Mechanically stacked photovoltaic cells, package assembly, and modules

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the concentration of the solar radiation. Thus, it would

MECHANCALLY STACKED PHOTOVOLTAC be highly desirable to have a mechanically stacked ap CELLS, PACKAGE ASSEMBLY, AND MODULES paratus which can interconnect two photovoltaic cells while minimizing the effects of heat generated by the

CROSS-REFERENCES TO RELATED 5 concentrated solar radiation.

APPLICATIONS In conventional mechanical stack designs, in particu This application is a continuation-in-part of our appli lar, those using thin top cells, the heat generated in the cation entitled "Photovoltaic Cell Package Assembly top cell must be transmitted through the transparent for Mechanically Stacked Photovoltaic Cells', filed on adhesive bonding the two cells together. This can lead June 3, 1985 and assigned U.S. Ser. No. 740,362 which O to undesirably high cell temperatures. To avoid this issued as U.S. Pat. No. 4,658,086. This application is difficulty, it would be highly desirable to have a pack also a continuation-in-part of our application entitled age design wherein heat spreaders incorporated therein "GaAs on GaSb Mechanically Stacked Photovoltaic are used both for the bottom and top cells. A further Cells, Package Assembly, and Modules', filed on Oct. 15 advantage would be to incorporate a wafer for the top 21, 1986 and assigned U.S. Ser. No. 921,566. Both of cell that it thick enough to conduct the heat laterally to said applications are completely incorporated herein by the top heat spreader. A still further advantage would reference for all purposes. be to have a design which isolates the cells so that the This invention relates to mechanically stacked photo effects of thermal expansion are reduced or minimized. voltaic cells. In monolithic cell designs, the top and bottom cells BACKGROUND OF THE INVENTION must generally be current matched or the performance of the cell is limited by the cell having the lower cur

Researchers have investigated various multi-color rent. Sine current matching different bandgap photo photovoltaic cells to increase the overall efficiency of voltaic cells can sometimes be difficult, it would be photovoltaic cells and to extract the maximum amount desirable to have a package which permits voltage of energy from solar radiation. These multi-color pho 25 matching of the two cells. Voltage matching is benefi tovoltaic cells can be divided into two general catego cial because the voltages of the cells change very little ries. The first category covers monolithic multi-color photovoltaic cells. A monolithic multi-color photovol with variations in solar spectrum or with the cell degra dation with space radiation damage. Thus, it would also taic cell is a photovoltaic cell which has distinct regions be highly desirable to have a package design which can optimized to absorb different portions of the solar radia dissipate the

heat and permit the easy wiring of numer tion spectrum in a single cell. U.S. Pat. Nos. 4,404,421 ous mechanically stacked cells into a module wiring and 4,451,691, incorporated herein by reference for all configuration for voltage matching instead of current purposes, describe suitable monolithic cells. Although matching.

these monolithic cells are attractive from a system and Furthermore, it would be desirable to specify two manufacturing point of view, they will require consid 35 component photovoltaic cell materials which will gen erable materials research to bring them to commercial erate voltages which are simple multiples of each other

A second approach involves tandem mechanically where one of these photovoltaic cells is a well devel stacked two-color photovoltaic cells. These cells com oped cell and the other can be rapidly developed to prise independent photovoltaic cells which are opti reach near its theoretical limit performance. Materials mized to the different portions of the solar spectrum and which can be rapidly developed are simple binary com are mechanically and electrically interconnected. These pounds with large optical absorption coefficients which tandem mechanically stacked two-color photovoltaic have already been used as photodetectors. cells offer a shorter path to commercialization primarily SUMMARY OF THE INVENTION because one of the cells can be an already developed 45 cell, such as Si or GaAs. U.S. application Ser. No. The present invention is an apparatus comprising 645,456 filed Aug. 28, 1984, incorporated herein by mechanically stacked photovoltaic cells. The apparatus reference for all purposes, describes a suitable high includes a bottom heat spreader and a top heat spreader, band gap photovoltaic cell. Examples of high-band gap a bottom photovoltaic cell and a top photovoltaic cell, photovoltaic cells are GaAsP or AlGaAs or GaAs or Si 50 and means for forming the necessary electrical contacts. photovoltaic cells, and the like. The heat spreaders are electrically insulated from These mechanically stacked cells often fall into the each other but are thermally connected to each other. category of photovoltaic cells known as concentrator The top heat spreader has a space therein to provide for photovoltaic cells. A concentrator photovoltaic cell is a the passage of solar radiation. high efficiency photovoltaic cell which utilizes some 55 The bottom photovoltaic cell has opposed major sort of focusing optics to concentrate solar radiation surfaces and has an anode and a cathode. The major from a strength of one sun to many suns, i.e., on the surface of the bottom photovoltaic cell opposed to solar order of 50 to 1000 or more suns. The concentration of radiation is thermally bonded to the bottom heat the solar radiation permits the photovoltaic cells to spreader. Preferably, the bottom photovoltaic cell is produce a greater amount of electricity per unit area 60 soldered to the bottom heat spreader. In one embodi than lower efficiency flat plate photovoltaic cells. This ment, the bottom photovoltaic cell comprises: makes them especially useful for space applications (a) a conductive substrate, where weight is of great concern and in jobs which (b) a layer of GaSb having regions of different con require maximum electrical output with a minimum ductivity forming a homojunction therein and contact amount of surface area. However, a drawback to con 65 ing the conductive substrate, and centrator photovoltaic cells is the problem associated (c) a bottom cell passivating layer contacting the with interconnecting the two mechanically stacked surface of the layer of GaSb opposite to the surface of photovoltaic cells and dissipating the heat generated by the GaSb layer contacting the substrate. One such bot

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tom cell passivating layer is a layer of AlyGa(1-)Sb, FIG. 6 illustrates a voltage matching module wiring such as Alo.85Gao.15Sb. diagram incorporating three mechanically stacked pho In another embodiment, the bottom photovoltaic cell tovoltaic assemblies.

can comprise: DETAILED DESCRIPTION OF THE (a) a conductive substrate, and INVENTION (b) a layer of Ge having regions of different conduc tivity forming a homojunction therein and contacting In its broadest aspect, the present invention is an the conductive substrate. In this embodiment, there is apparatus of mechanically stacked photovoltaic cells no requirement for a bottom cell passivating layer. having a top and bottom heat spreader, two photovol The top photovoltaic cell has opposed major sur 10 taic cells, and means for forming the necessary electri faces, has an anode and a cathode, and is electrically cal contacts.

insulated from the bottom photovoltaic cell. The major The invention will be more clearly illustrated by surface of the top photovoltaic cell incident to solar referring to the Figures. FIGS. 1, 2, and 3 illustrate an radiation is thermally bonded to the top heat spreader. exploded view, a cross sectional view, and a top view, Preferably, the top photovoltaic cell is soldered to the 15 respectively, of a mechanically stacked photovoltaic top heat spreader. The top photovoltaic cell has aband cell assembly 10. The assembly 10 has a package com gap energy responsive to solar radiation of shorter prising a bottom heat spreader 20 and a top heat wavelengths than the bandgap energy of the bottom spreader 50. The two heat spreaders are electrically photovoltaic cell. Preferably, the top photovoltaic cell 20 insulated from each other but are thermally connected comprises: to each other. Thermally bonded to bottom heat (a) a layer of GaAs having regions of different con spreader 20 is bottom photovoltaic cell 30 having an ductivity forming a homojunction therein, and anode and a cathode. A first electrically contacting (b) a top cell passivating layer contacting the surface means 31 electrically contacts the anode of bottom cell of the layer of GaAs which is incident to solar radiation. 30, and a second electrically contacting means 32 elec One such top cell passivating layer is a layer of Al 25 trically contacts the cathode of bottom cell 30. Ther Ga(1-xAs, such as Alo.85Ga0.15As, mally bonded to top heat spreader 50 is top photovol In another embodiment, the top photovoltaic cell can taic cell 40 having an anode and a cathode, and having comprise a layer of Si having regions of different con a bandgap energy responsive to solar radiation of ductivity forming a homojunction therein, and a top cell 30 shorter wavelengths than the bandgap energy of bottom passivating layer contacting the surface of the layer of photovoltaic cell 30. Bottom cell 30 and top cell 40 are Si which is incident to solar radiation. electrically insulated from each other. A third electri The means for forming the necessary electrical cally contacting means 41 electrically contacts the contacts include: anode of top cell 40 and a fourth electrically contacting a first means for electrically contacting the anode of 35 means 42 electrically contacts the cathode of top cell the bottom photovoltaic cell; 40.

a second means for electrically contacting the cath As shown in FIGS. 1, 2, and 3, the package comprises ... ode of the bottom photovoltaic cell; bottom heat spreader 20 and top heat spreader 50, a third means for electrically contacting the anode of which form a cavity. Within the cavity are two photo the top photovoltaic cell; and 40 voltaic cells. These two heat spreaders are electrically a fourth means for electrically contacting the cathode insulated from each other but are thermally connected of the top photovoltaic cell. Preferably, the first means to each other. Notice the hatched regions on both and the second means are 180' apart, the third means spreaders which represent electrically insulated re and the fourth means are 180' apart, and the first means gions. The top heat spreader 50 has a space therein to and the third means are 90' apart. 45 provide for the passage of solar radiation. In one embodiment, six GaSb cells are wired in series The major surface of bottom photovoltaic cell 30 for every two GaAs cells wired in series to form a opposed to solar radiation is bonded to bottom heat module of predetermined output voltage under illumi spreader 20 on an uninsulated region. Bottom cell 30 has nation. In another embodiment, three GaSb cells are opposed major surfaces and has an anode and a cathode. wired in series for every GaAs cell to form a module of 50 Preferably, bottom cell 30 is soldered to bottom heat predetermined output voltage under illumination. spreader 20.

BRIEF DESCRIPTION OF THE DRAWINGS A first electrically contacting means 31 electrically contacts the anode of bottom cell 30, and a second

In order to facilitate the understanding of this inven electrically contacting means 32 electrically contacts tion, reference will now be made to the appended draw 55 the cathode of bottom cell 30. As shown in FIGS. 1, 2, ings of preferred embodimens of the present invention. and 3, the first electrically contacting means 31 and the The drawings are exemplary only, and should not be second electrically contacting means 32 are 180 apart. construed as limiting the invention. Notice that the first electrically contacting means 31 is FIG. 1 illustrates an exploded view of the photovol insulated from the bottom heat spreader 20. taic cell package assembly for mechanically stacked The major surface of top photovoltaic cell 40 inci photovoltaic cells of our invention. dent to solar radiation is bonded to top heat spreader 50 FIG. 2 illustrates a side view of the assembly. on an uninsulated region. Top photovoltaic cell 40 has FIG. 3 illustrates a top view of the assembly. opposed major surfaces and has an anode and a cathode. FIG. 4 illustrates a side view of the photovoltaic cells Top cell 40 and bottom cell 30 are electrically insulated of one embodiment of the present invention. 65 from each other. The top cell 40 has a bandgap energy FIG. 5 illustrates a voltage matching module wiring responsive to solar radiation of shorter wavelengths diagram incorporating twelve mechanically stacked than the bandgap energy of bottom cell. Preferably, the photovoltaic assemblies. top cell 40 is soldered to top heat spreader 50.

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A third electrically contacting means 41 electrically tion therein and contacting said conductive substrate. contacts the anode of top photovoltaic cell 40 and a No bottom passivating layer is required for this embodi fourth electrically contacting means 42 electrically ment. w contacts the cathode of top photovoltaic cell 40. As A benefit of the package design 10 permits module shown in FIGS. 1, 2, and 3, the third electrically con wiring configurations illustrated in FIG. 5. The package tacting means 41 and the fourth electrically contacting design 10 avoids the requirement for current matching means 42 are 180' apart, and the first electrically con of the top and bottom cells and replaces it with a volt tacting means 31 and the third electrically contacting age matching configuration. This wiring scheme bene means 41 are 90' apart. Notice that the third electrically fits from the advantage that the voltage changes very contacting means 42 is insulated from the top heat O little with variations in the solar spectrum or device spreader 50. degradation. It requires four independent leads from the FIG. 4 illustrates a side view of the photovoltaic cells cell package which is consistent with the package de of one embodiment of the present invention. A top sign described above.

GaAs cell is above a botton GaSb cell. More specifically, if the stacked cells were designed The top GaAs cell comprises a layer of GaAs having 15 for series connection with currents matched at the be regions of different conductivity forming a homojunc tion therein, and a top passivating layer contacting the ginning of life, the currents would be mismatched at the end of life with perforamcne loss greater than the effi surface of the GaAs layer which is incident to solar ciency loss of the individual cells. Voltage matching radiation.

As shown in FIG. 4, the layer of GaAs has regions of 20 provides a longer life design since the device voltages vary only logarithmically with current changes. FIG. 5 different conductivity forming a homojunction therein. illustrates a six by two module wiring diagram for volt- - On the side of the homojunction incident to solar radia age matching of top and bottom cells whose output tion, the GaAs layer is doped with a p-type dopant. On voltages differ by a factor of 3. For the twelve element the opposing side, the GaAs layer is doped with a n configuration illustrated, six of the bottom and two of type dopant. A top passivating layer of AlxGa(1-xAs 25 the contacts the surface of the GaAs layer which is incident matching. Different voltageinratios top cells are connected series to provide voltage for different photo to solar radiation. In FIG. 4, x is equal to 0.85. voltaic cells would require other series and parallel The formation of GaAs photovoltaic cells is well connection schemes for voltage matching, e.g., four by known in the art. One preferred method of forming a three, seven by five, five by three, etc. The only limita GaAs cell is disclosed in the paper entitle "Epitaxial 30

Growth from Organometallic Sources in High Vac tions on the interconnection schemes are the voltages of uum' presented by L. M. Fraas, P. S. McLeod, L. D. the different cells. Of course, different modules having Partain, and J. A. Cape at the Electronic Materials specific output currents and voltages can be connected Conference in Boulder, Colorada on June 19, 1975. Said in series or parallel to provide any desired overall volt paper is incorporated herein by reference for all pur 35 age or current output for a particular application. poses. FIG. 6 illustrates a three by one module wiring dia The bottom GaSb cell comprises a conductive sub gram for voltage matching of top and bottom cells strate, a layer of GaSb having regions of different con whose output voltages differ by a factor of 3. For the ductivity forming a homojunction therein and contact three element configuration illustrated, three of the ing the conductive substrate, and a passivating layer bottom and one of the top cells are connected in series contacting the surface of the layer of GaSb opposite to to provide voltage matching.

the surface of the GaSb layer contacting the substrate. While the present invention has been described with The conductive substrate can be any material that reference to specific embodiments, this application is enables the subsequently deposited semiconductor lay intended to cover those various changes and substitu ers to grow in a crystal configuration suitable for photo- 45 tions which may be made by those skilled in the art voltaic cells. A suitable example is crystalline GaSb. without departing from the spirit and scope of the ap As shown in FIG. 4, the layer of GaSb has regions of pended claims. Modifications which would be obvious different conductivity forming a homojunction therein. to the ordinary skilled artisan, such as selection of par Below the homojunction is a n-type GaSb layer that ticular top and bottom photovoltaic cells and configura contacts the conductive substrate. Above the homo SO tion of the spreaders are contemplated to be within the junction, the GaSb layer is doped with a p-type dopant. scope of the invention.

A top passivating layer of AlyGa(1-Sb contacts the What is claimed is:

surface of the GaSb layer which is incident to solar 1. An apparatus comprising mechanically stacked radiation. In FIG. 4, y is equal to 0.85. photovoltaic cells, comprising: One would expect a GaSb cell fabrication technology 55 a package comprising a bottom heat spreader and a to closely parallel the GaAs cell fabrication technology top heat spreader, wherein said heat spreaders are in that AlGaSb window layers might be used to passiv electrically insulated from each other but are ther ate the top surface of a GaSb cell. In fact, F. Capasso et mally connected to each other, wherein said top al. Appl. Phys. Lett, 35, 165 (1980) have used liquid heat spreader has a space therein to provide for the phase epitaxy to grow p-type AlGaSb window-pass passage of solar radiation; ivated p-on-n GaSb photodiodes; and they have re a bottom photovoltaic cell having opposed major ported both the current versus voltage and spectral surfaces and having an anode and a cathode, response characteristics for these devices. wherein the major surface of said bottom photo In another embodiment of the present invention, a Ge voltaic cell opposed to solar radiation is thermally bottom photovoltaic cell is used instead of a GaSb cell. 65 bonded to said bottom heat spreader; In that embodiment, the bottom photovoltaic cell com first electrically contacting means for electrically prises a conductive substrate, and a layer of Ge having contacting the anode of said bottom photovoltaic regions of different conductivity forming a homojunc cell;

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second electrically contacting means for electrically 16. An apparatus according to claim 7 wherein said contacting the cathode of said bottom photovoltaic top photovoltaic cell comprises:

cell; (a) a layer of Sihaving regions of different conductiv a top photovoltaic cell having opposed major sur ity forming a homojunction therein, and faces, having an anode and a cathode, and being 5 (b) a top cell passivating layer contacting the surface electrically insulated from said bottom photovol of said layer of Si which is incident to solar radia taic cell, wherein the major surface of said top tion.

photovoltaic cell incident to solar radiation is ther bottomAnphotovoltaic 17. apparatus according to claim 7 wherein said cell comprises:

mally bonded to said top heat spreader, and (a) a conductive substrate, and wherein said top photovoltaic cell has a bandgap O energy responsive to solar radiation of shorter (b) a layer of Ge having regions of different conduc wavelengths than the bandgap energy to said bot tivity forming a homojunction therein and contact ton photovoltaic cell; ing said conductive substrate. 18. An apparatus comprising mechanically stacked third electrically contacting means for electrically photovoltaic cells, comprising:

contacting the anode of said top photovoltaic cell; 15 (a) a package comprising a bottom heat spreader and and a top heat spreader, wherein said heat spreaders are fourth electrically contacting means for electrically electrically insulated from each other but are ther contacting the cathode of said top photovoltaic mally connected to each other, wherein said top cell. heat spreader has a space therein to provide for the 2. An apparatus according to claim 1 wherein said 20 passage of solar radiation; first electrically contacting means and said second elec (b) a bottom photovoltaic cell having opposed major trically contacting means are 180 apart. surfaces and having an anode and a cathode, 3. An apparatus according to claim 2 wherein said wherein the major surface of said bottom photo third electrically contacting means and said fourth elec 25 voltaic cell opposed to solar radiation is soldered to trically contacting means are 180' apart. said bottom heat spreader, and wherein said bot 4. An apparatus according to claim 3 wherein said tom photovoltaic cell comprises: first electrically contacting means and said third electri (b1) a conductive substrate, cally contacting means are 90' apart. (b2) a layer of GaSb having regions of different 5. An apparatus according to claim 1 wherein said 30 conductivity forming a homojunction therein bottom photovoltaic cell is soldered to said bottom heat and contacting said conductive substrate, and spreader. (b3) a bottom cell passivating layer of Alo.8- 6. An apparatus according to claim 1 wherein said top 5Ga0.15Sb contacting the surface of said layer of photovoltaic cell is soldered to said top heat spreader. GaSb opposite to the surface of said GaSb layer contacting said substrate;

7. An apparatus according to claim 1 wherein said top 35 (c) first electrically contacting means for electrically photovoltaic cell comprises: contacting the anode of said bottom photovoltaic (a) a layer of GaAs having regions of different con cell;

ductivity forming a homojunction therein, and (d) second electrically contacting means for electri (b) a top cell passivating layer contacting the surface cally contacting the cathode of said bottom photo of said layer of GaAs which is incident to solar voltaic cell, wherein said first electrically contact radiation. ing means and said second electrically contacting 8. An apparatus according to claim 7 wherein said top means are 180° apart;

cell passivating layer is a layer of AlGa(1-xAs. (e) a top photovoltaic cell having opposed major 9. An apparatus according to claim 8 wherein x has a surface, having an anode and a cathode, and being value of about 0.85. 45 electrically insulated from said bottom photovol 10. An apparatus according to claim 7 wherein said taic cell, wherein the major surface of said top bottom photovoltaic cell comprises: photovoltaic cell incident to solar radiation is (a) a conductive substrate, and soldered to said top heat spreader, wherein said top (b) a layer of GaSb having regions of different con photovoltaic cell has a bandgap energy responsive ductivity forming a homojunction therein and con 50 to solar radiation of shorter wavelengths than the tacting said conductive substrate. bandgap energy of said bottom photovoltaic cell, 11. An apparatus according to claim 10 wherein said and wherein said top photovoltaic cell comprises: bottom photovoltaic cell further comprises a bottom (el) a layer of GaAs having regions of different cell passivating layer contacting the surface of said conductivity forming a honojunction therein, and layer of GaSb opposite to the surface of said GaSb layer 55 (e2), a top cell passivating layer of Alo,85Gao.15As contacting said substrate. contacting the surface of said layer of GaAs which 12. An apparatus according to claim 11 wherein said is incident to solar radiation; bottom cell passivating layer is a layer of AlGa(1-Sb. (f) third electrically contacting means for electrically 13. An apparatus according to claim 12 wherein y has contacting the anode of said top photovoltaic cell; a value of abut 0.85. and 14. An apparatus according to claim 10 further com (g) fourth electrically contacting means for electri prising wiring six bottom cells in series for every two cally contacting the cathode of said top photovol top cell wired in series to form a module of predeter taic cell, wherein said third electrically contacting mined output voltage under illumination. means and said fourth electrically contacting 15. An apparatus according to claim 10 further com 65 means are 180 apart, and wherein said first electri prising wiring three bottom cells in series for every top cally contacting means and said third electrically cell wired in series to form a module of predetermined contacting means are 90' apart. output voltage under illumination.