Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/04—Prisms
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F19/00—Integrated devices, or assemblies of multiple devices, comprising at least one photovoltaic cell covered by group H10F10/00, e.g. photovoltaic modules
  • H10F19/40—Integrated devices, or assemblies of multiple devices, comprising at least one photovoltaic cell covered by group H10F10/00, e.g. photovoltaic modules comprising photovoltaic cells in a mechanically stacked configuration
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F77/00—Constructional details of devices covered by this subclass
  • H10F77/40—Optical elements or arrangements
  • H10F77/42—Optical elements or arrangements directly associated or integrated with photovoltaic cells, e.g. light-reflecting means or light-concentrating means
  • H10F77/484—Refractive light-concentrating means, e.g. lenses
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F77/00—Constructional details of devices covered by this subclass
  • H10F77/40—Optical elements or arrangements
  • H10F77/42—Optical elements or arrangements directly associated or integrated with photovoltaic cells, e.g. light-reflecting means or light-concentrating means
  • H10F77/488—Reflecting light-concentrating means, e.g. parabolic mirrors or concentrators using total internal reflection
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy
  • Y02E10/52—PV systems with concentrators

Definitions

• This invention relates to photovoltaic assemblies, and, more particularly, to solar cell arrays utilizing solar cells optimally sensitive to differing wavelengths of light.
• a typical semiconductor solar cell includes an interface between n-type and p-type transparent semiconductor materials. Light shining on the interface creates hole- electron pairs in addition to those otherwise present, and the minority charge carriers migrate across the interface in opposite directions. There is no compensating flow of majority carriers, so that a net flow of electrical charge results. A useful electrical current is then obtained in an external electrical circuit by forming ohmic con ⁇ tacts to the materials on either side of the interface.
• a photovoltaic solar cell is fabricated by depositing the appropriate semiconductor layers onto a substrate, and then adding additional components to complete the cell.
• a conventional P-on-N gallium arsenide solar cell is fabricated by epitaxially depositing a layer of n-type gallium arsenide onto a single crystal gallium arsenide substrate, and depositing a layer of p-type gallium arsenide over the layer of n-type gallium arsenide.
• the interface between the p-type gallium arsenide and the n-type gallium arsenide forms the basic solar cell active structure.
• External ohmic electrical contacts to the n-type and p-type layers are applied, and a voltage is measured across the contacts when light is directed against the interface.
• gallium aluminum arsenide may be deposited over the layer of p-type gallium arsenide to limit recombination of charge carriers.
• All known types of solar cells are cha ⁇ racterized by an electrical current output which is ; dependent upon the wavelength of the light that is incident upon the solar cell, as may be determined in a laboratory experiment wherein the light wavelength is slowly varied and the output current is measured.
• This effect is thought to arise because of the quantum : nature of the conversion process wherein light photons of a particular wavelength or energy promote electron, transitions in the semiconductor materials used in forming the solar cell. That is, the number of excess charge carriers is dependent upon the wavelength or energy of the light photons, and the current produced by the solar cell in turn depends upon the number of excess charge carriers.
• the gallium arsenide solar cell discussed above has a band gap energy of about 1.4 eV.
• the conversion of light to > electrical current is optimized when the wavelength of the incident light is about 0.6 to about 0.9 micrometers. Light of lesser or greater wavelengths may have a minor effect on the production of electrical current, but for the most part is either reflected or transformed into unuseable heat energy.
• the wave ⁇ length of the light is determined by a light source which ordinarily is not controllable.
• solar cell arrays are furnished incident sunlight, which is a white light having a broad range of constituent wavelengths from many parts of the visible and invisible spectrum. Accordingly, most of the light falling upon a solar cell is outside the range of optimum sensitivity and conversion efficiency of the solar cell, so that the conversion efficiencies of most solar cells with incident sunlight are relatively low, on the order of about 5% to about 15%.
• a gallium aluminum arsenide solar cell could be fabricated epitaxially over a gallium arsenide solar cell.
• the gallium aluminum arsenide solar cell exhibits a band gap of about 1.8 eV, so that shorter wavelengths of light would be optimally converted by the gallium aluminum arsenide solar cell (as compared with the wavelengths optimally converted by the gallium arsenide solar cell) .
• conversion efficiencies of about 20% may be attained. It may be envisioned that even further solar cells could be epitaxially deposited into a single inte- grated structure, to obtain even higher efficiencies.
• the present invention resides in a semi ⁇ conductor solar cell array which achieves enhanced
• the solar cell array allows the interconnection of the individual solar cells in a series fashion to • 20 achieve the desired electrical voltages, and the inter ⁇ connection of the series-connected cells in a parallel fashion for high output currents.
• the solar cell arrays of the invention may be made in a compact form which is difficult to target and damage by iaser beam
• a semi ⁇ conductor solar cell array comprises at least two semiconductor solar cells, each solar cell having a different incident light wavelength of optimal con ⁇ s' version of light energy to electricity; and means for directing an incident light beam toward the solar cells, so that each cell receives the component of the incident light beam having its respective light wavelength of optimal conversion.
• the 0 means for directing includes a light wavelength ana ⁇ lyzer for splitting the incident light beam into component rays of various wavelengths, and means for positioning each of the solar cells to receive the component ray containing its respective wavelengths of 5 optimal conversion.
• the means for directing includes means for orienting the solar cells so that the incident light beam falls upon the solar cells serially, and further so that successive pairs of cells are not parallel to each other. Any Q desired number of different types of solar cells may be used together in such arrays, but as a practical matter, about four types of solar cells having different band gaps are usually enough to obtain an acceptably high conversion efficiency.
• the individual 5 solar cells may be electrically joined in any desired series or parallel arrangement to achieve particular output voltages and electrical currents.
• one embodiment provides a means for analyzing an incident light beam into component 0 rays of various wavelengths, and at least two semiconductor solar cells, each cell having a different incident light wavelength of optimum conversion of light energy to electricity, with each cell being positioned to receive from the light analyzer the 35 component ray containing its respective wavelength of optimal conversion of light to electricity.
• the means for analyzing is conveniently a prism or diffraction grating which splits a beam of light into its component wavelengths to form a spectrum, and then the individual solar cells are positioned to receive the portion of the spectrum that is optimally converted by the 5 respective solar cell.
• At least two semi ⁇ conductor solar cells having different incident light wavelengths of optimal conversion efficiency are supported in a mounting structure in a serially non- ⁇ coplanar arrangement, and there is provided means for directing an incident light beam serially from one of the solar cells to the next.
• Each solar cell then extracts energy from the incident light beam at its optimal conversion efficiency, passing onto the next , solar cell the portion of the incident light beam that is not converted to electricity.
• the individual solar cells may not be positioned in a layered, parallel arrangement, since internal reflection rapidly increases .the temperature of the solar cells to a point 0 where their conversion efficiency is drastically reduced.
• Successive pairs of solar cells may therefore not be parallel to each other, although, for example, a first cell and a third cell could be parallel to each other in a manner such that the light beam cannot be 5 reflected directly between the two.
• the incident light beam may be directed serially from one solar cell to the next with mirrors, preferably in the form of silvered back surfaces of the solar cells themselves.
• a particularly desirable angular 0 orientation between the successive semiconductor solar cells is 45 *, inasmuch as geometrically regular mounting structures can be fabricated in such orientations.
• One such mounting structure has a hollow triangular elevational crosssection, with at least one 5- solar cell on each side thereof.
• Another such solar cell array has a hollow parallelogram elevational cross-section.
• AU types of single-junction semiconductor solar cells may be utilized in conjunction with the present invention.
• Some such solar cells such as silicon and gallium arsenide, are already well established and in commercial use.
• Other types of single junction solar cells are known, but development work remains before they are commercially practical.
• the present invention allows the use of the solar cells already developed without substantial modification, and without the development of multi-junction structures in fabrication techniques. As other single-junction solar cells become commercially practical, these may be utilized in conjunction with the present invention also. With the arrays of the present invention, substantially improved photovoltaic assembly and solar cell array operating characteristics can be achieved.
• the arrays of the present invention may be made in a compact form, and utilized within surrounding support structure, which improves the survival characteristics of the solar cell arrays in a normal space environment and when subjected to attack in a space environment.
• FIGURE 1 is an elevational view of a con ⁇ ventional single-junction gallium arsenide solar cell
• FIGURE 2 is an elevational view of a dual- 5 junction solar cell
• FIGURE 3 is an elevational view of a pair of single-junction solar cells arranged one above the other in a coplanar fashion;
• FIGURE 4 is an elevational view of a solar I0> cell array wherein the incident light is split into its component rays and the component rays are directed to a number of solar cells;
• FIGURE 5 is an end elevational view of a solar cell triangular mounting structure and the solar cells 15 mounted thereupon;
• FIGURE 6 is an end elevational view of a solar cell parallelogram mounting structure and the solar cells mounted thereupon;
• FIGURE 7 is a schematic sectional view of a 20 solar cell array of the present invention mounted in conjunction with a parabolic reflector and lens to form a photovoltaic power supply.
• FIGURE 1 illustrates a conventional single- junction solar cell, here depicted as a gallium arse ⁇ nide solar cell 10 for purposes of illustration.
• the solar cell 10 comprises a single crystal gallium arsenide substrate 12, upon which the active elements of the solar cell are fabricated.
• Epitaxially overlying the substrate 12 is a single crystal layer of n-type gallium arsenide 14.
• Epitaxially overlying the ; single crystal layer 14 of n-type gallium arsenide is a layer of p-type gallium arsenide 16.
• the layers 14 and 16 together comprise an active gallium arsenide solar cell 18, with the interface between the layers 14 and 16 being the single- junction solar cell 18.
• a glass 5 window 20 is typically attached over the solar cell 18 to protect it from radiation and to support the solar cell 18.
• Other semiconductor layers may optionally be added, such as a P+ type gallium aluminum arsenide layer epitaxially overlying the p-type gallium arsenide 0 layer 16, as for the purpose of inhibiting surface charge recombination.
• Such a solar cell 18 has a band gap of about 1.4 electron volts, and is most sensitive to incident light radiation of about 0.6 to about 0.9 micrometers. 5 That is, light having such wavelengths is converted to electricity by the single-junction solar cell 10 with optimal efficiency. Light having other wavelengths may be converted to electrical energy, but at significantly reduced efficiencies.
• the gallium arsenide O solar cell 10 has been described in detail for the purpose of illustration, other solar cells based on other technologies such as silicon or cadmium telluride are similarly limited to particular incident light wavelengths of optimal conversion efficiency, although 5 the optimal wavelength ranges for other solar cells will be different from that of a gallium arsenide solar cell because of the differing band gaps.
• a single crystal gallium arsenide substrate 24 is first prepared.
• the layers 26 and 28" together form a gallium arsenide single- junction solar, cell. 30.
• a glass window 38 is then attached over the dual- junction solar cell 22.
• the dual-junction solar cell 22 thus comprises two solar cells 30 and 36 arrayed one on top of the other in a series arrangement.
• the interface between the layers 26 and 28 creates a first voltage
• the interface between the layers 32 and 34 creates a second voltage
• the total voltage produced by the dual- junction solar cell 22 being the sum of the voltages of the individual solar cells 30 and 36.
• the current which flows through the dual- junction solar cell 22 is limited to the maximum current produced by either the gallium arsenide solar cell 30, or the gallium aluminum arsenide solar cell 36.
• the band gap of the gallium arsenide solar cell 30 is about 1.4 eV, so that, as previously indicated, this solar cell junction is optimally sensitive to light having wavelengths of about 0.6 to about 0.9 micrometers.
• the gallium aluminum arsenide solar cell 36 has a band gap of about 1.8 eV, and is therefore optimally sensitive to light having wavelengths of from about 0.3 to about 0.6 micrometers.
• the two solar cells 30 and 36 together are sensitive to light having wavelengths of from about 0.3 to about 0.9 micrometers.
• the maximum current produced by the dual-junction solar cell 22 is limited to the lesser of the current flows produced by the gallium arsenide solar cell 30 and the gallium aluminum arsenide solar cell 36.
• Performance limitations with the multi- junction solar cell 22 arise because the individual single junction solar cells 30 and 36 which comprise the multi- junction solar cell 22 are necessarily linked in a series fashion, and cannot be individually connected by external connections. The problem of the multi- junction solar cell
• FIGURE 3 illustrates a pair of singlejuncti n solar cells 40 and 42, each of the same general type as illustrated in FIGURE 1 but constructed of different materials of different optimal conversion efficiencies, stacked one above the other to form a stacked solar cell 44 having a gap 50 therebetween.
• the individual single- junction solar cells 40 and 42 may be furnished with external connections independent of each other, thereby avoiding the internal series linking of solar cells found in the dual- junction solar cell 22.
• a beam of light incident upon the stacked solar cell 44 is indicated by the numeral 46. As the beam of light 46 passes through the first solar cell 40, the first solar cell responds most efficiently to a particular wavelength of light.
• the remainder of the beam of light 46 passes from the first solar cell 40 and into the second solar cell 42. However, a portion of a transmitted beam 48 is reflected from the top surface of the second solar cell 42. A portion of the reflected beam travels back into the first solar cell 40, and a portion is reflected from the bottom surface of the first solar cell 40. Multiple reflections in the gap 50 can then occur, resulting in heating of the solar cells 40 and 42. Under particularly demanding operating conditions, the heat build-up in the solar cells 40 and 42 due to the beams reflected in the gap 50 may heat the solar cells 40 and 42 above their optimum operating temperatures, resulting in a loss of efficiency of the conversion of light to electrical current.
• FIGURE 4 illustrates a semiconductor solar cell array 52 having an analyzer, here illustrated as a prism 54, for splitting an incident beam of light 56 into a spectrum of rays 58 of varying wavelengths.
• Three solar cells 60, 62 and 64 are placed into the path of the rays 58 so as to intercept and receive light waves of different wavelengths.
• the solar cells 60, 62 and 64 are chosen and placed so as to be optimally sensitive to the component rays intercepted.
• the following table illustrates, by way of example, a number of candidate solar cells, their band gaps, the corresponding light wavelengths, and the expected approximate range of optimal sensi- tivities and efficiency in converting light energy to electrical energy: TABLE I
• the semiconductor solar cell 60 might be a long wavelength cell such as those fabricated from doped germanium or silicon.
• the solar cells 62 might be of intermediate wavelength, such as a solar cell based upon doped indium phosphide or gallium arsenide.
• the solar cell 64 might be selected from those solar cells having greater sensitivity to short wavelengths, such as aluminum antimonide or cadmium sulfide.
• the solar cells 60, 62, and 64 produce electrical outputs independently of any of the other cells in the array 52.
• Each of the cells may be joined in series with other cells of the same or different types to produce greater voltages, or in parallel with solar cells of the same or different types to produce greater currents. It is not necessary that the solar cells 60, 62 or 64 be compatible in the sense that materials of one be capable of single crystal epitaxial deposition on the materials of the other.
• the cells 60, 62 and 64 are fabricated separately, using the appropriate procedures.
• FIGURES 5 and 6 Two other preferred embodiments of the present invention are illustrated in FIGURES 5 and 6.
• the incident light is not analyzed into component rays of different wavelengths, but instead is serially directed through a sequence of solar cells having optimal sensisvity and efficiency at various incident wavelengths, so that electrical energy may be converted from the full spectrum of wavelengths making up the incident light beam.
• the mounting structure supports the solar cells in a non-coplanar arrangement, and means is provided for directing an incident light beam from one solar cell to the next. More specifically, FIGURE 5 illustrates a triangular mounting structure 66 having a hollow tri ⁇ angular elevational cross-section.
• a first solar cell 68 is mounted on one side of the triangular support structure 66, so as to be intercepted by an incident Q light beam 70.
• a second solar cell 72 is mounted on an adjacent side of the triangular support structure 68, so that the portion of the incident light beam 70 which passes through the first solar cell 68 is directed against the second solar cell 72.
• the second solar 5 cell 72 is serially non-coplanar with the first solar cell 68.
• the term "serially non- coplanar" is used to refer to two solar cells inter ⁇ cepted by a light beam without passing through any intermediate solar cell, the two solar cells having Q. their active interfaces lying at an angle to each other, and not parallel or in the same plane.
• the second solar cell 72 which directly receives the transmitted portion of the incident beam 70 after it passes through 5 the first solar cell 68, lies at an angle of about
• the first solar cell 68 By constructing the array so that successive pairs of solar cells are non-coplanar, i.e., serially non- coplanar, internal reflection, such as described above in relation to the gap 50 and the stacked array 44, is avoided. Means is provided for directing the light beam serially from one of the solar cells to the next solar cell.
• the transmitted portion of the incident light beam 70 passes directly through the first solar cell 68 to the second solar cell 72.
• the second solar cell 72 preferably includes a silvered back surface 74, which acts as a mirror to reflect the light beam at an angle equal to its incident angle, thereby forming a first reflected light beam 76.
• a third solar cell 78 is mounted on the third ' side of the triangular support structure 66, and positioned to intercept the first reflected light beam 76.
• the third solar cell 78 is also preferably provided with a silvered back surface 80, which reflects the portion of the first reflected light beam 76 not converted to electricity in the third solar cell
• a fourth solar cell 84 may be mounted on the same side of the triangular support structure 66 as the first solar cell 68, and positioned so that the second reflected light beam 82 passes through the fourth solar cell 84 as it leaves the triangular support structure 66. In this way, a fourth wavelength range may be converted to electrical energy.
• the triangular support structure 66 thereby provides a support for four solar cells 68, 72, 78 and
• the solar cell array illustrated in FIGURE 5 achieves an efficiency of up to about 50% in converting the light energy of the incident beam 70 to electrical energy, through the use of the four solar cells.
• the four solar cells are provided with external connections allowing them to be connected with each other or with 5 cells in other arrays, in any selected series or parallel fashion to obtain a desired electrical voltage and current.
• the means for directing the incident light beam illustrated in FIGURE 5 is the silvered back surfaces 74 and 80. Mirrors, lenses, light pipes, or 0 ; other means for directing the light beams may also be provided.
• FIGURE 6 Another embodiment of the present invention is illustrated in FIGURE 6, wherein a parallelogram-shaped support structure 86 has an elevational cross- section
• the 20 converts electrical energy from a portion of the wavelengths contained within the incident light beam 88, and transmits the remaining wavelengths out of the first solar cell 90 in a transmitted beam 92.
• the transmitted beam 92 falls upon a second solar cell 94
• the second solar cell 94 includes a silvered back surface 96, which reflects the unconverted portion of the transmitted beam 92 out of the second solar cell 94, thereby forming a first reflected beam 98.
• the first reflected beam 98 impinges upon a third solar
• the third solar cell 100 is provided with a silvered-back surface 102, and the first reflected beam 98 is reflected from the silvered-back surface 102 to form a second reflected beam 104.
• the second reflected beam 104 impinges upon a fourth solar cell 106, which is supported by a fourth side of the parallelogram-shaped support structure 86.
• the fourth solar cell 106 converts the energy in an fourth range of wavelengths of the incident beam 88 to electrical energy.
• the fourth solar cell 106 is provided with a silvered-back surface 108, which reflects the second reflected beam 104 normal to the surface of the fourth solar cell 106, so that the beam traverses back along the path of the second reflected beam 104, the first reflected beam 98, the transmitted beam 92, and the incident beam 88.
• the light beam impinges twice upon the solar cells 100, 94 and 90 as it passes through the solar cell array.
• first solar cell 90 and the second solar cell 92 are serially non-coplanar
• the second solar cell 94 and the third solar cell 100 are serially non-coplanar
• the third solar cell 100 and the fourth solar cell 106 are serially non-coplanar.
• the triangular support structure 66 and the parralelogram-shaped support structure 86 have the important advantage that light beams cannot be trapped by multiple reflections between adjacent solar cells, to produce high heat loadings on the cells.
• the hollow core structure of the support structures 66 and 86 allows heat to be radiated away from the solar cells, or in an atmospheric environment, a coolant to be passed down the center of the support structure. It is particularly desirable that the solar cell arrays of the present invention be resistant to, and protect able from, damage caused by a laser or other energy beam directed against the solar cell array.
• FIGURE 7 illustrates a preferred photovoltaic assembly that exhibits such a damage resistance.
• a photovoltaic energy source 110 includes a solar cell array 112 in accordance with the present invention, and means for focusing a beam of incident light 114 upon the solar ; cells of the array 112.
• a parabolic reflector 116 is aimed at the incident light beam 114, so that components of the beam are reflected toward the focus of the parabola.
• a lens 118 at the focus directs the focused light beams into a single parallel beam 120, " which is then directed into the solar cell array 112, an can be converted to electrical energy in the manner previously described.
• the solar cell array 112 can be made compact due to its high conversion efficiency.
• the array 112 is placed in a protective covering 122 which protects the solar cell array 112 from the general radiation environment and also from damage induced by high energy beams directed against the solar- cell array 112.
• the compact solar cell array 112 is therefore more difficult to target and also more . defensible than prior solar cell arrays having much greater size.
• the solar cell arrays of the present invention are therefore more efficient than conventional solar cell arrays, and can be made more compact.
• the arrays can be constructed from known technologies, and do not require development of new multi- junction semiconductor solar cells.

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• 1986
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  • 1986-09-09 IL IL79995A patent/IL79995A0/xx unknown

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