Photovoltaic cell package assembly for mechanically stacked photovoltaic cells

A mechanically stacked module package is described. The package permits the effective mechanical stacking of two solar cells in a compact arrangement. The design also permits the easy wiring of the package into a voltage matching configuration for module wiring thus eliminating the problems of current matching the top and bottom cells. The package design can be used with any mechanically stacked cells although the design is most suitable for concentrator solar cell configurations where the removal of heat to avoid degradation and shortening of lifetime is a major concern.

This invention relates to photovoltaic cells. More specifically, this 
invention relates to mechanically stacked photovoltaic cells. 
BACKGROUND OF THE INVENTION 
To increase the overall efficiency of photovoltaic devices and extract the 
maximum amount of energy from solar radiation, researchers have 
investigated various multi-color photovoltaic devices. These multicolor 
photovoltaic devices can be divided into two general categories. The first 
category covers monolithic multicolor solar cells. A monolithic 
multi-color solar cell is a solar cell which has distinct regions 
optimized to absorb different portions of the solar radiation spectrum in 
a single device. U.S. Pat. Nos. 4,404,421 and 4,451,691, incorporated 
herein by reference for all purposes, describe suitable monolithic 
devices. Although these monolithic devices are attractive from a system 
and manufacturing point of view, they will require considerable materials 
research to bring them to commercialization. 
A second approach involves tandem mechanically stacked two-color solar 
cells. These devices comprise independent photovoltaic devices which 
optimized to different portions of the solar spectrum and are mechanically 
and electrically interconnected. These tandem mechanically stacked 
two-color solar cells offer a shorter path to commercialization primarily 
because the low-band gap cells, such as silicon solar cells, are already 
developed cells. U.S. application Ser. No. 645,456 filed Aug. 28, 1984, 
incorporated herein by reference for all purposes, describes a suitable 
high-band gap solar cell. Examples of high-band gap solar cells are GaAsP 
or AlGaAs or GaAs solar cells, and the like. 
These mechanically stacked cells often fall into the category of solar 
cells known as concentrator solar cells. A concentrator solar cell is a 
high efficiency solar cell which utilizes some sort of a focusing optics 
to concentrate solar radiation from a strength of one sun to many suns, 
i.e., on the order of 50 to 1000 or more suns. The concentration of the 
solar radiation permits the solar cells to produce a greater amount of 
electricity per unit area than lower efficiency flat plate solar cells. 
This makes them especially useful for space applications where weight is 
of great concern and in jobs which require maximum electrical output with 
a minimum amount of surface area. However, a drawback to concentrator 
solar cells is a means for interconnecting the two mechanically stacked 
solar cells and dissipating the heat generated by the concentration of the 
solar radiation. Thus, it would be highly desirable to have a mechanically 
stacked apparatus which can interconnect two solar cells while minimizing 
the effects of heat generated by the concentrated solar radiation. 
In conventional mechanical stack designs, in particular, those using thin 
top cells, the heat generated in the top cell must be transmitted through 
the transparent adhesive bonding the two cells together. This can lead to 
undesirably high cell temperatures. To avoid this difficulty, it would be 
highly desirable to have a package design wherein heat spreaders 
incorporated therein are used both for the bottom and top cells. A further 
advantage would be to incorporate a wafer for the top cell that is thick 
enough to conduct the heat laterally to the second heat spreader. A still 
further advantage or object would be to have a design which isolates the 
cells so that the effects of thermal expansion are reduced or minimized. 
In mechanical or monolithic cell designs, the top and bottom cells must 
generally be current matched or the performance of the device is limited 
by the cell having the lower current. Since current matching different 
bandgap solar cells can be extremely difficult, it would be desirable to 
have a package which permits voltage matching of the two cells. Voltage 
matching is beneficial because the voltages of the cells change very 
little with variations in solar spectrum or with the cell degradation with 
space radiation damage. Thus, it would also be highly desirable to have a 
package design which can dissipate the heat and permit the easy wiring of 
numerous mechanically stacked cells into a module wiring configuration for 
voltage matching instead of current matching. 
SUMMARY OF THE INVENTION 
We have invented a mechanically stacked photovoltaic cell package assembly 
for mechanically stacking solar cells which incorporates the above 
enumerated desirable features and other benefits which are readily 
apparent to the ordinary skilled artisan. The mechanically stacked 
apparatus includes top and bottom heat spreaders, insulating means to 
separate the two solar cells and means for forming electrical contacts to 
the exposed surfaces of each of the photovoltaic devices incorporated into 
the assembly. The means for forming electrical contact permit the easy 
interconnection of the completed device in a voltage matching scheme to 
avoid the requirements of current matching the individual photovoltaic 
cells.

DETAILED DESCRIPTION OF THE INVENTION 
The invention will be more clearly illustrated by referring to the figures. 
FIGS. 1, 2, and 3 illustrate an exploded view, a cross sectional view and 
a top view, respectively, of a mechanically stacked solar cell package 10. 
The package 10 includes a base support member 12 of a suitable insulating 
material such as alumina (Al.sub.2 O.sub.3). The base support member 12 
contains regions of metallization 12a and 12b. These regions provide a 
means for forming electrical contact to the bottom solar cell 30 through 
the bottom heat sink 14 and the electrode leads 32, 34, and 36. The 
metallization regions 12a and 12b can have any shape which permits the 
formation of electrical contact thereto, although the illustrated 
configuration is preferred for the contacts and heat sinks illustrated. A 
suitable metallization is gold, silver, nickel plating, and the like. The 
insulating base member can be soldered to a suitable heat sink, not 
illustrated, such as a nickel-plated heat sink. 
Contacting the base support means 12 through the metallization 12a is an 
electrically conductive heat spreader 14. A suitable bottom contact heat 
spreader is fabricated from a suitable thermal expansion matching 
conductor such as molybdenum. The heat spreader 14 also functions as a 
part of the means for forming an electrical contact to the major surface 
of the solar cell which is opposed to solar radiation. Surrounding this 
bottom contact heat spreader 14 is a spacer means 16. Preferably the 
spacer 16 is fabricated from an insulating material such as alumina or 
high temperature plastic and provides a space therein surrounding the 
bottom contact heat spreader 14 in which to position a low-band gap solar 
cell illustrated as photovoltaic device 30. The bottom cell 30 is in 
electrical contact with the heat spreader 14 and an electrode or lead 32 
for wiring into a module. 
The bottom solar cell 30 is electrically contacted with suitable means for 
forming an electrical contact to the portion of the solar cell opposite to 
the incident surface. This combination comprises the heat spreader 14, 
metallization 12a, and a lead 32. The incident surface of the solar cell 
is electrically wired into a module through top leads, i.e., contacts 34 
and 36. Lead 34 electrically contacts the incident surface of solar cell 
30 and the outer lead 36 through the metallization 12b. This preferred 
configuration permits the solar cell 30 to be isolated from the rest of 
the package 10 and thus minimizes the thermal expansion strains placed 
upon the cell 30 during the heating thereof caused by solar radiation. 
Although less preferred, the contacts 34 and 36 can be configured if 
desired as a unitary piece. Of course, if leads 34 and 36 were a single 
piece of metal then the metallization 12b would be unnecessary. The 
preferred configuration of the top and bottom leads to the bottom cell 30 
is illustrated. Of course, depending upon the packaging the leads 32, 34, 
and 36 need not be opposed but could be fabricated over but separated from 
each other. A further option, not illustrated, would be to have the 
metallization illustrated as 12b on support 12 applied instead on the 
insulating spacer 16. This option would avoid the tab portion of lead 34 
from having to bend down from the incident surface of solar cell 30 to the 
metallization 12b on support member 12. 
A second insulating spacer means 18 such as alumina, a high temperature 
plastic, and the like, separates the bottom cell 30 from the top cell 50. 
The top cell 50 is electrically contacted on the bottom, i.e., the major 
surface opposite the incident surface through leads 54 and 56 and the 
metallization 18a on the spacer 18. These bottom leads can also be 
configured as a single unitary piece. The incident surface of the solar 
cell is in electrical contact with an incident heat spreading means 20 for 
spreading the heat from the top solar cell 50 and the lead 52. A suitable 
heat spreader means 20 is a molybdenum heat sink in contact with top cell 
50. The lead 52 can be any suitable metal. Of course, the incident heat 
spreader 20 has a hole therein, as illustrated, for the passage of solar 
radiation. 
Although the package assembly 10 has been described and illustrated with 
spacers and heat spreaders, i.e., heat sinks, having a circular washer 
shaped configuration, any suitable configuration for the spacers and heat 
sinks is possible provided they contain a space therethrough for which the 
solar radiation to enter into the top cell 50 and pass to the bottom cell 
30. The contacts have preferably been illustrated having a Y-shaped 
configuration; however, any suitable configuration for contacting the 
appropriate tops and bottoms of the cells is possible such as a 
rectangular a square tab shape. Furthermore, their thicknesses can be 
adjusted to provide a more compact or robust assembly as required for a 
particular application. 
The selection of the top and bottom cells is a function of that portion of 
the solar radiation spectrum for which the package assembly is to be 
optimized. A preferred top solar cell would be a gallium arsenide 
phosphide (GaAs.sub.(x) P.sub.(1-x)) solar cell having a band gap of about 
1.75 eV fabricated on a gallium phosphide (GaP) substrate with a larger 
band gap, i.e., about 2.25 eV so that it appears transparent to solar 
radiation which would pass therethrough and into the bottom cell 30. A 
suitable preferred bottom cell would be an IR enhanced silicon solar cell 
having a band gap of about 1.1 eV. 
A benefit of the package design 10 permits module wiring configurations 
illustrated in FIG. 4. The package design 10 avoids the requirement for 
current matching of the top and bottom cells and replaces it with a 
voltage matching configuration. This wiring scheme benefits from the 
advantage that the voltage changes very little with variations in the 
solar spectrum or device degradation. It requires four independent leads 
from the cell package which is consistent with the package design 
described above. 
More specifically, if the stacked cells were designed for series connection 
with currents matched at the beginning of life, the currents would be 
mismatched at the end of life, with performance loss greater than the 
efficiency loss of the individual cells. Voltage matching provides a 
longer life design since the device voltages only vary logarithmically 
with current changes. FIG. 4 illustrates a four by two module wiring 
diagram for voltage matching of top and bottom cells whose output voltages 
differ by a factor of 2. In the arrangement of FIG. 4, four silicon solar 
cells are wired in series for every Ga As.sub.x P.sub.1-x solar cells 
wired in series. A voltage factor difference of 2 can be achieved with a 
gallium arsenide phosphide top solar cell 50 selected to have a band gap 
of about 1.75 eV and a bottom solar cell 30 of silicon selected to have a 
band gap of about 1.1 eV. For the eight element configuration illustrated, 
four of the bottom and two of the top cells are connected in series to 
provide voltage matching. Different voltage ratios for different solar 
cells would require other series and parallel connection schemes for 
voltage matching, e.g., four by three, seven by five, five by three, etc. 
The only limitations on the interconnection schemes are the voltages of 
the different cells. Of course, different modules having specific output 
currents and voltages can be connected in series or parallel to provide 
any desired overall voltage or current output for a particular 
application. 
The solar cell package design has been described with reference to 
particularly preferred configurations and embodiments. Modifications which 
would be obvious to the ordinary skilled artisan, such as selection of 
particular top and bottom solar cells and configuration of the spreaders 
and insulators are contemplated to be within the scope of the invention.