell lattice, thus providing optimal absorption of photonic energy for each of the element wavelength levels.

[00411 In a still further embodiment (not shown), the orientations of the receptors in each group changes in 1 degree increments through a North-South lateral plane, through about 30 degrees (depending upon the latitude at which the photovoltaic panel is disposed). In non-equatorial positions his further optimizes the collection of photonic energy and optimizes absorption taking in to account seasonal north south positional changes of the sun. Again the changing direction of the receptors may be staggered as between sub-layers in any particular cathode layer, and/or from one sequence of layers to the next, to maximize the probability of capturing photons within the intended wavelength range for that stage.

[00421 Each receptor is generally concave, with a radius of up to 100 nm. From one cathode layer to the next the angle of the receptors and their radii can be varied as the depth within the cell increases, increasing the tendency of successively lower layers to capture lower frequency photons.

100431 The receptor matrices can follow the same density patterning as the stages.
The receptor layers in the top stage cathode layer 22 may be more densely populated with receptors 23, to capture and concentrate shorter wavelength photonic energies;
and the receptor layers in the lower stages can be less densely populated with receptors to capture and concentrate progressively longer wavelength photons.
In other words, the receptor density within a row, and the row density, is preferably greater near the top of the cell to more effectively capture high frequency (short wavelength) photons, and the receptor density within a row, and the row density, preferably decrease as the depth into the cell increases, more effectively capturing increasingly lower frequency (longer wavelength) photons. The angling of 3 degree increments through a 180 degree East-West horizon plane and 1 degree increments through a 30 degree North-South seasonal plane may be fully replicated for each stage, but within each stage the rows of receptors are preferably staggered to maximize photonic capture and concentration.

[00441 In operation, sunlight strikes the top layer sequence 21 of the upper stage 20 of the photovoltaic cell 10. The mirror matrix layer 23 having mirrors angled to preferentially capture the highest energy photons deflects and guides the captured photons to the cathode layer 22, preferably with a particular concentration over the surfaces forming a junction between the semiconductor layer 22 and the contact grid 26.

100451 Photons striking the exposed surfaces 14 of the photovoltaic cells 10 release electrons from the cathode layer 22 of the first (uppermost) stage 20. These electrons cross the p-n junction attempting to unite with holes in the anode layer.
Since the p-n junction is forward-biased and only allows the electrons to move from the cathode layer 22 to the anode layer 24, an electrical potential is created between the terminals 28 and 32 of the cathode layer 22 and anode layer 24, respectively, and thus through conductors 18a, 18b leading to the substrate portions 12a, 12b, respectively.
This potential will cause electrons to flow through an external conductive path created between the substrate portions 12a, 12b, as the electrons try to reunite with holes left in the electron-depleted cathode layer 22.

[00461 The first stage 20 is formed primarily from iridium and iridium/titanium, which preferentially absorbs photons within the UV portion of the spectrum. As the unabsorbed spectral components of the sunlight progress downwardly through the first stage 20, and the compositions of the cathode and anode semiconductor layers 22, 24 changes such that the proportions of primary elements used in the lower stages increase and the proportion of iridium decreases in each sequence of layers, greater photonic absorption of lower energy photons starts to occur. Most of the longer wavelength photons in the remaining portion of the sunlight spectrum penetrate through the first stage 20 and into the photovoltaic cell. Unabsorbed photons in the short wavelength UV portion of the spectrum may continue deeper into the cell 10.
100471 The majority of photons in the short wavelength UV portion of the spectrum having been filtered out by the first stage 20, the remaining spectral components of the sunlight penetrate through to the second stage 40. In the embodiment shown the second stage 40 is composed primarily of gallium, which preferentially absorbs photons within the visible blue and long wavelength UV portions of the spectrum.
Through the mechanism described above, the absorbed photons create an electrical potential between the terminals 48 and 52 of the cathode layer 42 and anode layer 44, respectively, and thus through conductors 18a, 18b leading to the substrate portions 12a, 12b, respectively. Some of the longer wavelength photons in the remaining portion of the sunlight spectrum are absorbed by the second stage 40, particularly as the depth increases and the proportion of gallium in relation to the primary semiconductor components used in the other stages decreases, but most of the longer wavelength photons in the remaining portion of the sunlight spectrum penetrate through the second stage 40.

100481 The remaining spectral components of the sunlight penetrate through to the next lower stage 60. In the embodiment shown the third stage 60 is composed primarily of cadmium-beryllium which preferentially absorbs photons within the green through short wavelength infrared portions of the spectrum. Through the mechanism described above, the absorbed photons create an electrical potential between the terminals 68 and 72 of the cathode layer 62 and anode layer 64, respectively, and thus through conductors 18a, 18b leading to the substrate portions 12a, 12b, respectively. As in the second stage 40, some of the longer wavelength photons in the remaining portion of the sunlight spectrum are absorbed by the third stage 60, particularly as the depth increases and the proportion of cadmium-beryllium in relation to the primary semiconductor components used in the other stages decreases, but most of the longer wavelength photons in the remaining portion of the sunlight spectrum penetrate through the third stage 60 to the fourth stage 80.

[00491 The majority of photons having wavelengths shorter than the short wavelength IR portion of the spectrum having been filtered out by the first three stages 20, 40 and 60, the residual spectral components of the sunlight penetrate through to the fourth stage 80. In the embodiment shown the fourth stage 80 is composed of beryllium or beryllium/indium, which preferentially absorbs photons within the long wavelength UV portion of the spectrum. Through the mechanism described above, the absorbed photons create an electrical potential between the terminals 88 and 92 of the cathode layer 82 and anode layer 84, respectively, and thus through conductors 18a, 18b leading to the substrate portions 12a, 12b, respectively.

[00501 Staging the absorption of photons using different types of semiconductors in this fashion increases photon absorption, by effectively reducing the likelihood of photonic saturation at each stage (and at each sequence of layers within each stage).
Whereas a conventional silicon-based photovoltaic cell which absorbs the full spectrum of sunlight in a single stage can quickly become saturated in bright sunlight, resulting in a high proportion of unabsorbed photons, the present invention effectively frequency-divides the sunlight spectrum into components, which reduces the likelihood of reaching the saturation point in any particular stage and results in a significantly lower proportion of unabsorbed photons.

100511 Further, this arrangement extends the life of the photovoltaic cell because the most energetic photons, in the short wavelength UV portion of the spectrum, tend to degrade conventional silicon-based photovoltaic cells relatively quickly. In the preferred embodiment of the invention, the most energetic photons are largely filtered from the sunlight spectrum before the light penetrates into the photovoltaic cell.

[00521 The substrate portions 12a, 12b, respectively feeding a current to the conductor 18a and drawing a current from the conductor 18b, are respectively coupled to the positive and negative electrical buses leading to the output of the photovoltaic panel (not shown). The current from the plurality of photovoltaic cells in the photovoltaic panel is cumulative, and thus the power output of the photovoltaic panel comprises is determined by the number of photovoltaic cells 10 in the panel.

100531 Heat may be removed from the cell 10 via the heat sink function of the back contacts 30, 50, 70 and 90, increasing the efficiency of the cell 10.

[00541 Various embodiments of the present invention having been thus described in detail by way of example, it will be apparent to those skilled in the art that variations and modifications may be made without departing from the invention.

Claims (20)

1. A photovoltaic cell, comprising:
a non-conductive jacket; and at least two photon-absorbing stages arranged in a stacked relation within the jacket, each stage comprising:

a cathode layer formed from an n-type photon-absorbing material and comprising a cathode conductor in electrical contact with a terminal external to the cell;

an anode layer formed from a p-type material, in contact with the cathode layer to form a forward-biased p-n junction between the anode layer and the cathode layer; and a back contact comprising a conductor in contact with the anode layer and in electrical contact with a terminal external to the cell;

the cathode layer in each stage being composed of a material that preferentially absorbs photons having a wavelength within a selected spectral range, the spectral range being different as between at least two of the at least two stages.

2. The photovoltaic cell of claim 1 comprising four stages.

3. The photovoltaic cell of claim 2 wherein the stages are arranged so that the upper stages absorb the most energetic photons

4. The photovoltaic cell of claim 3 wherein the stages comprise an upper stage having a cathode composed primarily of iridium or an iridium/titanium mixture;
a next lower stage having a cathode composed primarily of gallium; a next lower stage having a cathode composed primarily of cadmium-beryllium; and a lowest stage having a cathode composed primarily of titanium.

5. The photovoltaic cell of claim 4 wherein the back contact also functions as an energy sink.

6. The photovoltaic cell of claim I wherein the cathode conductor comprises a conductive contact grid embedded in the photon-absorbing material.

7. The photovoltaic cell of claim 6 wherein the contact grid is formed in a substantially sinusoidal configuration.

8. The photovoltaic cell of claim 6 wherein the contact grid comprises at least one spine extending substantially across the cathode layer in one direction and a plurality of branches extending substantially across the cathode layer in a different direction.

9. The photovoltaic cell of claim 1 wherein the back contact extends over substantially the entire anode layer of the stage and conducts heat from the stage out of the cell.

10. The photovoltaic cell of claim 9 wherein the back contact is formed in a substantially sinusoidal configuration.

11. The photovoltaic cell of claim 1 wherein at least one of the stages is coated with an anti-reflective coating.

12. The photovoltaic cell of claim 8 wherein the cathode layers are formed from a plurality of concave receptors oriented to provide surfaces normal to the direction of incident sunlight when the sun is in different positions in the sky.

13. The photovoltaic cell of claim 12 wherein in at least one of the stages the cathode layer is composed of a plurality of sub-layers of concave receptors disposed in a staggered fashion such that the receptors are misaligned vertically within the cathode layer.

14. The photovoltaic cell of claim 13 wherein the concave receptors are arranged in groups oriented in different directions.

15. The photovoltaic cell of claim 12 wherein focal points of the receptors are directed toward the contact grid on the cathode layer of at least one stage.

16. The photovoltaic cell of claim 1 wherein the cell is mounted on a substrate.

17. A photovoltaic panel comprising a plurality of photovoltaic cells as defined in claim 1, electrically connected in parallel.

18. A method of converting sunlight to electricity in a photovoltaic cell having a plurality of stages arranged in stacked relation and comprising a cathode layer abutting an anode layer to form a p-n junction therebetween, comprising the steps of:
a. absorbing photons having a wavelength within a first selected spectral range in an upper stage, and b. absorbing photons having a wavelength within a second selected spectral range different from the first selected spectral range in at least one lower stage.

19. The method of claim 18 wherein the most energetic photons are absorbed primarily by the upper stage.

20. The method of claim 18 comprising, during steps a. and b., the step of removing heat from the cell as electrons are generated by the cell.