U.S. patent application number 11/956069 was filed with the patent office on 2009-06-18 for exponentially doped layers in inverted metamorphic multijunction solar cells.
This patent application is currently assigned to Emcore Corporation. Invention is credited to Arthur Cornfeld, Vance Ley, Mark A. Stan.
Application Number | 20090155952 11/956069 |
Document ID | / |
Family ID | 40622227 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090155952 |
Kind Code |
A1 |
Stan; Mark A. ; et
al. |
June 18, 2009 |
Exponentially Doped Layers In Inverted Metamorphic Multijunction
Solar Cells
Abstract
A method of forming a multifunction solar cell including an
upper subcell, a middle subcell, and a lower subcell, including
providing first substrate for the epitaxial growth of semiconductor
material; forming a first solar subcell on the substrate having a
first band gap; forming a second solar subcell over the first solar
subcell having a second band gap smaller than the first band gap;
forming a grading interlayer over the second subcell, the grading
interlayer having a third band gap greater than the second band
gap; and forming a third solar subcell over the grading interlayer
having a fourth band gap smaller than the second band gap such that
the third subcell is lattice mismatched with respect to the second
subcell, wherein at least one of the bases of a solar subcell has
an exponentially doped profile.
Inventors: |
Stan; Mark A.; (Albuquerque,
NM) ; Cornfeld; Arthur; (Sandia Park, NM) ;
Ley; Vance; (Albuquerque, NM) |
Correspondence
Address: |
Emcore Corporation
1600 Eubanks Blvd., SE
Albuquerque
NM
87123
US
|
Assignee: |
Emcore Corporation
Somerset
NJ
|
Family ID: |
40622227 |
Appl. No.: |
11/956069 |
Filed: |
December 13, 2007 |
Current U.S.
Class: |
438/94 ;
257/E21.002 |
Current CPC
Class: |
H01L 31/03046 20130101;
Y02P 70/521 20151101; Y02P 70/50 20151101; H01L 31/0693 20130101;
H01L 31/1844 20130101; H01L 31/06875 20130101; H01L 31/03042
20130101; Y02E 10/544 20130101 |
Class at
Publication: |
438/94 ;
257/E21.002 |
International
Class: |
H01L 21/02 20060101
H01L021/02 |
Goverment Interests
GOVERNMENT RIGHTS STATEMENT
[0005] This invention was made with government support under
Contract No. FA9453-06-C-0345 awarded by the U.S. Air Force. The
Government has certain rights in the invention.
Claims
1-20. (canceled)
21. A multifunction solar cell comprising an upper subcell, a
middle subcell, and a lower or bottom subcell, comprising: a first
solar subcell having a base and an emitter having a first band gap;
a second solar subcell having a base and an emitter disposed over
said first solar subcell having a second band gap smaller than said
first band gap; a grading interlayer disposed over said second
solar subcell, said grading interlayer having a third band gap
greater than said second band gap; and a third solar subcell having
a base and an emitter disposed over said grading interlayer having
a fourth band gap smaller than said second band gap such that said
third subcell is lattice mismatched with respect to said second
subcell, wherein at least one of the bases has an exponentially
doped profile.
22. A solar cell as defined in claim 21, wherein the base in said
first solar cell has an exponential gradation in doping from
1.times.10.sup.16 per cubic centimeter adjacent the base-emitter
junction to 1.times.10.sup.18 per cubic centimeter adjacent the
adjoining layer.
23. A solar cell as defined in claim 21, wherein the base in said
second solar cell has an exponential gradation in doping from
1.times.10.sup.16 per cubic centimeter adjacent the base-emitter
junction to 1.times.10.sup.18 per cubic centimeter adjacent the
adjoining layer.
24. A solar cell as defined in claim 21, wherein the base in said
third solar cell has an exponential gradation in doping from
1.times.10.sup.16 per cubic centimeter adjacent the base-emitter
junction to 1.times.10.sup.18 per cubic centimeter adjacent the
adjoining layer.
25. A solar cell as defined in claim 21, wherein the emitter layer
in at least one of the solar cells has an increasing gradation in
doping from 5.times.10.sup.17 per cubic centimeter adjacent the
base-emitter junction to 5.times.10.sup.18 per cubic centimeter
adjacent the adjoining layer.
26. A solar cell as defined in claim 24, wherein said third solar
cell is the bottom subcell and the exponential gradation in doping
results in an increase in short circuit current to a level
approximately equal to the short circuit current in a higher
subcell.
27. A solar cell as defined in claim 21, wherein the first solar
cell is formed on a first substrate composed of GaAs.
28. A solar cell as defined in claim 21, wherein said first solar
subcell is composed of an InGa(Al)P emitter region and an InGa(Al)P
base region.
29. A solar cell as defined in claim 21, wherein said second solar
subcell is composed of an InGaP emitter region and an GaAs base
region.
30. A solar cell as defined in claim 21, wherein said grading
interlayer is composed of InGaAlAs.
31. A solar cell as defined in claim 30, wherein said grading
interlayer is composed of nine steps of layers with monotonically
changing lattice constant.
32. A solar cell as defined in claim 21, wherein said third solar
subcell is composed of InGaAs.
33. A solar cell as defined in claim 21, further comprising a
barrier layer about one micron in thickness disposed adjacent said
grading interlayer for preventing threading dislocations from
propagating.
34. A semiconductor structure for use in manufacturing a solar cell
comprising: a first substrate; a sequence of layers of
semiconductor material forming a solar cell including at least one
base layer with exponential doping; mounting a surrogate substrate
on top of the sequence of layers disposed on said first substrate;
and a surrogate substrate mounted on top of the sequence of
layers.
35. A structure as defined in claim 34, wherein the sequence of
layers of semiconductor material forms a triple junction solar
cell, including first, second and third solar subcells.
36. A solar cell comprising: a top cell including base and emitter
layers composed of InGaP semiconductor material; a middle cell
emitter layer of GaAs semiconductor material and a base layer of
InGaP semiconductor material; and a bottom cell including an
emitter and base layer of InGaAs semiconductor material, wherein at
least one of the base layers has an exponentially doped
profile.
37. A solar cell as defined in claim 36, further comprising a first
substrate for the growth of semiconductor material, wherein said
top cell is directly grown over said first substrate and is lattice
matched thereto, and said bottom cell is lattice mismatched with
respect to said substrate.
38. A solar cell as defined in claim 37, further comprising a
second substrate for mounting the solar cell on a rigid support.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to co-pending U.S. patent
application Ser. No. 11/860,142 and 11/860,183 filed Sep. 24,
2007.
[0002] This application is related to co-pending U.S. patent
application Ser. No. 11/836,402 filed Aug. 8, 2007.
[0003] This application is also related to co-pending U.S. patent
application Ser. No. 11/616,596 filed Dec. 27, 2006.
[0004] This application is also related to co-pending U.S. patent
application Ser. No. 11/445,793 filed Jun. 2, 2006.
BACKGROUND OF THE INVENTION
[0006] 1. Field of the Invention
[0007] The present invention relates to the field of solar cell
semiconductor devices, and particularly to multifunction solar
cells including a metamorphic layer. Such devices also include
solar cells known as inverted metamorphic solar cells.
[0008] a. Description of the Related Art
[0009] Photovoltaic cells, also called solar cells, are one of the
most important new energy sources that have become available in the
past several years. Considerable effort has gone into solar cell
development. As a result, solar cells are currently being used in a
number of commercial and consumer-oriented applications. While
significant progress has been made in this area, the requirement
for solar cells to meet the needs of more sophisticated
applications has not kept pace with demand. Applications such as
satellites used in data communications have dramatically increased
the demand for solar cells with improved power and energy
conversion characteristics.
[0010] In satellite and other space related applications, the size,
mass and cost of a satellite power system are dependent on the
power and energy conversion efficiency of the solar cells used.
Putting it another way, the size of the payload and the
availability of on-board services are proportional to the amount of
power provided. Thus, as the payloads become more sophisticated,
solar cells, which act as the power conversion devices for the
on-board power systems, become increasingly more important.
[0011] Solar cells are often fabricated in vertical, multifunction
structures, and disposed in horizontal arrays, with the individual
solar cells connected together in a series. The shape and structure
of an array, as well as the number of cells it contains, are
determined in part by the desired output voltage and current.
[0012] Inverted metamorphic solar cell structures such as described
in M. W. Wanless et al., Lattice Mismatched Approaches for High
Performance, III-V Photovoltaic Energy Converters (Conference
Proceedings of the 31.sup.st IEEE Photovoltaic Specialists
Conference, Jan. 3-7, 2005, IEEE Press, 2005) present an important
starting point for the development of future commercial high
efficiency solar cells. The structures described in such prior art
present a number of practical difficulties relating to the
appropriate choice of materials and fabrication steps.
[0013] Prior to the present invention, the materials and
fabrication steps disclosed in the prior art have not been adequate
to produce a commercially viable and energy efficient solar cell
using an inverted metamorphic cell structure.
SUMMARY OF THE INVENTION
[0014] The present invention provides a method of forming a
multijunction solar cell comprising an upper subcell, a middle
subcell, and a lower subcell, the method comprising: providing
first substrate for the epitaxial growth of semiconductor material;
forming a first solar subcell having a base and an emitter on said
substrate having a first band gap; forming a second solar subcell
having a base and an emitter over said first solar subcell having a
second band gap smaller than said first band gap; forming a grading
interlayer over said second subcell, said grading interlayer having
a third band gap greater than said second band gap; and forming a
third solar subcell having a base and an emitter over said grading
interlayer having a fourth band gap smaller than said second band
gap such that said third subcell is lattice mismatched with respect
to said second subcell, wherein at least one of the bases has an
exponentially doped profile.
[0015] In another aspect, the present invention provides a method
of manufacturing a solar cell by providing a first substrate,
depositing on the first substrate a sequence of layers of
semiconductor material forming a solar cell, including at least one
base layer with exponential doping; mounting a surrogate substrate
on top of the sequence of layers; and removing the first
substrate.
[0016] In another aspect the present invention provides a method of
manufacturing a solar cell by providing a first substrate,
depositing on the first substrate a sequence of layers of
semiconductor material forming a solar cell, including at least one
base layer with exponential doping; mounting a surrogate substrate
on top of the sequence of layers; and removing the first substrate.
In another aspect, the present invention provides A method for
forming a solar cell comprising forming a top cell including base
and emitter layers composed of InGaP semiconductor material;
forming a middle cell emitter layer of InGaP semiconductor material
and a base layer of GaAs semiconductor material; and forming a
bottom cell including an emitter and base layer of InGaAs
semiconductor material, wherein at least one of the bases has an
exponentially doped profile.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The invention will be better and more fully appreciated by
reference to the following detailed description when considered in
conjunction with the accompanying drawings, wherein
[0018] FIG. 1 is an enlarged cross-sectional view of a solar cell
constructed according to the present invention;
[0019] FIG. 2 is a cross-sectional view of the solar cell of FIG. 1
after the next process step;
[0020] FIG. 3 is a cross-sectional view of the solar cell of FIG. 2
after the next process step;
[0021] FIG. 4 is a cross-sectional view of the solar cell of FIG. 3
after the next process step;
[0022] FIG. 5A is a cross-sectional view of the solar cell of FIG.
4 after the next process step in which the original substrate is
removed;
[0023] FIG. 5B is another cross-sectional view of the solar cell of
FIG. 5A with the surrogate substrate on the bottom of the
Figure;
[0024] FIG. 6A is a top plan view of a wafer in which the solar
cells are fabricated;
[0025] FIG. 6B is a bottom plan view of a wafer in which the solar
cells are fabricated;
[0026] FIG. 7 is a top plan view of the wafer of FIG. 6A after the
next process step;
[0027] FIG. 8 is a cross-sectional view of the solar cell of FIG.
5B after the next process step;
[0028] FIG. 9 is a cross-sectional view of the solar cell of FIG. 8
after the next process step;
[0029] FIG. 10 is a cross-sectional view of the solar cell of FIG.
9 after the next process step;
[0030] FIG. 11 is a cross-sectional view of the solar cell of FIG.
10 after the next process step;
[0031] FIG. 12 is a cross-sectional view of the solar cell of FIG.
11 after the next process step;
[0032] FIG. 13 is a cross-sectional view of the solar cell of FIG.
12 after the next process step;
[0033] FIG. 14 is a cross-sectional view of the solar cell of FIG.
13 after the next process step;
[0034] FIG. 15 is a cross-sectional view of the solar cell of FIG.
14 after the next process step; and
[0035] FIG. 16 is a graph of the doping profile between the emitter
and base layer in a subcell of the inverted metamorphic solar cell
according to the present invention.
DESCRIPTION OF THE PRIOR ART AND PREFERRED EMBODIMENT
[0036] Details of the present invention will now be described
including exemplary aspects and embodiments thereof. Referring to
the drawings and the following description, like reference numbers
are used to identify like or functionally similar elements, and are
intended to illustrate major features of exemplary embodiments in a
highly simplified diagrammatic manner. Moreover, the drawings are
not intended to depict every feature of the actual embodiment nor
the relative dimensions of the depicted elements, and are not drawn
to scale.
[0037] FIG. 1 depicts the multifunction solar cell according to the
present invention after formation of the three subcells A, B and C
on a substrate. More particularly, there is shown a substrate 101,
which may be either gallium arsenide (GaAs), germanium (Ge), or
other suitable material. In the case of a Ge substrate, a
nucleation layer 102 is deposited on the substrate. On the
substrate, or over the nucleation layer 102, a buffer layer 103,
and an etch stop layer 104 are further deposited. A contact layer
105 is then deposited on layer 104, and a window layer 106 is
deposited on the contact layer. The subcell A, consisting of an n+
emitter layer 107 and a p-type base layer 108, is then deposited on
the window layer 106.
[0038] It should be noted that the multifunction solar cell
structure could be formed by any suitable combination of group III
to V elements listed in the periodic table subject to lattice
constant and band gap requirements, wherein the group III includes
boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium
(T). The group IV includes carbon (C), silicon (Si), germanium
(Ge), and tin (Sn). The group V includes nitrogen (N), phosphorous
(P), arsenic (As), antimony (Sb), and bismuth (Bi).
[0039] In the preferred embodiment, the emitter layer 107 is
composed of InGa(Al)P and the base layer 108 is composed of
InGa(Al)P. The aluminum or Al term in parenthesis in the preceding
formula means that Al is an optional constituent, and in this
instance may be used in an amount ranging from 0% to 30%. The
doping profile of the emitter and base layers 107 and 108 according
to the present invention will be discussed in conjunction with FIG.
16.
[0040] On top of the base layer 108 is deposited a back surface
field ("BSF") layer 109 used to reduce recombination loss.
[0041] The BSF layer 109 drives minority carriers from the region
near the base/BSF interface surface to minimize the effect of
recombination loss. In other words, a BSF layer 109 reduces
recombination loss at the backside of the solar subcell A and
thereby reduces the recombination in the base.
[0042] On top of the BSF layer 109 is deposited a sequence of
heavily doped p-type and n-type layers 110 which forms a tunnel
diode which is a circuit element to connect subcell A to subcell
B.
[0043] On top of the tunnel diode layers 110 a window layer 111 is
deposited. The window layer 111 used in the subcell B also operates
to reduce the recombination loss. The window layer 111 also
improves the passivation of the cell surface of the underlying
junctions. It should be apparent to one skilled in the art, that
additional layer(s) may be added or deleted in the cell structure
without departing from the scope of the present invention.
[0044] On top of the window layer 111 the layers of subcell B are
deposited: the emitter layer 112, and the p-type base layer 113.
These layers are preferably composed of InGaP and In.sub.0.015GaAs
respectively, (for a Ge growth template) although any other
suitable materials consistent with lattice constant and band gap
requirements may be used as well. The doping profile of layers 112
and 113 according to the present invention will be discussed in
conjunction with FIG. 16.
[0045] On top of the cell B is deposited a BSF layer 114 which
performs the same function as the BSF layer 109. A p++/n++tunnel
diode 115 is deposited over the BSF layer 114 similar to the layers
110, again forming a circuit element to connect subcell B to
subcell C.
[0046] A barrier layer 116a, preferably composed of InGa(Al)P, is
deposited over the tunnel diode 115, to a thickness of about 1.0
micron. Such barrier layer is intended to prevent threading
dislocations from propagating, either opposite to the direction of
growth into the middle and top subcells B and C, or in the
direction of growth into the bottom subcell A, and are more
particularly described in copending U.S. patent application Ser.
No. 11/860,183, filed Sep. 24, 2007.
[0047] A metamorphic layer (grading interlayer) 116 is deposited
over the barrier layer 116a. Layer 116 is preferably a
compositionally step-graded series of InGaAlAs layers with
monotonically changing lattice constant that is intended to achieve
a transition in lattice constant from subcell B to subcell C. The
band gap of layer 116 is preferably 1.5 ev consistent with a value
slightly greater than the band gap of the middle subcell B.
[0048] In one embodiment, as suggested in the Wanless et al. paper,
the step grade contains nine compositionally graded InGaP steps
with each step layer having a thickness of 0.25 micron. In the
preferred embodiment, the layer 116 is composed of InGaAlAs, with
monotonically changing lattice constant.
[0049] In another embodiment of the present invention, an optional
second barrier layer 116b may be deposited over the InGaAlAs
metamorphic layer 116. The second barrier layer 116b will typically
have a slightly different composition than that of barrier layer
116a.
[0050] A window layer 117 is deposited over the barrier layer 116b,
this window layer operating to reduce the recombination loss in
subcell "C". It should be apparent to one skilled in the art that
additional layers may be added or deleted in the cell structure
without departing from the scope of the present invention.
[0051] On top of the window layer 117, the layers of cell C are
deposited: the n+ emitter layer 118, and the p-type base layer 119.
These layers are preferably composed of InGaAs, although an other
suitable materials consistent with lattice constant and band gap
requirements may be used as well. The doping profile of layers 118
and 119 will be discussed in connection with FIG. 16.
[0052] A BSF layer 120 is deposited on top of the cell C, the BSF
layer performing the same function as the BSF layers 109 and
114.
[0053] Finally a p+ contact layer 121 is deposited on the BSF layer
120.
[0054] It should be apparent to one skilled in the art, that
additional layer(s) may be added or deleted in the cell structure
without departing from the scope of the present invention.
[0055] FIG. 2 is a cross-sectional view of the solar cell of FIG. 1
after the next process step in which a metal contact layer 122 is
deposited over the p+ semiconductor contact layer 121. The metal is
preferably Ti/Au/Ag/Au.
[0056] FIG. 3 is a cross-sectional view of the solar cell of FIG. 2
after the next process step in which an adhesive layer 123 is
deposited over the metal layer 122. The adhesive is preferably
Wafer Bond (manufactured by Brewer Science, Inc. of Rolla,
Mo.).
[0057] FIG. 4 is a cross-sectional view of the solar cell of FIG. 3
after the next process step in which a surrogate substrate 124,
preferably sapphire, is attached. The surrogate substrate is about
40 mils in thickness, and is perforated with holes about 1 mm in
diameter, spaced 4 mm apart, to aid in subsequent removal of the
adhesive and the substrate.
[0058] FIG. 5A is a cross-sectional view of the solar cell of FIG.
4 after the next process step in which the original substrate is
removed by a sequence of lapping and/or etching steps in which the
substrate 101, the buffer layer 103, and the etch stop layer 104,
are removed. The choice of a particular etchant is growth substrate
dependent.
[0059] FIG. 5B is a cross-sectional view of the solar cell of FIG.
5A with the orientation with the surrogate substrate 124 being at
the bottom of the Figure. Subsequent Figures in this application
will assume such orientation.
[0060] FIG. 6A is a top plan view of a wafer in which for solar
cells are implemented. The depiction of four cells is for
illustration for purposes only, and the present invention is not
limited to any specific number of cells per wafer.
[0061] In each cell there are grid lines 501 (more particularly
shown in cross-section in FIG. 10), an interconnecting bus line
502, and a contact pad 503. The geometry and number of grid and bus
lines is illustrative and the present invention is not limited to
the illustrated embodiment.
[0062] FIG. 6B is a bottom plan view of the wafer with four solar
cells shown in FIG. 6A.
[0063] FIG. 7 is a top plan view of the wafer of FIG. 6A after the
next process step in which a mesa 510 is etched around the
periphery of each cell using phosphide and arsenide etchants.
[0064] FIG. 8 is a simplified cross-sectional view of the solar
cell of FIG. 5B depicting just a few of the top layers and lower
layers over the surrogate substrate 124.
[0065] FIG. 9 is a cross-sectional view of the solar cell of FIG. 8
after the next process step in which the etch stop layer 104 is
removed by a HCl/H.sub.2O solution.
[0066] FIG. 10 is a cross-sectional view of the solar cell of FIG.
9 after the next sequence of process steps in which a photoresist
mask (not shown) is placed over the contact layer 105 to form the
grid lines 501. The grid lines 501 are deposited via evaporation
and lithographically patterned and deposited over the contact layer
105. The mask is lifted off to form the metal grid lines 501.
[0067] FIG. 11 is a cross-sectional view of the solar cell of FIG.
10 after the next process step in which the grid lines are used as
a mask to etch down the surface to the window layer 106 using a
citric acid/peroxide etching mixture.
[0068] FIG. 12 is a cross-sectional view of the solar cell of FIG.
11 after the next process step in which an antireflective (ARC)
dielectric coating layer 130 is applied over the entire surface of
the "bottom" side of the wafer with the grid lines 501.
[0069] FIG. 13 is a cross-sectional view of the solar cell of FIG.
12 after the next process step in which the mesa 510 is etched down
to the metal layer 122 using phosphide and arsenide etchants. The
cross-section in the figure is depicted as seen from the A-A plane
shown in FIG. 7. One or more silver electrodes are then welded to
the contact pad(s).
[0070] FIG. 14 is a cross-sectional view of the solar cell of FIG.
13 after the next process step after the surrogate substrate 124
and adhesive 123 are removed by EKC 922. The preferred perforations
provided in the surrogate substrate have a diameter of 0.033
inches, and are separated by 0.152 inches.
[0071] FIG. 15 is a cross-sectional view of the solar cell of FIG.
14 In one embodiment after the next process step in which an
adhesive is applied over the ARC layer 130 and a rigid coverglass
attached thereto.
[0072] In a different embodiment, the solar cell of FIG. 13 may be
initially mounted on a support, and the surrogate substrate 124 and
adhesive 123 subsequently removed. Such a support may be the rigid
coverglass mounted by an adhesive, as depicted in FIG. 15.
[0073] FIG. 16 is a graph of the doping profile between the emitter
and base layer in a subcell of a metamorphic solar cell according
to the present invention in a first embodiment.
[0074] As noted above, the doping profile of the emitter and base
layers depicted in FIG. 16 may be implemented in any one or more of
the subcells of the triple junction solar cell of the present
invention.
[0075] The specific doping profile according to the present
invention is illustrated in the Figure: the emitter doping
decreases from approximately 5.times.10.sup.18 per cubic centimeter
in the region immediately adjacent the adjoining layer (e.g. layers
106, 111, or 117) to 5.times.10.sup.17 per cubic centimeter in the
region adjacent the p-n junction shown by the dotted line in FIG.
16. The base doping increases exponentially from 1.times.10.sup.16
per cubic centimeter adjacent the p-n junction to 1.times.10.sup.18
per cubic centimeter adjacent the adjoining layer (e.g., layer 109,
114, or 120).
[0076] The absolute value of the collection field generated by an
exponential doping gradient exp[-x/.lamda.] is given by the
constant electric field of magnitude
E=(kT/q(1/.lamda.))(exp[-X.sub.b/.lamda.]), where k is the
Boltzmann constant, T is the absolute temperature in degrees
Kelvin, q is the absolute value of electronic charge, and .lamda.
is a parameter characteristic of the doping decay.
[0077] The efficacy of the present invention has been demonstrated
in a test solar cell which incorporated an exponential doping
profile in the 3 .mu.m thick base layer of the bottom subcell,
according to the present invention. Following measurements of the
electrical parameters of the test cell, there was observed a 6.7%
increase in current collection. The measurements indicated an open
circuit voltage (V.sub.oc) equal to at least 3.014V, a short
circuit current (J.sub.sc) of at least 16.55 mA/cm, and a fill
factor (FF) of at least 0.86 at AMO.
[0078] The exponential doping profile taught by the present
invention produces a constant field in the doped region. In the
particular triple junction solar cell materials and structure of
the present invention, the bottom cell has the smallest short
circuit current among all the subcells. In a triple junction solar
cell, the individual subcells are stacked and form a series
circuit. The total current flow in the entire cell is therefore
limited by the smallest current produced in any one of the
subcells. Thus, by increasing the short circuit current in the
bottom cell by 6.7%, the current more closely approximates that of
the higher subcells, and the overall efficiency of the triple
junction solar cell is increased by 6.7% as well. In a solar triple
junction cell with approximately 30% efficiency, the implementation
of the present invention would increase efficiency by a factor of
1.067, i.e. to 32.01%. Such an increase in overall efficiency is
substantial in the field of solar cell technology. In addition to
an increase in efficiency, the collection field created by the
exponential doping profile will enhance the radiation hardness of
the solar cell, which is important for spacecraft applications.
[0079] Although the exponentially doped profile is the doping
design which has been implemented and verified, other doping
profiles may give rise to a linear varying collection field which
may offer yet other advantages. For example, a doping profile of
e.sup.-x.sup.2.sup./.lamda..sup.2 produces a linear field in the
doped region which would be advantageous for both minority carrier
collection and for radiation hardness at the end-of-life of the
solar cell. Such other doping profiles in one or more base layer
are within the scope of the present invention.
[0080] The doping profiles depicted herein are merely illustrative,
and other more complex profiles may be utilized as would be
apparent to those skilled in the art without departing from the
scope of the present invention.
[0081] It will be understood that each of the elements described
above, or two or more together, also may find a useful application
in other types of constructions differing from the types of
constructions differing from the types described above.
[0082] While the invention has been illustrated and described as
embodied in a inverted metamorphic multijunction solar cell, it is
not intended to be limited to the details shown, since various
modifications and structural changes may be made without departing
in any way from the spirit of the present invention.
[0083] Without further analysis, the foregoing will so fully reveal
the gist of the present invention that others can, by applying
current knowledge, readily adapt it for various applications
without omitting features that, from the standpoint of prior art,
fairly constitute essential characteristics of the generic or
specific aspects of this invention and, therefore, such adaptations
should and are intended to be comprehended within the meaning and
range of equivalence of the following claims.
* * * * *