High voltage planar multijunction solar cell

A high voltage multijunction solar cell is provided wherein a plurality of discrete voltage generating regions or unit cells are formed in a single generally planar semiconductor body (12). The unit cells comprise a doped regions (20, 22) of opposite conductivity type separated by a gap or undiffused region (24). Metal contacts (26) connect adjacent cells together in series so that the output voltages of the individual cells are additive. In some embodiments, doped field regions (14) separated by gap (16) overlie the unit cells but the cells may be formed in both faces of the wafer (FIG. 2).

TECHNICAL FIELD 
The present invention relates to solar cells and, in particular, to an 
improved solar cell construction capable of generating comparatively high 
voltages. 
BACKGROUND ART 
Photovoltaic power systems are undergoing rapid development as primary 
power sources for space as well as for terrestial uses. The early space 
shuttle flights will utilize a power extension pack (PEP) solar cell array 
to provide approximately 32 kilowatts of electrical power from over 
seventy thousand large area silicon solar cells. This solar cell array 
will measure approximately 240 feet in length and will be capable of being 
retrieved and returned to Earth between missions. Domestically, arrays 
producing up to one-half megawatt of electrical power are being produced. 
Further, in addition to these and other large power projects, there are 
many other specialized applications wherein there are unique power 
requirements. Thus, there exists a serious need for solar cell 
constructions which provides a high voltage output and can serve pressing 
demands both on the Earth and in space. 
Early work in this field began with the "solar battery" developed by Bell 
Telephone Laboratories in 1954. (See Chapin et al, "Bell Solar Battery", 
Bell Laboratories Record, Vol. 37, 1955, pp. 241 et seq.). This battery or 
cell basically comprised a "wraparound" junction device wherein the base 
was covered with a thin diffused layer over its entire surface except for 
a circular area etched on the back of the cell. Ohmic contacts were made 
to the respective "n" and "p" regions. This "wraparound" device evolved 
into a rectangular planar form with a diffused surface having a metal grid 
collector on the top side and a metal pad collector on the bottom. This 
form of cell is still the most commonly used for space solar cells. 
Reference is made to Matlow et al, "Ohmic Aluminum-n-Type Silicon 
Contact", Journal of Applied Physics, Volume 30, April 1959, pp. 541-543 
for a discussion of such cells. 
In the 1960's, a photovoltaic device, the thermophotovoltaic (TPV) cell, 
was developed which converted a narrow portion of the thermal spectrum 
into electrical energy. The device utilized an interdigitated contact 
structure covering the back of a germanium cell wafer. Because the device 
utilized narrow band heat radiation, photons are absorbed uniformly 
throughout the bulk of the wafer, and thus this type of back contact was 
satisfactory. However, for most visible light conversion, wherein most of 
the light is absorbed close to the surface and wherein migration of the 
carriers is effected by diffusion to the collectors, such a back contact 
arrangement requires several concessions. For example, the device must be 
thin and must be made of high quality semiconductor material having a very 
high lifetime. The TPV cell evolved into the interdigitated back contact 
silicon solar cell. 
In more recent times, a modification of the coplanar back contact solar 
cell was developed which is known as the Tandem Junction (TJC) solar cell. 
This device relies on a front surface field region in order to lower 
surface recombination losses and to provide an electrostatic field region 
which assists in sending the carriers to the rear surface of the cell for 
collection. Reference is made to Chiang et al, "Thin Tandem Junction Solar 
Cell", Conference Record, 13th IEEE Photovoltaic Specialists Conference, 
IEEE, Inc., New York, 1978, pp. 1290-1293 and to U.S. Pat. No. 4,113,698 
(Chiang et al) for a further discussion of TJC cells. The subject matter 
of these materials is hereby incorporated by reference. Other important 
developments include Vertical Multijunction (VMJ) solar cell (see Gorandia 
et al, IEEE Trans. on Electron Devices, Vol. ED-24, p. 342, April 1977), 
the Horizontal Multijunction (HMJ) solar cell (see U.S. Pat. No. 3,994,012 
(Warner)), and the V-Groove Multijunction (VGMJ) solar cell (see Chappell, 
IEEE Trans. on Electron Devices, Vol. ED-26, p. 1,091, July 1979). 
In general, all of the more recently developed devices, including the IBC, 
TJC, VMJ, HMJ, and VGMJ solar cells, represent improvements over the prior 
art particularly with respect to reduction or elimination of 
grid-shadowing and the sheet resistance component of series resistance. 
Further, all of the cells except for the IBC and TJC cells, operate as 
high-voltage low-current devices and thus provide a reduction in series 
resistance as well as in temperature effects due to I.sup.2 R heating. In 
addition, significant transparency to infrared photons further reduces 
high temperature effects in these cells. 
Other patents of possible interest in this field include U.S. Pat. Nos. 
3,150,990 (Rudenberg); 3,278,337 (Gault) 3,969,746 (Kendall et al); 
3,982,964 (Lindmayer et al); 4,052,228 (Russell); 4,072,541 (Meulenberg, 
Jr. et al); 4,099,199 (Wittry); 4,101,351 (Shah et al); 4,128,733 (Fraas 
et al); 4,131,486 (Brandhorst, Jr.); 4,135,950 (Rittner); 4,160,678 (Jain 
et al); 4,166,919 (Carlson); 4,167,015 (Hanak); 4,217,633 (Evans, Jr.). 
SUMMARY OF THE INVENTION 
In accordance with the invention, an improved solar cell of the 
photovoltaic type is provided which produces a high output voltage. 
Generally speaking, the cell comprises a plurality or array of discrete 
voltage-generating regions or unit cells formed within a semiconductor 
wafer and integrally interconnected in such a manner so as to provide a 
single device having a high voltage output. Stated differently, the high 
voltage multijunction solar cell of the invention comprises a generally 
planar semiconductor body and a plurality of unit cells formed in at least 
one surface of the semiconductor body, the unit cells each comprising a 
pair of doped regions of opposite conductivity type separated by a gap 
therebetween and the unit cells being connected together so that the 
voltage outputs of the individual cells are additive. 
The invention is particularly concerned with the problems of degradation 
and efficiency loss due to heating in the device. There are two primary 
causes of this heating, the trapping and absorption of infrared or heating 
photons, and heating due to internal electrical resistance, sometimes 
referred to as "Joule" heating. The invention provides a photovoltaic type 
cell which avoids both of these sources of energy conversion loss. More 
specifically, in the solar cell device of the invention, the photons 
capable of producing ionization are absorbed in a high-lifetime bulk 
region in the semiconductor wafer. The hole-electron pairs are collected 
at the doped regions which are preferably arranged in parallel and which 
form the unit cells within the wafer. In a specific embodiment, each unit 
cell comprises an n+ doped region separated by a gap from a p+ doped 
region. These regions may be formed by thermal diffusion or ion 
implantation. In this specific embodiment, the individual 
voltage-generating unit cells are preferably connected together within the 
bulk semiconductor by a narrow implanted or diffused ribbon of metal, such 
as aluminum, which serves to connect the unit cells in series so as to 
produce an overall output voltage which is the sum of the contributions of 
all of the unit cells within the semiconductor wafer. For embodiments 
wherein the device is rectangular in shape, the output of the solar cell 
is collected at opposite edges of the semiconductor wafer. 
Although a rectangular embodiment has been referred to above, it is to be 
understood that the planar multijunction solar cell of the invention can 
be made in a number of different configurations and, in this regard, a 
circular shape utilizing concentric voltage generating region is 
advantageous for some applications. Further, collection can be from one 
side only and this one side may be the front or back of the device, 
depending upon the desired results. Further, the solar cell of the 
invention can be fabricated as a double-sided device with voltage 
generating regions (unit cells) on both sides and collection may be from 
both surfaces. In addition, the device of the invention can be used at one 
Sun normal incidence or can be used in conjunction with lens or reflector 
concentration techniques that produce very high solar intensity levels. 
The heating problem discussed above is minimal with the device of the 
invention for two main reasons. First, the device has almost no surface 
metallization and thus permits substantially free passage of infrared 
energy completely through the solar cell. Thus, the device requires little 
or no heat-sinking or cooling. Second, the device of the invention 
exhibits very little internal electrical resistance, and the energy is 
extracted at high voltage and low current from the device. It will be 
understood that heating is related to the current flow and is proportional 
to the product of the square of the current flowing through the device and 
the internal impedance or resistance of the device (W=I.sup.2 R). 
It will be appreciated by those skilled in the photovoltaic art that the 
planar multijunction solar cell of the invention combines the better 
features of the interdigitated back contact (IBC) and tandem junction 
(TJC) cells with the vertical multijunction (VMJ) and horizontal 
multijunction (HMJ) solar cells discussed above. The device of the 
invention also is superior in a number of respects to these devices and to 
the V-groove multijunction solar cell (VGMJ) discussed above. For example, 
the IBC and TJC devices are not high voltage solar cells and require 
substantial surface metallization for collection, whereas the device of 
the present invention does not. Further, the HMJ device involves a rather 
complex fabrication process while the VGMJ device involves physically 
separate semiconductor unit cells held together on a glass plate. 
Moreover, none of the devices of the prior art provide high voltage in a 
single substrate construction of simple fabrication. 
Other features and advantages of the invention will be set forth in, or 
apparent from, the detailed description of the preferred embodiments found 
hereinbelow.

DETAILED DESCRIPTION 
Referring to FIG. 1, a schematic cross section of one embodiment of the 
planar multijunction high voltage solar cell (PMJ) of the invention is 
shown. The solar cell, which is generally denoted 10, includes a base 
region 12 preferably comprising a silicon crystal of a very high lifetime 
which may be suitably doped with, e.g., boron, to be of n or p type. In a 
specific example, a 100 ohm-cm p-type crystal having a diffusion length of 
400 micrometers and a thickness of 75 to 100 micrometers was utilized. 
Although experiments have shown an efficiency peak at around 100 ohm-cm 
p-type material, it is believed that the planar multijunction cell of the 
invention will operate at resistivities of between about 0.01 ohm-cm and 
30,000 ohm-cm and it is noted that advantages related to radiation 
resistance at higher resistivities may compensate for any conversion 
efficiency losses found at such higher resistance values. It is noted that 
although silicon is utilized in the preferred embodiment, the cell of the 
invention may be made from GaAs or other IIIA-VA semiconductors and from 
IIB-VIA compounds such as CdS. 
Formed in the illuminated top surface of the base region 12 are a plurality 
of n+ or p+ unit cell field regions denoted 14. Each of the regions 14 is 
separated from the adjacent regions by a gap 16 which may comprise an 
undiffused region in base region 12. The field regions 14 correspond to 
the unit cells which are described below and which are disposed directly 
beneath corresponding regions 22 on the bottom of the base region. The 
top, light-receiving surface of the cell 10 is preferably coated with an 
anti-reflective layer or coating 18 although other techniques such as 
texturizing can also be used. In addition, this top surface can also be 
treated or modified to reduce surface recombination velocity and thus 
increase conversion efficiency. This may be accomplished by the inclusion 
of a trapped charge oxide layer or by the inclusion of a front surface 
field of dopant ions. The oxide layer may be formed by wellknown MOS 
technology while the dopant layer may be thermally diffused or ion 
implanted. 
The unit cells each comprise a pair of doped regions separated by a 
precisely controlled gap or undiffused region; in FIG. 1, the doped 
regions are a relatively narrow p+ base region 20 and a relatively wide n+ 
collector region 22 separated by a gap 24. The unit cells, which have a 
total width W as indicated in FIG. 1, are connected together by metal 
(e.g., aluminum) contacts 26 located between the n+ collector regions and 
p+ base regions of adjacent cells. Contacts 26 serve to connect the 
individual unit cells or sub-cells within the base 12 in series so that 
the individual contributions of the unit cells can be summed together. 
In a specific exemplary embodiment, regions of phosphorous, boron and 
aluminum are implanted into the base crystal 12 to form the unit 
voltage-generating subcells, with the widths (denoted Wn) of the n+ 
regions 22 being about 100 micrometers, the widths (denoted Wp and Wg, 
respectively) of the p+ regions 20 and the gap or undoped regions 24 being 
about 25 micrometers. In this exemplary embodiment, the aluminum regions 
which electrically connect the sub-cells may be from about 5 micrometers 
to a greater width, depending on the forming technology used to fabricate 
the cells. 
Metal contacts or terminals 26 for connection to an external circuit or 
circuits are located at opposite edges of the cell 10. 
It will be appreciated that the exemplary values set forth above are merely 
exemplary and in no way limiting. Although the mathematical considerations 
that should be taken into account in the design of a solar cell of the 
invention have not been completely worked out, certain preliminary design 
values for key parameters can be arrived at from physical considerations 
and from theory developed in connection with the tandem junction solar 
cell to which reference was made above. From simply physical 
considerations, the longest path which a photo-generated electron must 
travel in a cell of thickness h in order to be collected is approximated 
by the formula 
##EQU1## 
This path length should be shorter than a diffusion length of the 
electrons in the base region to obtain high collection efficiency. Thus, 
the values of the parameters h and (Wp++Wg)/2 must be individually less, 
and preferably much less, than the electron diffusion length in the base 
region. With this constraint it has been determined experimentally for a 
tandem junction cell that "stripe" widths of Wn=0.85 W, Wp+=0.1 W and 
Wg=0.05W give high collection efficiency without sacrificing the full 
factor due to high series resistance and these same fractional stripe 
widths are thought to hold with the planar multijunction cell of the 
invention. For a low voltage PMJ solar cell, with only eight unit cells 
per centimeter, the relationships set forth above result in the following 
values for the parameters in question: W=1,250 .mu.m; Wn=1,063 .mu.m, 
Wp+=125 .mu.m, Wg=62 .mu.m. For a high voltage PMJ cell with 40 unit cells 
per centimeter the values are: W=250 .mu.m, Wn=213 .mu.m, Wp+=25 .mu.m and 
Wg=12 .mu.m. In these examples, if the cell thickness is 75 .mu.m, the 
path length for collection will be 120 .mu.m in the low voltage cell and 
77 .mu.m in the higher voltage cell. It is noted that a diffusion length 
in the base of 250 .mu.m or greater would ensure a high collection 
efficiency in either cell provided that surface recombination losses at 
the front and back of the cell are low. Based on related experimental 
results, the junction depth of the diffused regions 14 on the top or front 
surface of the cell 10 should be 0.2 to 0.3 .mu.m while the depths of the 
back surface junctions are preferably equal to or greater than 1 
micrometer in order to reduce series resistance. 
In early experimental cells, Czochralski grown silicon wafers, boron-doped 
of 2 and 10 ohm-cm resistivities and of circular shape were used as the 
base or substrate material. An n+/p junction was formed over the entire 
surface of each circular wafer with conventional phosphine diffusion in a 
tube furnace at 850.degree. C. for 30 minutes. The diffusion oxide was 
removed from one face of the wafer and a pattern of parallel bars of 
aluminum paste was screen-printed on the cleaned surface and fired in a 
belt furnace at 650.degree. C. with a 40 second pass. This firing time was 
sufficient to assure penetration of the aluminum through the diffused n+ 
region on this face. The wafer was then cut into a 1.5 cm by 1.5 cm 
device. A narrow portion of the n+ region was removed alongside of each 
aluminum bar on one side of the wafer by making a shallow cut with a 
dicing saw. This technique was used to form the required gap between 
corresponding regions 20 and 22. The contacts 26 were fabricated of fired 
silver paste. 
It is noted that initial testing has shown that the isolation of individual 
unit cell regions 14 over each of the unit cells formed by doped regions 
20, 22 is quite important with respect to the output voltages generated. 
This isolation can be effected, for example, by providing shallow grooves 
between the regions 14. 
Other cells have been made by modifying TJC solar cells wherein several 
regions were connected together in series on a single substrate, with each 
region, in effect, constituting a unit cell having a surface field and 
interdigitated back regions. In another approach involving TJC solar 
cells, the side bars of such cells were removed and every other gap was 
shorted out so as to produce unit cell interconnections. This shorting was 
accomplished by applying small dots of silver paste between the contact 
bars and firing. Leads were attached to the terminating n+ and p+ regions 
in a similar manner. The front field region of all of the unit cells were 
separated by shallow cuts through the diffused layer, using a dicing saw. 
These cuts were precisely located over the unit cell boundaries as 
described above. 
Two further embodiments of the invention are illustrated in FIGS. 2 and 3. 
The embodiment of FIG. 2 is similar to that of FIG. 1 and corresponding 
elements in FIG. 3 have been given the same reference numberals as in FIG. 
1 but with primes attached. The only basic difference between the 
embodiments of FIGS. 1 and 2 is that, in FIG. 2, the unit field regions 14 
of FIG. 1 have been replaced by collector regions similar to those in the 
other surface, i.e., collection regions are formed in both surfaces of the 
p-type base 10'. As in the embodiment of FIG. 1, unit cells are formed by 
p+ and n+ regions 20' and 22' separated by a gap, and metal contacts 26' 
are used to connect the unit cells together. The collector regions in the 
two surfaces overlie one another as illustrated and a pair of contacts 40 
located at opposite edges of the cell assembly are used to make connection 
to the "terminating" cells, i.e., those at the lateral edges of the 
assembly. 
The embodiment of FIG. 3 is also similar to that of FIG. 1 and 
corresponding elements in FIG. 3 have been given the same reference 
numerals as in FIG. 1 but with double primes attached. The basic 
difference between the embodiments of FIG. 1 and FIG. 3 is that, in the 
latter, the base 10" is circular in shape and the various regions are 
disposed in concentric circles. As in the embodiment of FIG. 1, the unit 
cells formed by doped regions 20" and 22" are disposed below a field 
region 14" which precisely overlies the unit cell, and contacts 26" are 
used to connect the unit cells together. The outermost p+ region 20" is 
connected to a circular positive terminal formed by metallization 44 while 
the innermost n+ region is connected to a negative or minus terminal 42. A 
circular solar cell such as shown in FIG. 3 has advantages when, for 
example, it is used in combination with a circular lens or 
concentrator-reflectior. It will be understood that this embodiment, 
similarly to the rectangular cell of FIG. 2, can also employ collector 
regions on both sides and that such double sided cells may also be 
illuminated from one or both sides. 
Briefly considering the operation of the planar multijunction solar cell of 
the invention, and referring to FIG. 1, when photons of the proper energy 
are absorbed in the silicon wafer 10, hole-electron pairs are often 
produced and these charge carriers are free to move under the field forces 
of the junction. Carriers are separated and collection and utilization 
through an external load circuit (not shown) will take place so that solar 
energy is thus converted to electricity. All photovoltaic solar cells 
follow this basic sequence of absorption, pair production, charge 
separation and collection. When the surface counter-doped region is moved 
to the bottom of the device, as in the IBC devices discussed above, the 
same principles apply, although the electric field configuration may be 
somewhat different. The collection and current flow is in a sheet close to 
the surface and along the surface, rather than through the thickness of 
the device. The presence of the dopant ions within the crystal lattice in 
the solar cell of the invention provides a built-in electrical field for 
the separation of the opposite charge carriers. The collected electrons, 
which comprise the carriers which move physically through the external 
circuit referred to above are driven by an electromotive force which is 
generated across the device as long as the photons are available and being 
used. The amount of electron current available in a properly matched 
external load is a function of several factors, including the solar 
conversion efficiency of the cell, the illuminated active area of the 
device and the solar intensity and it is possible to increase the latter 
greatly by the use of lenses or concentractor-reflectors as referred to 
above. The output voltage and the current of the device may increase 
dramatically with high light levels. 
In our concurrently filed, copending application Ser. No. 219,678 entitled 
HIGH VOLTAGE V-GROOVE SOLAR CELL, there is disclosed a method of making a 
planar multijunction solar cell wherein grooves are formed in the surface 
of the planar wafer and doped regions of opposite conductivity type formed 
in the faces of the grooves. It is to be understood that the term 
"surface", as applied to the semiconductor body, when used in this 
specification and in the claims, without further description, refers to a 
flat surface such as shown in FIGS. 1 to 3 as well as to a grooved or 
otherwise non-planar surface. 
Although the invention has been described in relation to exemplary 
embodiments thereof, it will be understood by those skilled in the art 
that variations and modifications can be effected in these exemplary 
embodiments without departing from the scope and spirit of the invention.