A monolithic multijunction solar cell was modified by fabricating an integrated circuit inverter on the back of the cell to produce a device capable of generating an alternating current output. In another embodiment, integrated curcuit power conditioning electronics was incorporated in a module containing a solar cell power supply

Evans, J. C., Jr.

English

NASA Technical Reports Server

United States Patent [191 11 11 4,217,633 
Evans, Jr. 1451 Aug. 12, 1980 
SOLAR CELL SYSTEM HAVING 
ALTERNATING CURRENT OUTPUT 
Inventor: John C. Evans, Jr., Ravenna, Ohio 
Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D. C. 
20546 
Appl. No.: 65,675 
Filed: Aug. 10, 1979 
Related U.S. Application Data 
Continuation-in-part of Ser. No. 915,050, Jun. 9, 1978, 
abandoned. 
Int. (3.3 ..................... H02M 3/315; HOlL 31/06 
U.S. Cl. ........................ 363/27; 136/249; 136/291; 
363/60: 363/147 
[58] Field of Search ........... 136/89 P, 89 MS, 89 AC; 
363/60, 147, 27, 61, 135; 58/23 C 
[561 References Cited 
U.S. PATENT DOCUMENTS 
4,080,221 3/1978 Manelas ........................... 136/89 PC 
451160 9/1975 U.S.S.R. .................................... 363/60 
FOREIGN PATENT DOCUMENTS 
OTHER PUBLICATIONS 
M.D. Lammert et al., “The Interdigitated Back Contact 
Solar Cell”, IEEE Trans. Electron Devices, vol. ED-24, 
Primary Examiner-Aaron Weisstuch 
Attorney, Agent, or Firm-N. T. Musial; J. R. Manning; 
G. E. Shook 
[571 ABSTRACT 
A monolithie multijunction solar cell is modified by 
fabricating an integrated circuit inverter on the back of 
the cell to produce a device capable of generating an 
alternating current output. In another embodiment, 
integrated circuit power conditioning electronics is 
incorporated in a module containing a solar cell power 
pp, 337-342 (1977). 
supply. 
5 Claims, 6 Drawing Figures 
13 
https://ntrs.nasa.gov/search.jsp?R=19810004031 2020-03-21T08:40:51+00:00Z
U.S. Patent Aug. 12,1980 Sheet 1 of 2 
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4,2 17,633 
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FIG. I 
FIG. 3 
35 
FIG. 2 
U.S. Patent Aug. 12, 1980 Sheet 2 of 2 
FIG. 4 
f 35 
-A- 
IS 
F53 
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4,217,633 
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FIG. 6 
7 FIG. 5 
4,2 17,633 
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gion for electronically converting the cell’s direct cur- 
rent output to alternating current of a desired frequency 
and amplitude. Arrays of such devices when commonly 
SOLAR CELL SYSTEM HAVING ALTERNATING 
CURRENTOUTPUT 
triggered, as by the aforementioned power company 
5 source, can produce energy with greater efficiency than 
present systems utilizing external power conditioners. 
The obviation of power conditioning equipment and the 
accomplishment Of built-in voltage control and fre- 
quency synchronization and stabilization can result in 
10 an estimated capital savings of over fifty percent in the 
system investment. Of course, the simplified embodi- 
meqt shown herein to depict the concept may be re- 
,placed in practice by a sophisticated integrated circuit 
microprocessor built into the solar cell wafer. 
In a second embodiment of the invention a module is 
ORIGIN OF THE INVENTION 
The invention described herein was made by an em- 
ployee of the United States Government and may be 
manufactured and used by or for the Government for 
governmental purposes without the payment of any 
royalties thereon or therefor. 
STATEMENT OF COPENDENCY 
This application is a continuation-in-part of applica- 
tion Ser. No. 915,050 which was filed June 9, 1978, now 
abandoned. l5 
BACKGROUND OF THE INVENTION 
The present invention relates generally to photovol- 
taic cells and more particularly to solar cell systems for 
power generation. 
The P-N solar cell is inherently a low voltage device 
taking the form of a wafer of semiconductor material, 
such as single crystal silicon. The usual silicon solar cell 
presently in use for terrestrial power arrays is a disc 
approximately 7.5 centimeters in diameter and 0.2 milli- 
meters in thickness. The front surface of the wafer con- 
tains a p-n junction in the proximity of the surface and 
has a current collecting metallic grid structure contact- 
ing the uppermost layer. The back of the solar cell has 
a metallic film covering its entire surface and acting as 
the counter or return electrode. The output of the sili- 
con photovoltaic solar cell is about 0.5 volts open cir- 
cuit with a current depending upon the exposed area 
and efficiency of the device, but which may amount to 
slightly over one ampere for the 7.5 cm. circular terres- 
trial cell mentioned. 
To produce useful quantities of electrical power from 
silicon solar cells, many must be interconnected in ar- 
rays directed at the sunlight. Since each cell by itself 
produces only a small fraction of the desired power, the 
cells are organized and interconnected in series-parallel 
networks which feed bus bars going to power condi- 
tioning equipment. The power conditioning equipment 
converts the direct current to alternating current for 
transfer by the power company grids to users. Some of 
the equipment used for power conditioning includes 
inverters or converters of the static or rotary form, 
transformers and storage batteries. Much of this equip- 
ment is expensive, cumbersome, and requires continual 
maintenance. In the conversion of direct current solar 
energy, appreciable losses occur in the power condi- 
tioning equipment largely due to hysteresis and joule 
heating effects. Also, solar energy depends upon sun- 
light which continually fluctuates in its intensity, partic- 
ularly in northern climes. Thus, it is desirable to design 
an ‘interactive’ solar power system which shares load 
responsibility with the local utility company power 
sources, such as nuclear or fossil fueled power sources. 
BRIEF SUMMARY OF THE INVENTION 
It is therefore one object of the present invention to 
provide an improved solar cell system. 
It is another object of the present invention to pro- 
vide an improved solar cell system in which efficiency 
of power conversion can be maximized. 
The objects of the present invention are achieved in a 
first embodiment by including in the back of a mono- 
lithic multijunction solar cell an integrated circuit re- 
20 
25 
30 
35 
provided including an array of series-connected solar 
cells for generating d.c. power and an integrated circuit 
connected to the solar cells for changing the low volt- 
age d.c. power to higher voltage a.c. power. The solar 
cells and the integrated circuit are mounted and encap- 
sulated on a rigid supporting structure, such as a heat 
sink which also contains a standard three prong outlet 
receptacle to deliver the a.c. power to a suitable load. 
The module can be used by itself, as for example, to 
supply energy to a small fluorescent light or to a televi- 
sion set at a remote site, or as part of a large system 
which, in turn, is permanently tied to a load system and 
a power company grid. 
The foregoing as well as other objects, features and 
advantages of the present invention will become more 
apparent from the following detailed description when 
taken in conjunction with the appended drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a bottom plan view of a solar cell having an 
inverter integrated into its back surface according to a 
first embodiment of the invention. 
FIG. 2 is a schematic diagram of the inverter of FIG. 
45 
50 
55 
60 
65 
FIG. 3 is a plan view of a heat sink supporting a solar 
cell power supply and power conditioning circuitry 
according to a second embodiment of the invention. 
FIG. 4 is a schematic diagram of the power condi- 
tioning circuitry of FIG. 3. 
FIG. 5 is a top plan view of a solar cell showing the 
wraparound front collector, and 
FIG. 6 is a side view showing the dielectric or insulat- 
ing layer over the base region. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENTS 
Referring now to the drawings, wherein like refer- 
ence numerals designate identical or corresponding 
parts, there is illustrated in FIG. 1 one embodiment of 
the invention. FIG. 1 shows a bottom plan view of a 
monolithic multijunction solar cell 11 having an inte- 
grated circuit inverter 13 shown in detail in FIG. 2 
fabricated on its back surface and having contact pads 
15 and 17 connected to the output terminals of the in- 
verter. The solar cell 11 is of a new type comprising 
preferably six interdigitated sub-cells acting as voltage- 
generating regions 20 all electrically connected in series 
as shown in FIG. 1. The output so connected is a poten- 
tial of approximately three volts in bright sunlight. This 
voltage is sufficient to drive controlled rectifiers and 
transistors so that oscillation occurs, and inversion from 
direct to alternating current is accomplished. Included 
4.2 17.633 
3 
on this same single crystal silicon wafer along one edge 
thereof are the attendant triggering and control circuit- 
ries to regulate the alternating current output of the 
integrated circuit inverter. 
The silicon wafer body on which the device is fabri- 
cated i s  of p-type, high carrier lifetime single crystal, 
and boron doped. The bulk resistivity of the material 
may range from one ohm-centimeter to ten thousand 
ohms-centimeter. The corresponding excess carrier 
lifetimes for this material correspondingly will range 
from one microsecond to ten thousand microseconds or 
greater. It is necessary to use the greatest lifetime mate- 
rial which can be obtained in order to assure that an 
efficient device may be made, this being a requirement 
for interdigitated structures. Relatively high efficiency 
devices have been made using two ohm-centimeter 
boron-doped silicon with a measured lifetime between 
ten and twenty microseconds. 
In the described preferred embodiment, the wafer 
thickness may range between fifty micrometers, or 
nominally two mils, to two-hundred-and fifty microme- 
ters, or nominally ten mils. Satisfactory solar cells have 
been fabricated from material five mils in thickness. 
The solar cell voltage-generating regions 20 are fabri- 
cated by well-known microelectronic and furnace diffu- 
sion techniques. They may also be made by ion implan- 
tation with proper masking techniques. In practice, 
phosphorus may be used to form the front and back n+ 
junctions and these may be diffused simultaneously. 
Front texturization may be used to cut reflection losses 
or an anti-reflection coating of tantalum pentoxide or 
similar AR material may be deposited on the completed 
devices. Thermal masking has been used to form the 
interdigitated finger regions with either boron or alumi- 
num for the p + diffusions. Aluminum requires a drive- 
in temperature in excess of six-hundred-and-fifty de- 
grees Celsius after deposition by vacuum evaporation or 
screen printing. 
Front and back contact metallization is accomplished 
in a conventional manner. Titanium. palladium-silver 
has proved excellent for both p and n regions. A typical 
front contact of wraparound configuration which cov- 
ers the n regions is shown in FIGS. 5 and 6 while a 
plurality of interdigitated diffused n and p regions are 
covered by the back metallization pattern shown in 
FIG. 1 to collect additional current. 
The respective front n + collectors 19 shown in FIG. 
5 are joined by their metallization shown in FIG. 6 to 
their corresponding rear or back n+ contacts 19 shown 
in FIG. 1 while the remaining portions 21 of the metalli- 
zation pattern cover the p regions of the back surface. It 
is necessary to isolate the wraparound contact metal 
from the p-type base region over which it passes at the 
wafer edge, and a dielectric layer 22 shown in FIG. 5 is 
applied under the metal contact material. Similarly, it is 
also necessary to series interconnect one voltage- 
generating region to the next as well as the other surface 
conductors shown in the figures by implanting a narrow 
band of metal such as aluminum into the wafer to the 
depth of the diffused regions, thus linking the p+ of one 
region to the n+ of the succeeding region. Also, screen 
printed glass as well as oxide layer isolation have been 
used successfully for such metallizations. 
The components required tz form the integrated 
circuit inverter 13 to change the direct current of the 
solar cell 11 to the desired alternating current output are 
shown in their respective locations in FIG. 1. In this 
plan view is shown a greatly simplified but functional 
4 
arrangement using integrated circuit fabrication. FIG. 2 
is the schematic diagram of the inverter 13. 
In the embodiment shown, capacitor 23 and inductor 
25 may be formed by diffusion and/or thin film deposi- 
5 tion techniques, while a conventional triggering circuit 
24 can be likewise integrated by any well-known inte- 
grated circuit technique. The controlled rectifiers 27 
and 29 may be formed by diffusion techniques well 
known in the art. 
The interconnections are shown in FIGS. 1 and 2. 
SCR 27 has its anode connected to metallization pattern 
21; its gate connected to triggering circuit 24 at contact 
28; and its cathode connected to inductor 25 and to the 
anode of SCR 29. SCR 29 has its gate connected to 
15 triggering circuit 24 at contact 30; and its cathode con- 
nected to metallization pattern 19 and to capacitor 23. 
Capacitor 23 is connected to contact pad 15 and to 
inductor 25 and contact pad 17. 
The inverter 13 changes the d.c. power generated by 
20 the solar cell 11 to an a.c. power output at the contact 
pads 15 and 17 which can then be transferred to a suit- 
able load 32. A contact pad for synchronized triggering 
control may be optionally provided. 
Referring to FIG. 3, there is illustrated a second 
25 embodiment of the invention. FIG. 3 shows in plan 
view a rigid structure such as a heat sink 31 supporting 
a solar cell power supply 33 and power conditioning 
circuitry 35. The solar cell power supply comprises a 
plurality of series-connected solar cells. The power 
30 conditioning circuitry 35 (FIG. 4) comprises a high 
frequency inverter 37 of the type illustrated in FIG. 2 
for changing the d.c. power output of the solar cell 
power supply to high frequency ax. power; a cascade 
voltage multiplier 39 connected to the inverter 37 for 
35 converting the a.c. power to d.c. power of a higher 
amplitude; and a 60 Hz. inverter 41 of the type illus- 
trated in FIG. 2 and connected to the multiplier 39 for 
changing the high voltage d.c. power output of the 
multiplier to a.c. power at 60 Hz. The multiplier illus- 
40 trated is a Cockcroft WaIton cascade multiplier. Such 
multipliers have been made for many devices using 
discrete circuit components. The custom multiplier is 
constructed on a single wafer with integrated technol- 
ogy well known in the art. It will be tailored to produce 
45 one-hundred-and-twenty volts at a frequency of sixty 
Hertz delivered across the output capacitor 81 and the 
load 82 shown in FIG. 4. A three-prong receptacle (not 
shown) is disposed in the heat sink and is connected to 
the output of the 60 Hz. inverter 41 for delivering the 
The interconnections are shown in FIG. 4. The SCR 
43 has its anode connected to the positive side of the 
power supply 33; its gate connected to a triggering 
circuit at contact 44, and its cathode connected to in- 
55 ductor 45 and to the anode of SCR 47. SCR 47 has its 
gate connected to the triggering circuit at contact 48; 
and its cathode connected to the negative side of the 
power supply 33. Capacitor 49 is connected across in- 
ductor 45 and the cathode of SCR 47. One stack of 
60 capacitors 51, 53, 55 is connected at one end of the 
negative side of the power supply 33 and at the other 
end to the anode of SCR 57. The second stack com- 
prises capacitors 59, 61, 63 connected at one end to 
inductor 45. Diodes 67, 69, 71, 73, 75, and 77 intercon- 
65 nect the capacitors of the two stacks. SCR 57 has its 
gate connected to a second triggering circuit at contact 
58 and its cathode connected to inductor 65 and to the 
anode of SCR 79. SCR 79 has its gate connected to the 
10 
50 generated a.c. power to a suitable load 82. 
4,217,633 
5 6 
second triggering circuit at contact 80; and its cathode 
connected to the negative side of the power supply 33. 
Capacitor 81 is connected to the inductor 65 and the 
cathode of SCR 79. The output load 82 is connected 
across capacitor 81. 
Obviously, numerous additional modifications and 
variations of the present invention are possible in light 
of the above teachings. It is, therefore, to be understood 
that within the scope of the appended claims the inven- 10 
tion may be practiced otherwise than as specifically 
described herein. verter. 
integrated circuit means incorporated in said Semi- 
conductor body along an edge thereof and includ- 
ing a plurality of rectifiers and transistors for 
changing the direct current power to alternating 
current power. 
2. The solar cell system recited in claim 1 including 
a heat sink; and 
wherein the solar cell and the integrated circuit 
3. The solar cell system recited in claim 1 wherein 
the integrated circuit means comprises a SCR in- 
4. The solar cell system recited in claim 1 wherein the 
means are mounted on the heat sink. 
What is claimed is: 
1. A solar cell system having an alternating current integrated circuit means includes 15 a first inverter; 
a voltage multiplier connected to the inverter; and 
5. The solar cell system recited in claim 4 wherein the 
output comprising 
a monolithic solar cell comprising a semiconductor a second inverter connected to the voltage multiplier. 
body having a Plurality of voltage generating re- 
gions connected in series to provide a direct cur- voltage multiplier is a Cockcroft-Walton circuit. * * * * *  rent potential, and 20 
25 
30 
35 
45 
50 
55 
60 
65 


Solar cell system having alternating current output

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