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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to solar cells for converting solar energy into
usable electrical energy, and more particularly, to packages for reliably
housing a plurality of solar cells.
2. Brief Description of the Prior Art:
Various types of solar cells are known in the art. For example, silicon
solar cells may be provided on silicon wafers having a large-area PN
junction. During bright sun light, a 3-inch diameter silicon solar cell
may provide about 1.0-1.5 amps of current at approximately 0.5 volts. In
order to provide a suitably sized solar panel, a plurality of such solar
cells must be housed within a single package. It is necessary that the
electrodes of each silicon solar cell make low resistance electrical
contact to the P-region and N-region forming the large area PN junction.
It is also necessary that the means of electrically contacting the
semiconductor material upon which the incident solar radiation falls be
such that semiconductor active area is shielded as little as possible by
the contacting material. To date, no solar package for solar cells has
been provided which provides the combined features of minimizing shading
of solar radiation from the cells, and provides high packing density of
the solar cells such that each solar cell is very closely located to the
adjacent solar cells, and which is easily manufacturable and which
provides a satisfactorily low amount of resistance between the terminals
of each individual solar cell and the external terminals of the solar cell
package.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a solar cell package which
maximizes active solar cell are exposed to solar radiation and which has a
minimum amount of internal resistance to the active regions of the solar
cell from the external terminals of the solar cell package.
It is another object of the invention to provide a solar cell package
wherein conducting members coupled to the external terminal of the solar
cell package are located on the opposite side of the solar cells from
which solar radiation is received.
It is another object of the invention to provide a readily manufacturable
solar cell package.
Briefly described, the invention is a solar cell package including a back
member and a transparent cover plate member forming and enclosing a
substantially filled region. A plurality of solar cells are located within
the region, each of the solar cells having a first surface adjacent to the
cover plate member and a second surface adjacent to the back member. All
of the power conductors coupled to the external terminals of the solar
cell package are located on the sides of the solar cells opposite to the
cover plate member. Electrically insulative and termally conductive
material is located between the power conductors and the solar cells and
also between the power conductors and the back member. In one embodiment,
the back member itself is connected to one side of each solar cell and
serves as a power conductor.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top view diagram of an embodiment of the invention.
FIG. 2 is a side view diagram through line 2--2 of the embodiment of the
invention shown in FIG. 1.
FIG. 3 is a top view diagram of another embodiment of the invention.
DESCRIPTION OF THE INVENTION
A presently preferred embodiment of the invention is described by reference
to FIGS. 1 and 2. Solar package 10 includes back member 12, which is
essentially a shallow pan having a bottom 14 and sides 13. It should be
noted that the dimensions in FIGS. 1 and 2 are not drawn to scale. The
inner surface of bottom 14 of back member 12 may include ridges 58 to
provide structural strength. Back member 12 may be made of a metal such as
aluminum or other substance which may not be metallic. Cover plate 16 is
transparent to incident radiation which is converted to electrical energy
by the solar cells. Cover plate 16 may advantageously be an
anti-reflection treated glass material. A plurality of solar cells such as
18, 20, 22 and 24 are included within cavity 16 sealed off by cover plate
16 and back member 12. Although various types of solar cells may be
suitably packaged in solar cell package 10, semiconductor solar cells are
referred to hereinafter. Each solar cell includes a wafer having a body
region of one conductivity type and a second region of the opposite
conductivity type forming a large area junction therebetween. For example,
solar cell 18 includes P-type substrate region 28 and N-type region 26
forming PN junction 29.
Each of the solar cell wafers, may, for example, be three inches in
diameter. The doping level of the N-type region 26 may be close to solid
solubility levels at the top surface of the wafer, and the doping level of
the P-type region may be, for example, in the range from 10 ohm
centimeters to 0.1 ohm centimeters. This range of doping levels however,
is not at all inclusive. When incident sunlight, indicated by the arrows I
in FIG. 2 falls on solar cell package 10, it passes through cover plate 16
and strikes the exposed surface of each of the solar cells. This causes a
substantial amount of current to flow across the PN junctions, such as
junction 29. A metallized pattern is provided on the upper surface of each
of the wafers electrically contacting N-type region 26. The metallized
pattern, of course, shields active semiconductor surface area from the
incident sunlight, and decreases the efficiency of the energy conversion
process. Therefore, it is highly desirable that the metallization pattern
utilize the narrowest conductor possible while nevertheless having the
lowest resistance so as to decrease power dissipation and minimize the
voltage drop due to high current flowing in the metallization from the
various portions of the semiconductor wafer. The opposite surface of each
wafer, i.e., the bottom surface of each wafer in FIG. 2, is coated with an
electrically conductive layer which extends over the entire bottom surface
so that current flowing into P-type substrate region 28 is distributed
through very low resistance to all portions of P-type region 28. (Of
course, the bottom conductive layer may be suitably patterned). Thus, the
solar cell metallization is selected such that the voltage drop at each
solar cell is minimized. The metallization on the top surface of each of
the solar cells is connected to a first conductor 30 located beneath all
of the solar cells, and isolated therefrom by a thermally conductive
electrically insulative material 34, which may for example, be an alumina
filled silicone. For example, a metallized surface of solar cell 18 is
connected to conductor 30 by means of conductor 36. Conductor 36 may be a
wire or a strap which is electrically attached to conductor 30 and the
metallized surface of solar cell 18. Or, it may more preferably be a
punched out tab portion of conductor 30 which is bent upward and soldered
or otherwise attached to the metallized upper surface of solar cell 18.
Similarly, conductor tabs 38, 40 and 42 couple the upper metallized
surfaces of solar cells 20, 22 and 24, respectively, to conductor 30.
Conductor 30 is electrically connected to terminal 54, which may end
through the bottom of solar package 12, as shown in FIG. 2 or may even
extend to the side or even to or through the cover plate in any suitable
manner. The second thermally conductive but electrically insulative layer
52 is located between conductor 30 and support member 12. A second
conductive layer 32 is substantially coplanar with conductor 30 is
sandwiched between insulators 34 and 52. As shown in FIG. 1, conductive
layer 32 has the general appearance of an H which has been separated from
conductor 30 by removal of conductor material forming a gap 62 between
conductors 30 and 32, so that they nowhere electrically contact each
other. The metallized lower surfaces of solar cells 18, 20, 22 and 24
electrically contact conductor 32 by means of members 44, 46, 48 and 50,
respectively. The contact members such as member 44 may advantageously be
dimples or punched out tab portions of conductor 32 which extend through
accommodating openings in insulative layer 34 in the same manner that
conductive members or tabs 36 contacts the upper metallized surfaces
through accommodating apertures in insulator material 34. Alternatively,
all of the area of the subtended conductor lying directly beneath said
solar cell may be contacted by the metal of that solar cell. Conductor 32
is connected to external terminal 56 which extends through the backside of
back member 12 in the same manner as terminal 54. Of course, terminals 54
and 56 are electrically insulated by appropriate grommets of washers from
back member 12 if back member 12 is an electrically conductive material.
Referring to FIG. 1, it is seen that solar package 10 includes 16 solar
cells, each having its lower metallized surface electrically contacting
each sheet conductor 32. For example, wafer 18 has its bottom metallized
layer contacting conductor 32 by means of contact member 44 extending
through an aperture in insulator 34, as explained above. The upper
metallized pattern on solar cell 18 contacts conductor 30 by means of four
tabs 36, 361, 362 and 363. For convenience, the metallized pattern is not
shown in FIG. 1. However, in order to minimize both the resistance of the
metallization and the amount of active surface area of solar cell 18 that
it covers, it is highly advantageous to provide electrical contact at at
least four places around its perimeter. Of course, if it were necessary,
additional contact members or tabs such as 36, etc., could be provided. It
will be noted that the side view shown in FIG. 2 does not correspond
exactly to the top view in order to clarify the explanation of a
structure. For example, contact member 36 as shown in FIG. 1 does not
really appear across section line 2--2, but is nevertheless shown in FIG.
2 for clarification. For simplicity, the other contact members 361, 362
and 363 have been omitted. The same holds true for the remaining solar
cells shown in FIG. 2.
By referring to FIG. 1, it is seen that the "H-shaped" structure of
conductor 32 and the complementary structure of conductor 30 permit
contacts to the heavily metallized low resistance back surfaces of each of
the solar cells to a single conductor 32, and permit four peripheral
contact points of each of the partially metallized upper solar cell
surfaces to conductor 30. A parallel electrical connection of all 16 solar
cells between terminals 54 and 56 is thus achieved. Of course, the number
of rows and columns could be diminished or expanded in either direction in
FIG. 1, an additional cross region such as 32 could be provided as a
number of rows is increased, and additional members such as 62 could be
added as additional columns of solar cells are added. It should be
emphasized that the conductor topography can be chosen such that the
shapes of conductors 30 and 32 may accommodate more or less the four front
surface contact points between conductor 30 and each solar cell.
FIG. 3 is a partial top view diagram showing an alternate method of
connecting solar cells in series rather than in parallel. The structural
features of packaging series connected solar cells are similar to the
features shown in FIGS. 1 and 2, except that the conductor shapes are
different as shown in FIG. 3. In FIG. 3 it is seen that the lower
metallized surface of each solar cell, such as solar cell 108 is connected
to a tab like portion of such as 114 of a metal conductor such 104. The
upper metallized pattern of each solar cell is connected by means of four
symmetrically positioned contact members 118, 120, etc., to a second
conductor 100 which is separate and spaced from the first conductor 104
and separated therefrom by means of a narrow space 111. Of course, the
topograph of conductor 100 also could be chosen to accommodate more or
less than four front surface contact points. There are 16 such conductors,
each of which has a tablike protrusion such as 114 and also has an open
region such as 122 which essentially accommodates the tablike protrusion
of another of the conductors. As in FIG. 2, all of the conductors are
essentially coplanar and are sandwiched between insulators 34 and 52. The
conductive members extend through openings in such insulator as 34 as
shown in FIG. 2. However, only the "end" conductors in FIG. 3 forming the
two ends of the "chain" formed by the "tongue-in-groove" arrangement of
the conductors in FIG. 3 are connected to external terminals 54 and 56.
For example, portion 131 of conductor 132 could be connected to external
terminal 54 and a tongue-like conductor 133 fitting in a groove portion of
conductor 134 could be connected to the other external terminal 56 of a
package similar to that in FIG. 2.
Referring to FIG. 2, the remaining part of space 60 could be filled with a
transparent filler, which could be some sort of silicone material.
It should also be recognized that various series-parallel combinations of
solar cells could be provided within a single package such as 10, to
provide both stepped up voltages characteristic of the series connected
solar cells and high current capability and inherent reliability of the
parallel connected solar cells.
The package as described in FIGS. 1-3 provides a readily manufacturable
package for solar cells which has a number of important advantages. The
conductive sheets can all be punches out of the same piece of foil-like
material, so that very little material is wasted. Positioning of the power
conducting busses beneath the solar cells eliminates any shading of the
active exposed surface of the solar cells that would occur if one of the
power conductors were above the solar cells. The general shape of the
support member 12 is relatively rigid, which is important, since the solar
cell wafers may only be from 10 to 20 mils in thickness, and consequently,
the vertical dimension of the package may be rather small. The insulative
and conductive elements of the package are all easily formed of relatively
inexpensive materials and the elements are easily assembled.
Of course, the structure of the solar cell package is not dependent on the
slope of the solar cells, i.e., they may be in the form of square or
rectangular or otherwise sloped wafers as well as round wafers.
Also, it should be noted that the contacts of the power conductors to the
P- and N-regions of each solar cell need not be on opposite sides of the
wafer. For example, the solar cell could be provided with wrap-around
metal contact conductors extending around the edge of the wafer from the
top surface to the back surface of the wafer, where contact to the power
conductors could be made. Or, heavily doped diffused regions of the same
conductivity type as the top surface could extend through to the back of
the wafer, where they could contact patterned metallization which in turn
could contact an appropriate power conductor.
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