U.S. patent application number 16/196578 was filed with the patent office on 2019-03-21 for method for forming multijunction metamorphic solar cells for space applications.
This patent application is currently assigned to SolAero Technologies Corp.. The applicant listed for this patent is SolAero Technologies Corp.. Invention is credited to Daniel Derkacs.
Application Number | 20190088811 16/196578 |
Document ID | / |
Family ID | 57137975 |
Filed Date | 2019-03-21 |
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United States Patent
Application |
20190088811 |
Kind Code |
A1 |
Derkacs; Daniel |
March 21, 2019 |
METHOD FOR FORMING MULTIJUNCTION METAMORPHIC SOLAR CELLS FOR SPACE
APPLICATIONS
Abstract
A method of manufacturing a multijunction solar cell including
growing interconnected first and second discrete semiconductor
regions disposed adjacent and parallel to each other in a single
semiconductor body, including first top subcell, second (and
possibly third) lattice matched middle subcells; a graded
interlayer adjacent to the last middle solar subcell; and a bottom
solar subcell adjacent to said graded interlayer being lattice
mismatched with respect to the last middle solar subcell; wherein
the interconnected regions form at least a four junction solar cell
by a series connection being formed between the bottom solar
subcell in the first semiconductor region and the bottom solar
subcell in the second semiconductor region.
Inventors: |
Derkacs; Daniel;
(Albuquerque, NM) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SolAero Technologies Corp. |
Albuquerque |
NM |
US |
|
|
Assignee: |
SolAero Technologies Corp.
Albuquerque
NM
|
Family ID: |
57137975 |
Appl. No.: |
16/196578 |
Filed: |
November 20, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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15250643 |
Aug 29, 2016 |
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16196578 |
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62288181 |
Jan 28, 2016 |
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62243239 |
Oct 19, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/0735 20130101;
Y02E 10/544 20130101; H01L 31/0687 20130101; H01L 31/03046
20130101; H01L 31/02008 20130101; H01L 31/1844 20130101; H01L
31/0547 20141201; H01L 31/0504 20130101; H01L 31/078 20130101; H01L
31/0725 20130101; Y02E 10/52 20130101 |
International
Class: |
H01L 31/0725 20120101
H01L031/0725; H01L 31/0735 20120101 H01L031/0735; H01L 31/054
20140101 H01L031/054; H01L 31/18 20060101 H01L031/18; H01L 31/02
20060101 H01L031/02; H01L 31/0304 20060101 H01L031/0304; H01L
31/078 20120101 H01L031/078; H01L 31/05 20140101 H01L031/05; H01L
31/0687 20120101 H01L031/0687 |
Claims
1. A method of fabricating a multijunction solar cell having a
terminal of first polarity and a terminal of second polarity
comprising: (a) forming a semiconductor body by growing a sequence
of semiconductor layers on a substrate, the sequence of layers
including: an upper first solar subcell composed of a semiconductor
material having a first band gap, and including a top contact
region on the top surface thereof; a second solar subcell adjacent
to said first solar subcell and composed of a semiconductor
material having a second band gap smaller than the first band gap
and being lattice matched with the upper first solar subcell; a
third solar subcell adjacent to said second solar subcell and
composed of a semiconductor material having a third band gap
smaller than the second band gap and being lattice matched with the
second solar subcell; a graded interlayer adjacent to said third
solar subcell, said graded interlayer having a fourth band gap
greater than said third band gap; and a fourth solar subcell
adjacent to said graded interlayer and composed of a semiconductor
material having a fifth band gap smaller than the fourth band gap
and being lattice mismatched with the third solar subcell, and
including a first electrode on the top surface thereof, and a
second electrode on the bottom surface thereof; wherein the graded
interlayer is compositionally graded to lattice match the third
solar subcell on one side and the lower fourth solar subcell on the
other side, and is composed of any of the As, P, N, Sb based III-V
compound semiconductors subject to the constraints of having the
in-plane lattice parameter less than or equal to that of the third
solar subcell and greater than or equal to that of the lower fourth
solar subcell; wherein the semiconductor body includes first and
second adjacent and parallel semiconductor regions, each region
including a top contact; and further including the upper first
solar subcell; the second solar subcell adjacent to said first
solar subcell; the third solar subcell adjacent to said second
solar subcell; the graded interlayer adjacent to said third solar
subcell; and a fourth solar subcell (D.sub.1 and D.sub.2
respectively) adjacent to the graded interlayer; and (b) etching
the semiconductor body from the substrate side so as to form a
first opening in the first and second semiconductor regions
providing a spatial separation between the two fourth solar
subcells (D.sub.1) and (D.sub.2) disposed in the first and second
regions respectively.
2. The method as defined in claim 1, wherein the fourth solar
subcell is lattice mismatched from the third solar subcell.
3. The method as defined in claim 1, wherein the fourth solar
subcell (D.sub.2) in the second semiconductor region includes a
first contact on the top surface thereof, and a second contact on
the bottom surface thereof connected to the terminal of a second
polarity; (i) wherein the top contact of the first semiconductor
region is electrically coupled with the top contact of the second
semiconductor region and to the terminal of first polarity; wherein
the first contact on the top surface of the fourth solar subcell
(D.sub.1) of the first semiconductor region is electrically coupled
with the bottom contact of the third solar subcell; and a second
contact on the bottom surface of the fourth solar subcell (D.sub.1)
of the first semiconductor region is electrically coupled with a
contact on the top surface of the fourth solar subcell (D.sub.2) of
the second semiconductor region thereof so as to form a five
junction solar cell.
4. The method as defined in claim 1, wherein the fourth subcell has
a band gap of approximately 0.67 eV, the third subcell has a band
gap in the range of approximately 1.41 eV and 1.31 eV, the second
subcell has a band gap in the range of approximately 1.65 to 1.8 eV
and the upper first subcell has a band gap in the range of 2.0 to
2.20 eV.
5. The method as defined in claim 4, wherein the third subcell has
a band gap of approximately 1.37 eV, the second subcell has a band
gap of approximately 1.73 eV and the upper first subcell has a band
gap of approximately 2.10 eV.
6. The method as defined in claim 1, wherein: the upper first
subcell is composed of indium gallium aluminum phosphide; the
second solar subcell includes an emitter layer composed of indium
gallium phosphide or aluminum gallium arsenide, and a base layer
composed of aluminum gallium arsenide or indium gallium arsenide
phosphide; the third solar subcell is composed of indium gallium
arsenide; the fourth subcell is composed of germanium or InGaAs,
GaSb, GaAsSb, InAsP, InAlAs, SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi,
InGaAsNSbBi, InGaSbN, InGaBiN, or InGaSbBiN; and the graded
interlayer is composed of (Al)In.sub.xGa.sub.1-xAs or
In.sub.xGa.sub.1-xP with 0<x<1, and (Al) designates that
aluminum is an optional constituent.
7. The method as defined in claim 1, wherein the band gap of the
interlayer is in the range of 1.41 eV to 1.6 eV throughout its
thickness.
8. The method as defined in claim 1, further comprising: forming a
distributed Bragg reflector (DBR) layer adjacent to and between the
third and the fourth solar subcells and arranged so that light can
enter and pass through the third solar subcell and at least a
portion of which can be reflected back into the third solar subcell
by the DBR layer, and the distributed Bragg reflector layer is
composed of a plurality of alternating layers of lattice matched
materials with discontinuities in their respective indices of
refraction and the difference in refractive indices between
alternating layers is maximized in order to minimize the number of
periods required to achieve a given reflectivity, and the thickness
and refractive index of each period determines the stop band and
its limiting wavelength, and wherein the DBR layer includes a first
DBR layer composed of a plurality of p type
Al.sub.xGa.sub.1-x(In)As layers, and a second DBR layer disposed
over the first DBR layer and composed of a plurality of n type or p
type Al.sub.yGa.sub.1-y(In)As layers, where 0<x<1,
0<y<1, and y is greater than x, and (In) designates that
indium is an optional constituent.
9. The method as defined in claim 1, wherein the respective
selection of the composition, band gaps, open circuit voltage, and
short circuit current of each of the subcells (i) maximizes the
efficiency of the assembly at high temperature (in the range of 40
to 100 degrees Centigrade) in deployment in space at a
predetermined time after the initial deployment (referred to as the
"beginning of life" or BOL), such predetermined time being referred
to as the "end-of-life" (EOL), wherein such predetermined time is
in the range of one to twenty-five years; or (ii) maximizes the
efficiency of the solar cell at a predetermined low intensity (less
than 0.1 suns) and low temperature value (less than minus 80
degrees Centigrade) in deployment in space at a predetermined time
after the initial deployment in space, or the "beginning of life"
(BOL), such predetermined time being referred to as the
"end-of-life" (EOL) time, and being at least one year.
10. The method as defined in claim 1, wherein the amount of
aluminum in the upper first subcell is at least 10% by mole
fraction.
11. The method as defined in claim 1, wherein the semiconductor
body further comprises a first highly doped lateral conduction
layer disposed adjacent to and above the fourth solar subcell and a
blocking p-n diode or insulating layer disposed adjacent to and
above the first highly doped lateral conduction layer.
12. The method as defined in claim 11, wherein the first and second
semiconductor bodies further comprises a second highly doped
lateral conduction layer disposed adjacent to and above the
blocking p-n diode or insulating layer.
13. The method as defined in claim 12, further comprising forming a
first alpha layer disposed above the second lateral conduction
layer and having a different composition from the second lateral
conduction layer, having a thickness of between 0.25 and 1.0
microns, and functioning to prevent threading dislocations from
propagating, either opposite to the direction of growth or in the
direction of growth into the second subcell.
14. The method as defined in claim 5, wherein the short circuit
current density (J.sub.sc) of the first, second and third solar
subcells are approximately 11 mA/cm.sup.2, and the short circuit
current density (J.sub.sc) of the fourth subcell is approximately
34 mA/cm.sup.2.
15. The method as defined in claim 1, wherein the short circuit
density (J.sub.sc) of the fourth subcell is at least three times
that of the first, second and third subcells, with the base region
of such subcell having a gradation in doping that increases from
the base-emitter junction to the bottom of the base region in the
range of 1.times.10.sup.15 to 5.times.10.sup.18 per cubic
centimeter.
16. The method as defined in claim 13 comprising: forming a first
opening in the first semiconductor body extending from a bottom
surface of the semiconductor body to the first lateral conduction
layer; forming a second opening in the first semiconductor body
extending from the bottom surface of the semiconductor body to the
second lateral conduction layer; and forming a third opening in the
first semiconductor body extending from the bottom surface of the
first semiconductor body to the p-type semiconductor material of
the bottom subcell.
17. The method as defined in claim 16, further comprising:
providing a first metallic contact pad disposed on the first
lateral conduction layer of each of the first and second
semiconductor bodies; and providing a second metallic contact pad
disposed on the second lateral conduction layer of the first
semiconductor body; and providing an electrical interconnect
connecting the first and second contact pads.
18. The method as defined in claim 17, further comprising:
providing a third metallic contact pad disposed on the second
lateral conduction layer of the second semiconductor body;
providing a fourth metallic contact pad disposed on the p-type
semiconductor material of the bottom subcell of the first
semiconductor body; and providing an electrical interconnect
connecting the third and fourth contact pads.
19. A method of forming a multijunction solar cell including a
terminal of first polarity and a terminal of second polarity
comprising: forming first and second semiconductor regions in a
single monolithic semiconductor body, each region including
substantially identical tandem vertical stacks of at least an upper
and a bottom solar subcell in which the second semiconductor region
is disposed adjacent to and with respect to the incoming
illumination, parallel to the first semiconductor region with the
bottom subcells of the first and second semiconductor regions being
spatially separated such that the layers are electrically isolated;
providing a bottom contact on the bottom subcell of the second
semiconductor region connected to the terminal of second polarity;
providing a top electric contact on both the upper subcells of the
first and second semiconductor regions electrically connected to
the terminal of first polarity; and providing an electrical
interconnect connecting the bottom electrode on the bottom second
subcell of the first semiconductor region to the top electrode of
the bottom second subcell of the second semiconductor region so
that the bottom subcell of the first semiconductor region and the
bottom subcell of the second semiconductor region are connected in
an electrical series, and at least a three junction solar cell is
formed by the electrically interconnected semiconductor
regions.
20. A method as defined in claim 19, wherein the first and second
semiconductor regions comprise: an upper first solar subcell
composed of a semiconductor material having a first band gap, and
including a top contact region on the top surface thereof; a second
solar subcell adjacent to said first solar subcell and composed of
a semiconductor material having a second band gap smaller than the
first band gap and being lattice matched with the upper first solar
subcell; a third solar subcell adjacent to said second solar
subcell and composed of a semiconductor material having a third
band gap smaller than the second band gap and being lattice matched
with the second solar subcell; a graded interlayer adjacent to said
third solar subcell, said graded interlayer having a fourth band
gap greater than said third band gap; and a fourth solar subcell
adjacent to said graded interlayer and composed of a semiconductor
material having a fifth band gap smaller than the fourth band gap
and being lattice mismatched with the third solar subcell.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. patent application
Ser. No. 15/250,643 filed Aug. 29, 2016.
[0002] The Ser. No. 15/250,643 application claims the benefit of
U.S. Provisional Application No. 62/288,181 filed Jan. 28, 2016,
and U.S. Provisional Patent Application Ser. No. 62/243,239 filed
Oct. 19, 2015.
[0003] This application is related to co-pending U.S. patent
application Ser. Nos. 14/828,197 and 14/828,206 filed Aug. 17,
2015; Ser. No. 15/210,532 filed Jul. 14, 2016; and Ser. No.
15/213,594 filed Jul. 19, 2016, and Ser. No. 15/250,673, now U.S.
Pat. No. ______ filed Aug. 29, 2016.
[0004] This application is also related to co-pending U.S. patent
application Ser. No. 14/660,092 filed Mar. 17, 2015, which is a
division of U.S. patent application Ser. No. 12/716,814 filed Mar.
3, 2010, now U.S. Pat. No. 9,018,521; which was a continuation in
part of U.S. patent application Ser. No. 12/337,043 filed Dec. 17,
2008.
[0005] This application is also related to co-pending U.S. patent
application Ser. No. 13/872,663 filed Apr. 29, 2012, which was also
a continuation-in-part of application Ser. No. 12/337,043, filed
Dec. 17, 2008.
[0006] All of the above related applications are incorporated
herein by reference in their entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0007] The present disclosure relates to solar cells and the
fabrication of solar cells, and more particularly the design and
specification of a multijunction solar cell using electrically
coupled but spatially separated semiconductor regions in a
semiconductor body based on III-V semiconductor compounds.
Description of the Related Art
[0008] Solar power from photovoltaic cells, also called solar
cells, has been predominantly provided by silicon semiconductor
technology. In the past several years, however, high-volume
manufacturing of III-V compound semiconductor multijunction solar
cells for space applications has accelerated the development of
such technology not only for use in space but also for terrestrial
solar power applications. Compared to silicon, III-V compound
semiconductor multijunction devices have greater energy conversion
efficiencies and generally more radiation resistance, although they
tend to be more complex to properly specify and manufacture.
Typical commercial III-V compound semiconductor multijunction solar
cells have energy efficiencies that exceed 27% under one sun, air
mass 0 (AM0) illumination, whereas even the most efficient silicon
technologies generally reach only about 18% efficiency under
comparable conditions. The higher conversion efficiency of III-V
compound semiconductor solar cells compared to silicon solar cells
is in part based on the ability to achieve spectral splitting of
the incident radiation through the use of a plurality of
photovoltaic regions with different band gap energies, and
accumulating the current from each of the regions.
[0009] 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 payloads become more sophisticated, and
applications anticipated for five, ten, twenty or more years, the
power-to-weight ratio and lifetime efficiency of a solar cell
becomes increasingly more important, and there is increasing
interest not only the amount of power provided at initial
deployment, but over the entire service life of the satellite
system, or in terms of a design specification, the amount of power
provided at the "end of life" (EOL).
[0010] The efficiency of energy conversion, which converts solar
energy (or photons) to electrical energy, depends on various
factors such as the design of solar cell structures, the choice of
semiconductor materials, and the thickness of each subcell. In
short, the energy conversion efficiency for each solar cell is
dependent on the optimum utilization of the available sunlight
across the solar spectrum as well as the "age" of the solar cell,
i.e. the length of time it has been deployed and subject to
degradation associated with the temperature and radiation in the
deployed space environment. As such, the characteristic of sunlight
absorption in semiconductor material, also known as photovoltaic
properties, is critical to determine the most efficient
semiconductor to achieve the optimum energy conversion to meet
customer requirements of intended orbit and lifetime.
[0011] Typical III-V compound semiconductor solar cells are
fabricated on a semiconductor wafer in vertical, multijunction
structures or stacked sequence of solar subcells, each subcell
formed with appropriate semiconductor layers and including a p-n
photoactive junction. Each subcell is designed to convert photons
over different spectral or wavelength bands to electrical current.
After the sunlight impinges on the front of the solar cell, and
photons pass through the subcells, the photons in a wavelength band
that are not absorbed and converted to electrical energy in the
region of one subcell propagate to the next subcell, where such
photons are intended to be captured and converted to electrical
energy, assuming the downstream subcell is designed for the
photon's particular wavelength or energy band.
[0012] The individual solar cells or wafers are then disposed in
horizontal arrays or panels, with the individual solar cells
connected together in an electrical series and/or parallel circuit.
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.
[0013] The energy conversion efficiency of multijunction solar
cells is affected by such factors as the number of subcells, the
thickness of each subcell, the composition and doping of each
active layer in a subcell, and the consequential band structure,
electron energy levels, conduction, and absorption of each subcell,
as well as its exposure to radiation in the ambient environment
over time. Factors such as the short circuit current density
(J.sub.sc), the open circuit voltage (V.sub.oc), and the fill
factor are thereby affected and are also important. Another
parameter of consideration taught by the present disclosure is the
difference between the band gap and the open circuit voltage, or
(E.sub.g/q-V.sub.oc), of a particular active layer, and such
parameters may vary depending on subcell layer thicknesses, doping,
the composition of adjacent layers (such as tunnel diodes), and
even the specific wafer being examined from a set of wafers
processed on a single supporting platter in a reactor run. Such
factors also over time (i.e. during the operational life of the
system). Accordingly, such parameters are NOT simple "result
effective" variables (as discussed and emphasized below) to those
skilled in the art confronted with complex design specifications
and practical operational considerations.
[0014] One of the important mechanical or structural considerations
in the choice of semiconductor layers for a solar cell is the
desirability of the adjacent layers of semiconductor materials in
the solar cell, i.e. each layer of crystalline semiconductor
material that is deposited and grown to form a solar subcell, have
similar crystal lattice constants or parameters. The present
application is directed to solar cells with several substantially
lattice matched subcells, but including at least one subcell which
is lattice mismatched, and in a particular embodiment to a five
junction (5J) solar cell using electrically coupled but spatially
separated four junction (4J) semiconductor devices in a single
semiconductor body.
SUMMARY OF THE DISCLOSURE
Objects of the Disclosure
[0015] It is an object of the present disclosure to provide
increased photoconversion efficiency in a multijunction solar cell
for space applications over the operational life of the
photovoltaic power system.
[0016] It is another object of the present disclosure to provide in
a multijunction solar cell in which the selection of the
composition of the subcells and their band gaps maximizes the
efficiency of the solar cell at a predetermined high temperature
(in the range of 40 to 100 degrees Centigrade) in deployment in
space at AM0 at a predetermined time after the initial deployment,
such time being at least one year, and in the range of one to
twenty-five years.
[0017] It is another object of the present disclosure to provide a
four junction solar cell subassembly in which the average band gap
of all four cells in the subassembly is greater than 1.44 eV, and
to couple the subassembly in electrical series with at least one
additional subcell in an adjacent solar cell subassembly.
[0018] It is another object of the present disclosure to provide a
lattice mis-matched five junction solar cell in which the bottom
subcell is intentionally designed to have a short circuit current
that is substantially greater than current through the top three
subcells when measured at the "beginning-of-life" or time of
initial deployment.
[0019] It is another object of the present disclosure to provide a
five-junction (5J) solar assembly assembled from two four-junction
(4J) solar cell subassemblies in an integrated semiconductor
structure so that the total current provided by the two
subassemblies matches the total current handling capability of the
bottom subcell of the assembly.
[0020] It is another object of the present disclosure to match the
larger short circuit current of the bottom subcell of the solar
cell assembly with two or three parallel stacks of solar subcells,
i.e. a configuration in which the value of the short circuit
current of the bottom subcell is at least twice, or at least three
times, that of the solar subcells in each parallel stack which are
connected in a series with the bottom subcell. Stated another way,
given the choice of the composition of the bottom subcell, and
there by the short circuit current of the bottom subcell, it is an
object of the disclosure that the upper subcell stack be specified
and designed to have a short circuit which is one-half or less than
that of the bottom subcell.
[0021] Some implementations of the present disclosure may
incorporate or implement fewer of the aspects and features noted in
the foregoing objects.
Features of the Invention
[0022] The present application is directed to solar cells with
several substantially lattice matched subcells, but in some
embodiments including at least one subcell which is lattice
mismatched, and in a particular embodiment to a five junction (5J)
solar cell using electrically coupled but spatially separated four
junction (4J) semiconductor regions in a semiconductor body based
on III-V semiconductor compounds.
[0023] All ranges of numerical parameters set forth in this
disclosure are to be understood to encompass any and all subranges
or "intermediate generalizations" subsumed herein. For example, a
stated range of "1.0 to 2.0 eV" for a band gap value should be
considered to include any and all subranges beginning with a
minimum value of 1.0 eV or more and ending with a maximum value of
2.0 eV or less, e.g., 1.0 to 2.0, or 1.3 to 1.4, or 1.5 to 1.9
eV.
[0024] Briefly, and in general terms, the present disclosure
describes solar cells that include a solar cell assembly of two or
more solar cell subassemblies in a single monolithic semiconductor
body composed of a tandem stack of solar subcells, where the
subassemblies are interconnected electrically to one another.
[0025] As described in greater detail, the present application
discloses that interconnecting two or more spatially split
multijunction solar cell regions or subassemblies can be
advantageous. The spatial split can be provided for multiple solar
cell subassemblies monolithically formed on a single substrate and
remaining as a monolithic semiconductor body with distinct
characteristics. Alternatively, the solar cell subassemblies can be
physically separated or fabricated individually as separate
semiconductor chips that can be coupled together electrically.
(Such alternative embodiments are covered in parallel applications,
such as Ser. No. 15/213,594, noted in the Reference to Related
Applications).
[0026] One advantage of interconnecting two or more spatially split
multijunction solar cell subassemblies is that such an arrangement
can allow accumulation of the current from the upper subcells in
the adjacent semiconductor bodies into the bottom subcells which
have higher current generation ability.
[0027] One advantage of interconnecting two or more spatially split
multijunction solar cell subassemblies is that such an arrangement
can allow the bottom subcells of different subassemblies to be
connected in electrical series, this boosting the maximum
operational voltage and open circuit voltage associated with the
solar cell assembly, and thereby improving efficiency.
[0028] Further, selection of relatively high band gap semiconductor
materials for the top subcells can provide for increased
photoconversion efficiency in a multijunction solar cell for outer
space or other applications over the operational life of the
photovoltaic power system. For example, increased photoconversion
efficiency at a predetermined time (measured in terms of five, ten,
fifteen or more years) after initial deployment of the solar cell
can be achieved.
[0029] Thus, in one aspect, a monolithic solar cell subassembly
includes a first semiconductor body including an upper first solar
subcell composed of (aluminum) indium gallium phosphide
((Al)InGaP); a second solar subcell disposed adjacent to and
lattice matched to said upper first subcell, the second solar
subcell composed of (aluminum) (indium) gallium arsenide
((Al)(In)GaAs) or indium gallium arsenide phosphide (InGaAsP); and
a bottom subcell lattice matched to said second subcell and
composed of (indium) gallium arsenide (In)GaAs.
[0030] The aluminum (or Al) constituent element, or indium (or In),
shown in parenthesis in the preceding formula means that Al or In
(as the case may be) is an optional constituent, and in the case of
Al, in this instance may be used in an amount ranging from 0% to
40% by mole fraction. In some embodiments, the amount of aluminum
may be between 20% and 30%. The subcells are configured so that the
current density of the upper first subcell and the second subcell
have a substantially equal predetermined first value, and the
current density of the bottom subcell is at least twice that of the
predetermined first value.
[0031] Briefly, and in general terms, the present disclosure
provides a five junction solar cell comprising a semiconductor body
including:
(a) a first semiconductor region including:
[0032] an upper first solar subcell composed of a semiconductor
material having a first band gap, and including a top contact on
the top surface thereof;
[0033] a second solar subcell adjacent to said first solar subcell
and composed of a semiconductor material having a second band gap
smaller than the first band gap and being lattice matched with the
upper first solar subcell;
[0034] a third solar subcell adjacent to said second solar subcell
and composed of a semiconductor material having a third band gap
smaller than the second band gap and being lattice matched with the
second solar subcell;
[0035] an interlayer adjacent to said third solar subcell, said
interlayer having a fourth band gap or band gaps greater than said
third band gap; and
[0036] a fourth solar subcell adjacent to said interlayer and
composed of a semiconductor material having a fifth band gap
smaller than the fourth band gap and being lattice mismatched with
the third solar subcell, and including a first contact on the top
surface thereof, and a second contact on the bottom surface
thereof;
(b) a second semiconductor region disposed adjacent and parallel to
the first semiconductor region and including:
[0037] an upper first solar subcell composed of a semiconductor
material having a first band gap, and including a top contact on
the top surface thereof;
[0038] a second solar subcell adjacent to said first solar subcell
and composed of a semiconductor material having a second band gap
smaller than the first band gap and being lattice matched with the
upper first solar subcell;
[0039] a third solar subcell adjacent to said second solar subcell
and composed of a semiconductor material having a third band gap
smaller than the second band gap and being lattice matched with the
second solar subcell and having a bottom contact;
[0040] an interlayer adjacent to said third solar subcell, said
interlayer having a fourth band gap greater than said third band
gap; and
[0041] a fourth solar subcell adjacent to said interlayer and
composed of a semiconductor material having a fifth band gap
smaller than the fourth band gap and being lattice mismatched with
the third solar subcell, and including a first contact on the top
surface thereof, and a second contact on the bottom surface thereof
connected to the terminal of a second polarity;
(c) wherein the top contact of the first semiconductor region is
electrically coupled with the top contact of the second
semiconductor region and to a terminal of first polarity;
[0042] wherein the first contact on the top surface of the fourth
solar subcell of the first semiconductor region is electrically
coupled with the bottom contact of the third solar subcell of the
second semiconductor region; and
[0043] the second contact on the bottom surface of the fourth solar
subcell of the first semiconductor region is electrically coupled
with the first contact on the top surface of the fourth solar
subcell of the second semiconductor region thereof so as to form a
five junction solar cell.
[0044] In some embodiments, the interlayer in each of the first and
second semiconductor region is compositionally graded to
substantially lattice match the upper solar subcell on one side and
the adjacent lower solar subcell on the other side, and is composed
of any of the As, P, N, Sb based III-V compound semiconductors
subject to the constraints of having the in-plane lattice parameter
less than or equal to that of the third solar subcell on the first
surface and greater than or equal to that of the lower fourth solar
subcell on the other opposing surface.
[0045] In some embodiments, the interlayer in each of the first and
second semiconductor bodies is compositionally graded to
substantially lattice match the third solar subcell on one side and
the lower fourth solar subcell on the other side, and is composed
of (In.sub.xGa.sub.1-x)Al.sub.1-yAs.sub.y, In.sub.xGa.sub.1-xP, or
(Al)In.sub.xGa.sub.1-xAs compound semiconductors subject to the
constraints of having the in-plane lattice parameter less than or
equal to that of the third solar subcell and greater than or equal
to that of the lower fourth solar subcell.
[0046] In some embodiments, the fourth subcell has a band gap of
approximately 0.67 eV, the third subcell has a band gap in the
range of 1.41 eV and 1.31 eV, the second subcell has a band gap in
the range of 1.65 to 1.8 eV and the upper first subcell has a band
gap in the range of 2.0 to 2.20 eV.
[0047] In some embodiments, the third subcell has a band gap of
approximately 1.37 eV, the second subcell has a band gap of
approximately 1.73 eV and the upper first subcell has a band gap of
approximately 2.10 eV.
[0048] In some embodiments, the upper first subcell is composed of
indium gallium aluminum phosphide; the second solar subcell
includes an emitter layer composed of indium gallium phosphide or
aluminum gallium arsenide, and a base layer composed of aluminum
gallium arsenide; the third solar subcell is composed of indium
gallium arsenide; the fourth subcell is composed of germanium or
InGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb,
InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN, InGaSbBiN, and the graded
interlayer is composed of (Al)In.sub.xGa.sub.1-xAs or
In.sub.xGa.sub.1-xP with 0<x<1, and (Al) denotes that
aluminum is an optional constituent.
[0049] In some embodiments, the upper first solar subcell, the
second solar subcell, the third solar subcell, and the interlayer
in the first and second semiconductor region form an integral
monolithic semiconductor body. The semiconductor regions are
"parallel" to each other in that the regions are disposed adjacent
and parallel to one another so that the incoming light illuminates
both the upper first solar subcell of the first semiconductor
region and the first solar subcell of the second semiconductor
region, and that parallel light beams traverses the stack of
subcells of the entire semiconductor body.
[0050] In some embodiments, the first and second semiconductor
region constitute a single semiconductor body that has been etched
from the backside so that the substrate is separated into two
spatially separated interconnected regions.
[0051] In some embodiments, the band gap of the interlayer is in
the range of 1.41 eV to 1.6 eV throughout its thickness.
[0052] In some embodiments, the first and second semiconductor
regions constitute a single semiconductor body that has been
isolated to form two spatially separated and electrically
interconnected solar cell subassemblies.
[0053] In some embodiments, the respective selection of the
composition, band gaps, open circuit voltage, and short circuit
current of each of the subcells (i) maximizes the efficiency of the
assembly at high temperature (in the range of 40 to 100 degrees
Centigrade) in deployment in space at a predetermined time after
the initial deployment (referred to as the "beginning of life" or
BOL), such predetermined time being referred to as the
"end-of-life" (EOL), wherein such predetermined time is in the
range of one to twenty-five years; or (ii) maximizes the efficiency
of the solar cell at a predetermined low intensity (less than 0.1
suns) and low temperature value (less than minus 80 degrees
Centigrade) in deployment in space at a predetermined time after
the initial deployment in space, or the "beginning of life" (BOL),
such predetermined time being referred to as the "end-of-life"
(EOL) time, and being at least one year.
[0054] In some embodiments, the amount of aluminum in the upper
first subcell is at least 10% by mole fraction.
[0055] In some embodiments, the semiconductor body further
comprises a first highly doped lateral conduction layer disposed
adjacent to and above the fourth solar subcell and a blocking p-n
diode or insulating layer disposed adjacent to and above the first
highly doped lateral conduction layer.
[0056] In some embodiments, the semiconductor body further
comprises a second highly doped lateral conduction layer disposed
adjacent to and above the blocking p-n diode or insulating
layer.
[0057] In some embodiments, there further comprises a first alpha
layer disposed above the second lateral conduction layer and having
a different composition and a thickness of between 0.25 and 1.0
microns and functioning to prevent threading dislocations from
propagating, either opposite to the direction of growth or in the
direction of growth into the second subcell.
[0058] In some embodiments, the short circuit current density
(J.sub.sc) of the first, second and third middle subcells are
approximately 11 mA/cm.sup.2, and the short circuit current density
(J.sub.sc) of the bottom subcell is approximately 34
mA/cm.sup.2.
[0059] In some embodiments, the short circuit density (J.sub.sc) of
the bottom subcell is at least three times that of the first,
second and third subcells, with the base region of such subcell
having a gradation in doping that increases from the base-emitter
junction to the bottom of the base region in the range of
1.times.10.sup.15 to 5.times.10.sup.18 per cubic centimeter.
[0060] In another aspect, the present disclosure provides a
multijunction solar cell including a terminal of first polarity and
a terminal of second polarity comprising first and second
semiconductor regions in a single semiconductor body, each region
including substantially identical tandem vertical stacks of at
least an upper first and a second bottom solar subcell in which the
second semiconductor region is disposed adjacent to and with
respect to the incoming illumination, parallel to the first
semiconductor region; a bottom contact on the bottom subcell of the
second semiconductor region connected to the terminal of second
polarity; a top electric contact on both the upper first subcells
of the first and second semiconductor regions electrically
connected to the top electrical contacts to the terminal of first
polarity; and an electrical interconnect connecting the bottom
second subcell of the first semiconductor region in a series
electrical circuit with the bottom second subcell of the second
semiconductor region so that at least a three junction solar cell
is formed by the electrically interconnected semiconductor
regions.
[0061] In another aspect, the present disclosure provides a method
comprising:
(a) growing a sequence of semiconductor layers on a substrate
forming a semiconductor body, the sequence of layers including an
upper first solar subcell composed of a semiconductor material
having a first band gap, and including a top contact region on the
top surface thereof;
[0062] a second solar subcell adjacent to said first solar subcell
and composed of a semiconductor material having a second band gap
smaller than the first band gap and being lattice matched with the
upper first solar subcell;
[0063] a third solar subcell adjacent to said second solar subcell
and composed of a semiconductor material having a third band gap
smaller than the second band gap and being lattice matched with the
second solar subcell;
[0064] a graded interlayer adjacent to said third solar subcell,
said graded interlayer having a fourth band gap greater than said
third band gap; and
[0065] a fourth solar subcell adjacent to said third solar subcell
and composed of a semiconductor material having a fifth band gap
smaller than the fourth band gap and being lattice mismatched with
the third solar subcell, and including a first contact on the top
surface thereof, and a second contact on the bottom surface
thereof;
[0066] wherein the graded interlayer is compositionally graded to
lattice match the third solar subcell on one side and the lower
fourth solar subcell on the other side, and is composed of any of
the As, P, N, Sb based III-V compound semiconductors subject to the
constraints of having the in-plane lattice parameter less than or
equal to that of the third solar subcell and greater than or equal
to that of the lower fourth solar subcell,
(b) etching the semiconductor body from the substrate side to form
first and second adjacent but electrically isolated semiconductor
regions, each region including:
[0067] the upper first solar subcell;
[0068] the second solar subcell;
[0069] the third solar subcell;
[0070] the graded interlayer; and
[0071] in each region, a distinct and spatially separated fourth
solar subcell.
[0072] In some embodiments, there further comprises:
(c) forming electrical connections so that the top contact region
of the first semiconductor region is electrically coupled with the
top contact of the second semiconductor region;
[0073] the first contact region on the top surface of the fourth
solar subcell of the first semiconductor region is electrically
coupled with the first contact region on the top surface of the
fourth solar subcell of the second semiconductor region; and
[0074] the second contact region on the bottom surface of the
fourth solar subcell of the first semiconductor region is
electrically coupled with the first contact region on the top
surface of the fourth solar subcell of the second semiconductor
region thereof.
[0075] In some embodiments, the short circuit density (J.sub.sc) of
the bottom subcell is at least three times that of the first,
second and third subcells.
[0076] In some embodiments, the semiconductor body further
comprises a first opening in the backside of the body extending
from a bottom surface of the semiconductor body to the first
lateral conduction layer; a second opening in the semiconductor
body extending from the bottom surface of the first semiconductor
body to the second lateral conduction layer; and a third opening in
the first semiconductor body extending from a surface of the
semiconductor body to the p-type semiconductor material of the
bottom subcell of the semiconductor body.
[0077] In some embodiments, the solar cell assembly further
comprises a first metallic contact pad disposed on the first
lateral conduction layer of each of the semiconductor regions; and
a second metallic contact pad disposed on the second lateral
conduction layer of the semiconductor body; and an electrical
interconnect connecting the first and second contact pads.
[0078] In some embodiments, the solar cell assembly further
comprises a third metallic contact pad disposed on the second
lateral conduction layer of the semiconductor regions; a fourth
metallic contact pad disposed on the p-type semiconductor material
of the bottom subcell of the semiconductor body; and an electrical
interconnect connecting the third and fourth contact pads.
[0079] In another aspect, the present disclosure provides a
multijunction solar cell including a terminal of first polarity and
a terminal of second polarity comprising first and second
semiconductor regions including substantially identical tandem
vertical stacks of at least an upper first and a bottom second
solar subcell lattice mismatched to the upper first solar subcell
in which the second semiconductor region is disposed adjacent and
parallel to the first semiconductor region; a bottom contact on the
bottom second subcell of the second semiconductor region connected
to the terminal of second polarity; a top electric contact on both
the upper first subcells of the first and second semiconductor
region electrically connected to the top electrical contacts to the
terminal of first polarity; and an electrical interconnect
connecting the bottom second subcell of the first semiconductor
region in a series electrical circuit with the bottom second
subcell of the second semiconductor region so that at least a
multijunction solar cell is formed by the electrically
interconnected semiconductor region. The series connection of the
bottom subcell of the second semiconductor region incrementally
increases the aggregate open circuit voltage of the assembly by
0.25 volts and the operating voltage by 0.21 volts, thereby
increasing the power output of the solar cell.
[0080] In some embodiments, the average band gap of all four
subcells (i.e., the sum of the four band gaps of each subcell
divided by four) in the semiconductor body is greater than 1.44 eV,
and the fourth subcell is comprised of a direct or indirect band
gap material such that the lowest direct band gap of the material
is greater than 0.75 eV.
[0081] In some embodiments, the fourth subcell is comprised of a
direct or indirect band gap material such that the lowest direct
band gap of the material is less than 0.90 eV.
[0082] In some instances, the upper first subcell of the first
semiconductor body is composed of aluminium indium gallium
phosphide (AlInGaP); the second solar subcell of the first
semiconductor body is disposed adjacent to and lattice matched to
said upper first subcell, and is composed of aluminum indium
gallium arsenide (Al(In)GaAs); and the third subcell is disposed
adjacent to said second subcell and is composed of indium gallium
arsenide (In)GaAs.
[0083] In some cases (e.g., for an assembly having two
subassemblies), the short circuit density (J.sub.sc/cm.sup.2) of
each of the first and second subcells is approximately 12
mA/cm.sup.2. In other instances (e.g., for an assembly having three
subassemblies), the short circuit density (J.sub.sc/cm.sup.2) of
each of the first, second and third middle subcells is
approximately 10 mA/cm.sup.2. The short circuit density
(J.sub.sc/cm.sup.2) of the bottom subcell in the foregoing cases
can be approximately greater than 24 mA/cm.sup.2 or greater than 30
mA/cm.sup.2. However, the short circuit densities
(J.sub.sc/cm.sup.2) may have different values in some
implementations.
[0084] In some embodiments, the band gap of the graded interlayer
may be either constant or may vary throughout the thickness of the
interlayer.
[0085] In some embodiments, there further comprises a distributed
Bragg reflector (DBR) layer adjacent to and between the third and
the fourth solar subcells and arranged so that light can enter and
pass through the third solar subcell and at least a portion of
which can be reflected back into the third solar subcell by the DBR
layer.
[0086] In some embodiments, the distributed Bragg reflector layer
is composed of a plurality of alternating layers of lattice matched
materials with discontinuities in their respective indices of
refraction.
[0087] In some embodiments, the difference in refractive indices
between alternating layers is maximized in order to minimize the
number of periods required to achieve a given reflectivity, and the
thickness and refractive index of each period determines the stop
band and its limiting wavelength.
[0088] In some embodiments, the DBR layer includes a first DBR
layer composed of a plurality of n type or p type
Al.sub.xGa.sub.1-x(In)As layers, and a second DBR layer disposed
over the first DBR layer and composed of a plurality of n type or p
type Al.sub.yGa.sub.1-y(In)As layers, where 0<x<1,
0<y<1, and y is greater than x, and (In) designates that
indium is an optional constituent.
[0089] In another aspect, the present disclosure provides a
multijunction solar cell and its method of manufacture including
interconnected first and second discrete semiconductor regions
disposed adjacent and parallel to each other in a single
semiconductor body, including first top subcell, second (and
possibly third) lattice matched middle subcells; a graded
interlayer adjacent to the last middle solar subcell; and a bottom
solar subcell adjacent to said graded interlayer being lattice
mismatched with respect to the last middle solar subcell; wherein
the interconnected regions form at least a four junction solar cell
by a series connection being formed between the bottom solar
subcell in the first semiconductor region and the bottom solar
subcell in the second semiconductor region.
[0090] In another aspect, the present disclosure provides a method
of manufacturing a five junction solar cell comprising providing a
germanium substrate; growing on the germanium substrate a sequence
of layers of semiconductor material using a MOCVD semiconductor
disposition process to form a solar cell comprising a plurality of
subcells including a metamorphic layer, growing a third subcell
over the metamorphic layer having a band gap of approximately 1.30
eV to 1.41 eV, growing a second subcell over the third subcell
having a band gap in the range of approximately 1.65 to 1.8 eV, and
growing an upper first subcell over the second subcell having a
band gap in the range of 2.0 to 2.20 eV.
[0091] In some embodiments, there further comprises (i) a back
surface field (BSF) layer disposed directly adjacent to the bottom
surface of the third subcell, and (ii) at least one distributed
Bragg reflector (DBR) layer directly below the BSF layer so that
light can enter and pass through the first, second and third
subcells and at least a portion of which be reflected back into the
third subcell by the DBR layer.
[0092] In some embodiments, the fourth (i.e., bottom) subcell of
each of the solar cell subassemblies is composed of germanium. The
indirect band gap of the germanium at room temperature is about
0.66 eV, while the direct band gap of germanium at room temperature
is 0.8 eV. Those skilled in the art with normally refer to the
"band gap" of germanium as 0.66 eV, since it is lower than the
direct band gap value of 0.8 eV. Thus, in some implementations, the
fourth subcell has a direct band gap of greater than 0.75 eV.
Reference to the fourth subcell having a direct band gap of greater
than 0.75 eV is expressly meant to include germanium as a possible
semiconductor material for the fourth subcell, although other
semiconductor materials can be used as well. In other instances,
the fourth subcell may have a direct band gap of less than 0.90 eV.
For example, the fourth subcell may be composed of InGaAs, GaAsSb,
InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, or other
III-V or II-VI compound semiconductor materials.
[0093] In some embodiments, additional layer(s) may be added or
deleted in the cell structure without departing from the scope of
the present disclosure.
[0094] Some implementations can include additional solar subcells
in one or more of the semiconductor bodies.
[0095] Some implementations of the present disclosure may
incorporate or implement fewer of the aspects and features noted in
the foregoing summaries.
[0096] Additional aspects, advantages, and novel features of the
present disclosure will become apparent to those skilled in the art
from this disclosure, including the following detailed description
as well as by practice of the disclosure. While the disclosure is
described below with reference to preferred embodiments, it should
be understood that the disclosure is not limited thereto. Those of
ordinary skill in the art having access to the teachings herein
will recognize additional applications, modifications and
embodiments in other fields, which are within the scope of the
disclosure as disclosed and claimed herein and with respect to
which the disclosure could be of utility.
BRIEF DESCRIPTION OF THE DRAWINGS
[0097] 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:
[0098] FIG. 1 is a graph representing the BOL value of the
parameter E.sub.g/q-V.sub.oc at 28.degree. C. plotted against the
band gap of certain ternary and quaternary materials defined along
the x-axis;
[0099] FIG. 2A is a cross-sectional view of a first embodiment of a
semiconductor body including a four solar subcells after several
stages of fabrication including the growth of certain semiconductor
layers on the growth substrate up to the contact layer, according
to the present disclosure;
[0100] FIG. 2B is a cross-sectional view of a second embodiment of
a semiconductor body including a four solar subcells including two
lattice mismatched subcells with a metamorphic layer between them,
after several stages of fabrication including the growth of certain
semiconductor layers on the growth substrate up to the contact
layer, according to the present disclosure;
[0101] FIG. 2C is a cross-sectional view of the embodiment of FIG.
2B following the steps of etching contact ledges on various
semiconductor layers according to a first implementation in the
present disclosure;
[0102] FIG. 2D is a cross-sectional view of the embodiment of FIG.
2B following the steps of etching contact ledges on various
semiconductor layers according to a second implementation in the
present disclosure;
[0103] FIG. 2E is a cross-sectional view of the embodiment of FIG.
2D following electrical connection of the first and second
semiconductor regions by discrete electrical interconnects
according to the present disclosure;
[0104] FIG. 3 is a bottom plan view of the solar cell of FIG. 2E
depicting the electrical interconnects;
[0105] FIG. 4 is a graph of the doping profile in the base and
emitter layers of a subcell in the solar cell according to the
present disclosure; and
[0106] FIG. 5 is a schematic diagram of the five junction solar
cell of FIG. 2E.
GLOSSARY OF TERMS
[0107] "III-V compound semiconductor" refers to a compound
semiconductor formed using at least one elements from group III of
the periodic table and at least one element from group V of the
periodic table. III-V compound semiconductors include binary,
tertiary and quaternary compounds. Group III includes boron (B),
aluminum (Al), gallium (Ga), indium (In) and thallium (T). Group V
includes nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb)
and bismuth (Bi).
[0108] "Band gap" refers to an energy difference (e.g., in electron
volts (eV)) separating the top of the valence band and the bottom
of the conduction band of a semiconductor material.
[0109] "Beginning of Life (BOL)" refers to the time at which a
photovoltaic power system is initially deployed in operation.
[0110] "Bottom subcell" refers to the subcell in a multijunction
solar cell which is furthest from the primary light source for the
solar cell.
[0111] "Compound semiconductor" refers to a semiconductor formed
using two or more chemical elements.
[0112] "Current density" refers to the short circuit current
density J.sub.sc through a solar subcell through a given planar
area, or volume, of semiconductor material constituting the solar
subcell.
[0113] "Deposited", with respect to a layer of semiconductor
material, refers to a layer of material which is epitaxially grown
over another semiconductor layer.
[0114] "End of Life (EOL)" refers to a predetermined time or times
after the Beginning of Life, during which the photovoltaic power
system has been deployed and has been operational. The EOL time or
times may, for example, be specified by the customer as part of the
required technical performance specifications of the photovoltaic
power system to allow the solar cell designer to define the solar
cell subcells and sublayer compositions of the solar cell to meet
the technical performance requirement at the specified time or
times, in addition to other design objectives. The terminology
"EOL" is not meant to suggest that the photovoltaic power system is
not operational or does not produce power after the EOL time.
[0115] "Graded interlayer" (or "grading interlayer")--see
"metamorphic layer".
[0116] "Inverted metamorphic multijunction solar cell" or "IMM
solar cell" refers to a solar cell in which the subcells are
deposited or grown on a substrate in a "reverse" sequence such that
the higher band gap subcells, which would normally be the "top"
subcells facing the solar radiation in the final deployment
configuration, are deposited or grown on a growth substrate prior
to depositing or growing the lower band gap subcells.
[0117] "Layer" refers to a relatively planar sheet or thickness of
semiconductor or other material. The layer may be deposited or
grown, e.g., by epitaxial or other techniques.
[0118] "Lattice mismatched" refers to two adjacently disposed
materials or layers (with thicknesses of greater than 100 nm)
having in-plane lattice constants of the materials in their fully
relaxed state differing from one another by less than 0.02% in
lattice constant. (Applicant expressly adopts this definition for
the purpose of this disclosure, and notes that this definition is
considerably more stringent than that proposed, for example, in
U.S. Pat. No. 8,962,993, which suggests less than 0.6% lattice
constant difference).
[0119] "Metamorphic layer" or "graded interlayer" refers to a layer
that achieves a gradual transition in lattice constant generally
throughout its thickness in a semiconductor structure.
[0120] "Middle subcell" refers to a subcell in a multijunction
solar cell which is neither a Top Subcell (as defined herein) nor a
Bottom Subcell (as defined herein).
[0121] "Short circuit current (I.sub.sc)" refers to the amount of
electrical current through a solar cell or solar subcell when the
voltage across the solar cell is zero volts, as represented and
measured, for example, in units of milliamps.
[0122] "Short circuit current density"--see "current density".
[0123] "Solar cell" refers to an electronic device operable to
convert the energy of light directly into electricity by the
photovoltaic effect.
[0124] "Solar cell assembly" refers to two or more solar cell
subassemblies interconnected electrically with one another.
[0125] "Solar cell subassembly" refers to a stacked sequence of
layers including one or more solar subcells.
[0126] "Solar subcell" refers to a stacked sequence of layers
including a p-n photoactive junction composed of semiconductor
materials. A solar subcell is designed to convert photons over
different spectral or wavelength bands to electrical current.
[0127] "Substantially current matched" refers to the short circuit
current through adjacent solar subcells being substantially
identical (i.e. within plus or minus 1%).
[0128] "Top subcell" or "upper subcell" refers to the subcell in a
multijunction solar cell which is closest to the primary light
source for the solar cell.
[0129] "ZTJ" refers to the product designation of a commercially
available SolAero Technologies Corp. triple junction solar
cell.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0130] 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.
[0131] A variety of different features of multijunction solar cells
(as well as inverted metamorphic multijunction solar cells) are
disclosed in the related applications noted above. Some, many or
all of such features may be included in the structures and
processes associated with the non-inverted or "upright" solar cells
of the present disclosure. However, more particularly, the present
disclosure is directed to the fabrication of a multijunction
lattice matched solar cell grown over a metamorphic layer which is
grown on a single growth substrate which comprises two or more
interconnected solar cell subassemblies. More specifically,
however, in some embodiments, the present disclosure relates to a
multijunction solar cell with direct band gaps in the range of 2.0
to 2.15 eV (or higher) for the top subcell, and (i) 1.65 to 1.8 eV,
and (ii) 1.41 eV for the middle subcells, and 0.6 to 0.9 eV direct
or indirect band gaps, for the bottom subcell(s), respectively, and
the connection of two or more such subassemblies to form a solar
cell assembly.
[0132] The conventional wisdom for many years has been that in a
monolithic multijunction tandem solar cell, " . . . the desired
optical transparency and current conductivity between the top and
bottom cells . . . would be best achieved by lattice matching the
top cell material to the bottom cell material. Mismatches in the
lattice constants create defects or dislocations in the crystal
lattice where recombination centers can occur to cause the loss of
photogenerated minority carriers, thus significantly degrading the
photovoltaic quality of the device. More specifically, such effects
will decrease the open circuit voltage (V.sub.oc), short circuit
current (J.sub.sc), and fill factor (FF), which represents the
relationship or balance between current and voltage for effective
output" (Jerry M. Olson, U.S. Pat. No. 4,667,059, "Current and
Lattice Matched Tandem Solar Cell").
[0133] As progress has been made toward higher efficiency
multijunction solar cells with four or more subcells, nevertheless,
"it is conventionally assumed that substantially lattice-matched
designs are desirable because they have proven reliability and
because they use less semiconductor material than metamorphic solar
cells, which require relatively thick buffer layers to accommodate
differences in the lattice constants of the various materials"
(Rebecca Elizabeth Jones-Albertus et al., U.S. Pat. No.
8,962,993).
[0134] Even more recently " . . . current output in each subcell
must be the same for optimum efficiency in the series-connected
configuration" (Richard R. King et al., U.S. Pat. No.
9,099,595).
[0135] The present disclosure provides a solar cell subassembly
with an unconventional four junction design (with three grown
lattice matched subcells, which are lattice mismatched to the Ge
substrate) that leads to significant performance improvement over
that of traditional three junction solar cell on Ge despite the
substantial current mismatch present between the top three
junctions and the bottom Ge junction. This performance gain is
especially realized at high temperature and after high exposure to
space radiation by the proposal of incorporating high band gap
semiconductors that are inherently more resistant to radiation and
temperature.
[0136] As described in greater detail, the present application
further notes that interconnecting two or more spatially split
multijunction solar cell subassemblies (with each subassembly
incorporating Applicant's unconventional design) can be even more
advantageous. The spatial split can be provided for multiple solar
cell subassemblies monolithically formed on the same substrate
according to the present disclosure. Alternatively, the solar cell
subassemblies can be fabricated as separate semiconductor chips
that can be coupled together electrically, as described in related
applications.
[0137] In general terms, a solar cell assembly in accordance with
one aspect of the present disclosure, can include a terminal of
first polarity and a terminal of second polarity. The solar cell
assembly includes a first semiconductor subassembly including a
tandem vertical stack of at least a first upper, a second, third
and fourth bottom solar subcells, the first upper subcell having a
top contact connected to the terminal of first polarity. A second
semiconductor subassembly is disposed adjacent to the first
semiconductor subassembly and includes a tandem vertical stack of
at least a first upper, a second, third, and fourth bottom solar
subcells, the fourth bottom subcell having a back side contact
connected to the terminal of second polarity. The fourth subcell of
the first semiconductor subassembly is connected in a series
electrical circuit with the third subcell of the second
semiconductor subassembly. Thus, a five-junction solar assembly is
assembled from two four-junction solar cell subassemblies.
[0138] In some cases, the foregoing solar cell assembly can provide
increased photoconversion efficiency in a multijunction solar cell
for outer space or other applications over the operational life of
the photovoltaic power system.
[0139] Another aspect of the present disclosure is that to provide
a five junction solar cell assembly composed of an integral
semiconductor body with two interconnected spatially separated four
junction solar cell subassemblies or regions, the average band gap
of all four subcells (i.e., the sum of the four band gaps of each
subcell divided by 4) in each solar cell subassembly being greater
than 1.44 eV.
[0140] Another descriptive aspect of the present disclosure is to
characterize the fourth subcell as being composed of an indirect or
direct band gap material such that the lowest direct band gap is
greater than 0.75 eV, in some embodiments.
[0141] Another descriptive aspect of the present disclosure is to
characterize the fourth subcell as being composed of a direct band
gap material such that the lowest direct band gap is less than 0.90
eV, in some embodiments.
[0142] In some embodiments, the fourth subcell in each solar cell
subassembly is germanium, while in other embodiments the fourth
subcell is InGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn, InGaAsN,
InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN, InGaSbBiN or
other III-V or II-VI compound semiconductor material.
[0143] The indirect band gap of germanium at room temperature is
about 0.66 eV, while the direct band gap of germanium at room
temperature is 0.8 eV. Those skilled in the art will normally refer
to the "band gap" of germanium as 0.66 eV, since it is lower than
the direct band gap value of 0.8 eV.
[0144] The recitation that "the fourth subcell has a direct band
gap of greater than 0.75 eV" is therefore expressly meant to
include germanium as a possible semiconductor for the fourth
subcell, although other semiconductor materials can be used as
well.
[0145] More specifically, the present disclosure intends to provide
a relatively simple and reproducible technique that does not employ
inverted processing associated with inverted metamorphic
multijunction solar cells, and is suitable for use in a high volume
production environment in which various semiconductor layers are
grown on a growth substrate in an MOCVD reactor, and subsequent
processing steps are defined and selected to minimize any physical
damage to the quality of the deposited layers, thereby ensuring a
relatively high yield of operable solar cells meeting
specifications at the conclusion of the fabrication processes.
[0146] The lattice constants and electrical properties of the
layers in the semiconductor structure are preferably controlled by
specification of appropriate reactor growth temperatures and times,
and by use of appropriate chemical composition and dopants. The use
of a deposition method, such as Molecular Beam Epitaxy (MBE),
Organo Metallic Vapor Phase Epitaxy (OMVPE), Metal Organic Chemical
Vapor Deposition (MOCVD), or other vapor deposition methods for the
growth may enable the layers in the monolithic semiconductor
structure forming the cell to be grown with the required thickness,
elemental composition, dopant concentration and grading and
conductivity type.
[0147] The present disclosure is directed to, in one embodiment, a
growth process using a metal organic chemical vapor deposition
(MOCVD) process in a standard, commercially available reactor
suitable for high volume production. More particularly, the present
disclosure is directed to the materials and fabrication steps that
are particularly suitable for producing commercially viable
multijunction solar cells using commercially available equipment
and established high-volume fabrication processes, as contrasted
with merely academic expositions of laboratory or experimental
results.
[0148] Prior to discussing the specific embodiments of the present
disclosure, a brief discussion of some of the issues associated
with the design of multijunction solar cells, and in particular
metamorphic solar cells, and the context of the composition or
deposition of various specific layers in embodiments of the product
as specified and defined by Applicant is in order.
[0149] There are a multitude of properties that should be
considered in specifying and selecting the composition of, inter
alia, a specific semiconductor layer, the back metal layer, the
adhesive or bonding material, or the composition of the supporting
material for mounting a solar cell thereon. For example, some of
the properties that should be considered when selecting a
particular layer or material are electrical properties (e.g.
conductivity), optical properties (e.g., band gap, absorbance and
reflectance), structural properties (e.g., thickness, strength,
flexibility, Young's modulus, etc.), chemical properties (e.g.,
growth rates, the "sticking coefficient" or ability of one layer to
adhere to another, stability of dopants and constituent materials
with respect to adjacent layers and subsequent processes, etc.),
thermal properties (e.g., thermal stability under temperature
changes, coefficient of thermal expansion), and manufacturability
(e.g., availability of materials, process complexity, process
variability and tolerances, reproducibility of results over high
volume, reliability and quality control issues).
[0150] In view of the trade-offs among these properties, it is not
always evident that the selection of a material based on one of its
characteristic properties is always or typically "the best" or
"optimum" from a commercial standpoint or for Applicant's purposes.
For example, theoretical studies may suggest the use of a
quaternary material with a certain band gap for a particular
subcell would be the optimum choice for that subcell layer based on
fundamental semiconductor physics. As an example, the teachings of
academic papers and related proposals for the design of very high
efficiency (over 40%) solar cells may therefore suggest that a
solar cell designer specify the use of a quaternary material (e.g.,
InGaAsP) for the active layer of a subcell. A few such devices may
actually be fabricated by other researchers, efficiency
measurements made, and the results published as an example of the
ability of such researchers to advance the progress of science by
increasing the demonstrated efficiency of a compound semiconductor
multijunction solar cell. Although such experiments and
publications are of "academic" interest, from the practical
perspective of the Applicants in designing a compound semiconductor
multijunction solar cell to be produced in high volume at
reasonable cost and subject to manufacturing tolerances and
variability inherent in the production processes and suited for
specific applications such as the space environment where the
efficiency over the entire operational life is an important goal,
such an "optimum" design from an academic perspective is not
necessarily the most desirable design in practice, and the
teachings of such studies more likely than not point in the wrong
direction and lead away from the proper design direction. Stated
another way, such references may actually "teach away" from
Applicant's research efforts and the ultimate solar cell design
proposed by the Applicants.
[0151] In view of the foregoing, it is further evident that the
identification of one particular constituent element (e.g. indium,
or aluminum) in a particular subcell, or the thickness, band gap,
doping, or other characteristic of the incorporation of that
material in a particular subcell, is not a single, simple "result
effective variable" that one skilled in the art can simply specify
and incrementally adjust to a particular level and thereby increase
the efficiency of a solar cell at the beginning of life or the end
of life, or over a particular time span of operational use. The
efficiency of a solar cell is not a simple linear algebraic
equation as a function of band gap, or the amount of gallium or
aluminum or other element in a particular layer. The growth of each
of the epitaxial layers of a solar cell in an MOCVD reactor is a
non-equilibrium thermodynamic process with dynamically changing
spatial and temporal boundary conditions that is not readily or
predictably modeled. The formulation and solution of the relevant
simultaneous partial differential equations covering such processes
are not within the ambit of those of ordinary skill in the art in
the field of solar cell design.
[0152] Another aspect of the disclosure is to match the larger
short circuit current of the bottom subcell of the solar cell
assembly with two or three parallel stacks of solar subcells, i.e.
a configuration in which the value of the short circuit current of
the bottom subcell is at least twice, or at least three times, that
of the solar subcells in each parallel stack which are connected in
series with the bottom subcell. Stated another way, given the
choice of the composition of the bottom subcell, and thereby the
short circuit current of the bottom subcell, the upper subcell
stack is specified and designated to have a short circuit current
which is one-third or less or is one-half or less than that of the
bottom subcell.
[0153] Even when it is known that particular variables have an
impact on electrical, optical, chemical, thermal or other
characteristics, the nature of the impact often cannot be predicted
with much accuracy, particularly when the variables interact in
complex ways, leading to unexpected results and unintended
consequences. Thus, significant trial and error, which may include
the fabrication and evaluative testing of many prototype devices,
often over a period of time of months if not years, is required to
determine whether a proposed structure with layers of particular
compositions, actually will operate as intended, in a given
environment over the operational life, let alone whether it can be
fabricated in a reproducible high volume manner within the
manufacturing tolerances and variability inherent in the production
process, and necessary for the design of a commercially viable
device.
[0154] Furthermore, as in the case here, where multiple variables
interact in unpredictable ways, the proper choice of the
combination of variables can produce new and "unexpected results",
and constitute an "inventive step" in designing and specifying a
solar cell to operate in a predetermined environment (such as
space), not only at the beginning of life, but over the entire
defined operational lifetime.
[0155] Reference throughout this specification to "one embodiment"
or "an embodiment" means that a particular feature, structure, or
characteristic described in connection with the embodiment is
included in at least one embodiment of the present invention. Thus,
the appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily all referring to the same embodiment. Furthermore, the
particular features, structures, or characteristics may be combined
in any suitable manner in one or more embodiments.
[0156] One aspect of the present disclosure relates to the use and
amount of aluminum in the active layers of the upper subcells in a
multijunction solar cell (i.e. the subcells that are closest to the
primary light source). The effects of increasing amounts of
aluminum as a constituent element in an active layer of a subcell
affects the photovoltaic device performance. One measure of the
"quality" or "goodness" of a solar cell subcell or junction is the
difference between the band gap of the semiconductor material in
that subcell or junction and the V.sub.oc, or open circuit voltage,
of that same junction. The smaller the difference, the higher the
V.sub.oc of the solar cell junction relative to the band gap, and
the better the performance of the device. V.sub.oc is very
sensitive to semiconductor material quality, so the smaller the
E.sub.g/q-V.sub.oc of a device, the higher the quality of the
material in that device. There is a theoretical limit to this
difference, known as the Shockley-Queisser limit. That is the best
voltage that a solar cell junction can produce under a given
concentration of light at a given temperature.
[0157] The experimental data obtained for single junction (Al)GaInP
solar cells indicates that increasing the Al content of the
junction leads to a larger E.sub.g/q-V.sub.oc difference,
indicating that the material quality of the junction decreases with
increasing Al content. FIG. 1 shows this effect. The three
compositions cited in the Figure are all lattice matched to GaAs,
but have differing Al composition. As seen by the different
compositions represented, with increasing amount of aluminum
represented by the x-axis, adding more Al to the semiconductor
composition increases the band gap of the junction, but in so doing
also increases E.sub.g/q-V.sub.oc. Hence, we draw the conclusion
that adding Al to a semiconductor material degrades that material
such that a solar cell device made out of that material does not
perform relatively as well as a junction with less Al.
[0158] Thus, contrary to the conventional wisdom as indicated
above, the present application utilizes a substantial amount of
aluminum, i.e., over 20% aluminum by mole fraction in at least the
top subcell, and in some embodiments in one or more of the middle
subcells.
[0159] Turning to the fabrication of the multijunction solar cell
assembly of the present disclosure, and in particular a
five-junction solar cell assembly, FIG. 2A is a cross-sectional
view of a first embodiment of a semiconductor body 500 after
several stages of fabrication including the growth of certain
semiconductor layers on the growth substrate, and formation of
grids and contacts on the contact layer of the top side (i.e., the
light-facing side) of the semiconductor body.
[0160] As shown in the illustrated example of FIG. 2A, the bottom
subcell (which we refer to initially as subcell D) includes a
substrate 600 formed of p-type germanium ("Ge") in some
embodiments, which also serves as a base layer of the subcell
(i.e., the p-polarity layer of a "base-emitter" photovoltaic
junction formed by adjacent layers of opposite conductivity
type).
[0161] In some embodiments, the bottom subcell D is germanium,
while in other embodiments the fourth subcell is InGaAs, GaAsSb,
InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi,
InGaAsNSbBi, InGaSbN, InGaBiN, InGaSbBiN or other III-V or II-VI
compound semiconductor material.
[0162] The bottom subcell D, further includes, for example, a
highly doped n-type Ge emitter layer 601, and an n-type indium
gallium arsenide ("InGaAs") nucleation layer 602. The nucleation
layer or buffer 602 is deposited over the base layer, and the
emitter layer 601 is formed in the substrate by diffusion of atoms
from the nucleation layer 602 into the Ge substrate, thereby
forming the n-type Ge layer 601b.
[0163] A highly doped first lateral conduction layer 603 is
deposited over layer 602, and a blocking p-n diode or insulating
layer 604 is deposited over the layer 603. A second highly doped
lateral conduction layer 605 is then deposited over layer 604.
[0164] Heavily doped p-type aluminum gallium arsenide ("AlGaAs")
and heavily doped n-type indium gallium arsenide ("(In)GaAs")
tunneling junction layers 606, 607 may be deposited over the second
lateral conduction layer 605 to provide a low resistance pathway
between the bottom D and the middle subcell C.
[0165] In some embodiments, distributed Bragg reflector (DBR)
layers 608 are then grown adjacent to and between the tunnel
junction 606/607 and the third solar subcell C.sub.1. The DBR
layers 608 are arranged so that light can enter and pass through
the third solar subcell C.sub.1 and at least a portion of which can
be reflected back into the third solar subcell C.sub.1 by the DBR
layers 608. In the embodiment depicted in FIG. 2A, the distributed
Bragg reflector (DBR) layers 608 are specifically located between
the third solar subcell C and tunnel junction layers 607; in other
embodiments, the distributed Bragg reflector tunnel diode layers
606/607 may be located between DBR layer 608 and the third subcell
C.sub.1. In another embodiment, depicted in FIG. 2C, the tunnel
diode layer 670/671 are located between the lateral conduction
layer 602d and the first alpha layer 606.
[0166] For some embodiments, distributed Bragg reflector (DBR)
layers 608 can be composed of a plurality of alternating layers
608a through 608z of lattice matched materials with discontinuities
in their respective indices of refraction. For certain embodiments,
the difference in refractive indices between alternating layers is
maximized in order to minimize the number of periods required to
achieve a given reflectivity, and the thickness and refractive
index of each period determines the stop band and its limiting
wavelength.
[0167] For some embodiments, distributed Bragg reflector (DBR)
layers 608a through 608z includes a first DBR layer composed of a
plurality of p type Al.sub.x(In)Ga.sub.1-xAs layers, and a second
DBR layer disposed over the first DBR layer and composed of a
plurality of p type Al.sub.y(In)Ga.sub.1-yAs layers, where y is
greater than x, with 0<x<1, 0<y<1.
[0168] On top of the DBR layers 608 the subcell C.sub.1 is
grown.
[0169] In the illustrated example of FIG. 2A, the subcell C.sub.1
includes a highly doped p-type aluminum gallium arsenide
("Al(In)GaAs") back surface field ("BSF") layer 609, a p-type
InGaAs base layer 610, a highly doped n-type indium gallium
phosphide ("InGaP2") emitter layer 611 and a highly doped n-type
indium aluminum phosphide ("AlInP2") window layer 612. The InGaAs
base layer 610 of the subcell C.sub.1 can include, for example,
approximately 1.5% In. Other compositions may be used as well. The
base layer 610 is formed over the BSF layer 609 after the BSF layer
is deposited over the DBR layers 608a through 608z.
[0170] The window layer 612 is deposited on the emitter layer 611
of the subcell C.sub.1. The window layer 612 in the subcell C.sub.1
also helps reduce the recombination loss and improves passivation
of the cell surface of the underlying junctions. Before depositing
the layers of the subcell B, heavily doped n-type InGaP and p-type
Al(In)GaAs (or other suitable compositions) tunneling junction
layers 613, 614 may be deposited over the subcell C.sub.1.
[0171] The middle subcell B.sub.1 includes a highly doped p-type
aluminum (indium) gallium arsenide ("Al(In)GaAs") back surface
field ("BSF") layer 615, a p-type Al(In)GaAs base layer 616, a
highly doped n-type indium gallium phosphide ("InGaP2") or
Al(In)GaAs layer 617 and a highly doped n-type indium gallium
aluminum phosphide ("AlGaAlP") window layer 618. The InGaP emitter
layer 617 of the subcell B.sub.1 can include, for example,
approximately 50% In. Other compositions may be used as well.
[0172] Before depositing the layers of the top cell A.sub.1,
heavily doped n-type InGaP and p-type Al(In)GaAs tunneling junction
layers 619, 620 may be deposited over the subcell B.
[0173] In the illustrated example, the top subcell A.sub.1 includes
a highly doped p-type indium aluminum phosphide ("InAlP") BSF layer
621, a p-type InGaAlP base layer 622, a highly doped n-type InGaAlP
emitter layer 623 and a highly doped n-type InAlP2 window layer
624. The base layer 623 of the top subcell A.sub.1 is deposited
over the BSF layer 621 after the BSF layer 621 is formed over the
tunneling junction layers 619, 620 of the subcell B.sub.1. The
window layer 624 is deposited over the emitter layer 623 of the top
subcell A.sub.1 after the emitter layer 623 is formed over the base
layer 622.
[0174] In some embodiments, the amount of aluminum in the top
subcell A, is 20% or more by mole fraction.
[0175] A cap or contact layer 625 may be deposited and patterned
into separate contact regions over the window layer 624 of the top
subcell A.sub.1. After further processing to be described in
subsequent Figures, the solar cell assembly 500 can be provided
with grid lines, interconnecting bus lines, and contact pads on the
top surface. The geometry and number of the grid lines, bus lines
and/or contacts may vary in different implementations.
[0176] The cap or contact layer 625 serves as an electrical contact
from the top subcell A.sub.1 to metal grid 626. The doped cap or
contact layer 625 can be a semiconductor layer such as, for
example, a GaAs or InGaAs layer.
[0177] After the cap or contact layer 625 is deposited, the grid
lines 626 are formed via evaporation and lithographically patterned
and deposited over the cap or contact layer 625.
[0178] A contact pad 627 connected to the grid 626 is formed on one
edge of the subassembly 500 to allow an electrical interconnection
to be made, inter alia, to an adjacent subassembly.
[0179] The subcells A.sub.2, B.sub.2, C.sub.2 of the solar cell
subassembly 500 can be configured so that the short circuit current
densities of the three subcells A.sub.2, B.sub.2, C.sub.2 have a
substantially equal predetermined first value (i.e., J1=J2=J3), and
the short circuit current density (J4) of the bottom subcell
D.sub.2 is at least twice that of the predetermined first
value.
[0180] FIG. 2B is a cross-sectional view of a second embodiment of
a semiconductor body including a four solar subcells including two
lattice mismatched subcells with a metamorphic layer between them,
after several stages of fabrication including the growth of certain
semiconductor layers on the growth substrate up to the contact
layer, according to the present disclosure.
[0181] As shown in the previously illustrated example of FIG. 2A,
the bottom subcell (which we refer to initially as subcell D)
includes a substrate 600 formed of p-type germanium ("Ge") in some
embodiments, which also serves as a base layer, and since the
layers 601 through 605 are substantially the same as described in
connection with FIG. 2A, they will not be described in detail here
for brevity.
[0182] In the embodiment of FIG. 2B, a first alpha layer 606a,
composed of n-type (Al)GaIn(As)P, is deposited over the first
lateral conduction layer 605, to a thickness of between 0.25 and
1.0 micron. Such an alpha layer is intended to prevent threading
dislocations from propagating, either opposite to the direction of
growth into the bottom subcell D, or in the direction of growth
into the subcell C, and is more particularly described in U.S.
Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.).
[0183] A metamorphic layer (or graded interlayer) 606b is deposited
over the first alpha layer 606a using a surfactant. Layer 604 is
preferably a compositionally step-graded series of InGaAlAs layers,
preferably with monotonically changing lattice constant, so as to
achieve a gradual transition in lattice constant in the
semiconductor structure from subcell D.sub.1 to subcell C.sub.1
while minimizing threading dislocations from occurring. The band
gap of layer 606b is either constant throughout its thickness, in
one embodiment approximately equal to 1.42 to 1.62 eV, or otherwise
consistent with a value slightly greater than the band gap of the
middle subcell C.sub.1, or may vary within the above noted region.
One embodiment of the graded interlayer may also be expressed as
being composed of (Al)In.sub.xGa.sub.1-xAs, with 0<x<1, and x
selected such that the band gap of the interlayer is in the range
of at approximately 1.42 to 1.62 eV or other appropriate band
gap.
[0184] In the surfactant assisted growth of the metamorphic layer
606b, a suitable chemical element is introduced into the reactor
during the growth of layer 606b to improve the surface
characteristics of the layer. In one embodiment, such element may
be a dopant or donor atom such as selenium (Se) or tellurium (Te).
Small amounts of Se or Te are therefore incorporated in the
metamorphic layer 604, and remain in the finished solar cell.
Although Se or Te are the preferred n-type dopant atoms, other
non-isoelectronic surfactants may be used as well.
[0185] Surfactant assisted growth results in a much smoother or
planarized surface. Since the surface topography affects the bulk
properties of the semiconductor material as it grows and the layer
becomes thicker, the use of the surfactants minimizes threading
dislocations in the active regions, and therefore improves overall
solar cell efficiency.
[0186] As an alternative to the use of non-isoelectronic one may
use an isoelectronic surfactant. The term "isoelectronic" refers to
surfactants such as antimony (Sb) or bismuth (Bi), since such
elements have the same number of valence electrons as the P atom of
InGaP, or the As atom in InGaAlAs, in the metamorphic buffer layer.
Such Sb or Bi surfactants will not typically be incorporated into
the metamorphic layer 606b.
[0187] In one embodiment of the present disclosure, the layer 606b
is composed of a plurality of layers of (Al)InGaAs, with
monotonically changing lattice constant, each layer having a band
gap, approximately in the range of 1.42 to 1.62 eV. In some
embodiments, the band gap is in the range of 1.45 to 1.55 eV. In
some embodiments, the band gap is in the range of 1.5 to 1.52
eV.
[0188] The advantage of utilizing the embodiment of a constant
bandgap material such as InGaAs is that arsenide-based
semiconductor material is much easier to process in standard
commercial MOCVD reactors.
[0189] Although the preferred embodiment of the present disclosure
utilizes a plurality of layers of (Al)InGaAs for the metamorphic
layer 606b for reasons of manufacturability and radiation
transparency, other embodiments of the present disclosure may
utilize different material systems to achieve a change in lattice
constant from subcell C.sub.1 to subcell D. Other embodiments of
the present disclosure may utilize continuously graded, as opposed
to step graded, materials. More generally, the graded interlayer
may be composed of any of the As, P, N, Sb based III-V compound
semiconductors subject to the constraints of having the in-plane
lattice parameter greater than or equal to that of the third solar
cell and less than or equal to that of the fourth solar cell, and
having a bandgap energy greater than that of the third solar
cell.
[0190] A second alpha layer 606c, composed of n+ type GaInP, is
deposited over metamorphic buffer layer 606b, to a thickness of
between 0.25 and about 1.0 micron. Such second alpha layer is
intended to prevent threading dislocations from propagating, either
opposite to the direction of growth into the subcell D, or in the
direction of growth into the subcell C.sub.1, and is more
particularly described in U.S. Patent Application Pub. No.
2009/0078309 A1 (Cornfeld et al.).
[0191] A second embodiment of the second solar cell subassembly
similar to that of FIG. 2C is another configuration (not shown)
with that the metamorphic buffer layer 604 is disposed above the
tunnel diode layers 706, 707 and below the DBR layers 708 (not
illustrated).
[0192] Heavily doped p-type aluminum gallium arsenide ("AlGaAs")
and heavily doped n-type indium gallium arsenide ("(In)GaAs")
tunneling junction layers 607a, 607b may be deposited over the
alpha layer 606c to provide a low resistance pathway between the
bottom and middle subcells D and C.sub.1.
[0193] Since the tunnel diode layers 607a, 607b, and the
subsequently grown layers 608 through 625 are substantially the
same as described in connection with FIG. 2A, they will not be
described here in detail for brevity.
[0194] Turing to FIG. 2C, following the deposition of the
semiconductor layers 602 through 625, the semiconductor body 501 is
partially etched from the backside (i.e., through the substrate
600) to form a channel 670 that partially bisects the wafer or
semiconductor body, and several ledges or platforms are formed on
intermediate layers so that electrical contact may be made thereto,
in particular, in one embodiment depicted in this Figure, ledges
666, and 667.
[0195] To this end, the solar cell assembly can include a plurality
of openings in the semiconductor body, each of the openings
extending from a bottom surface of the semiconductor body to a
different respective layer in the semiconductor body. Such
"openings" may include recesses, cavities, holes, gaps, cut-outs,
or similar structures, but for simplicity we will subsequently just
use the term "opening" throughout this disclosure. In other
implementations, we can etch through the top or the side of the
substrate and have some or all the openings come from one or more
sides. This approach may be more efficient than etching from the
top side as it does not shadow the top two or top three solar
subcells, and results in a solar epitaxial structure of only a few
tens of microns in thickness.
[0196] FIG. 2D is a cross-sectional view of the embodiment of FIG.
2B following the steps of etching contact ledges on various
semiconductor layers according to a second implementation in the
present disclosure. In particular, in addition to the two ledges
667 and 666 depicted in FIG. 2C, there is a third ledge 668 in the
substrate 600 which is etched to allow electrical contact to be
made to the p terminal of subcell D.
[0197] As a result of the etching process depicted in FIG. 2C or
FIG. 2D, the semiconductor body 501 is divided into two
semiconductor regions, with one depicted on the left hand side of
the Figure and one on the right hand side. The bottom surface of a
portion of the highly doped second lateral conduction layer 605 is
exposed by the etching process and forms a ledge 667. The blocking
p-n diode or insulating layer 604 is divided into two portions,
with one in each of the respective semiconductor region, one
portion 604a being on the left and one portion 604b being on the
right of the Figure. Similarly, the first highly doped lateral
conduction layer 603 is divided into two parts, with one in each
respective semiconductor region, one portion 603a on the left and
one portion 603b on the right of the Figure. A ledge 666 is formed
on the left portion 603a of the first highly doped lateral
conduction layer 603, and a ledge 669 is formed on the right
portion 603b of the first highly doped lateral conduction layer
603.
[0198] The buffer layer 602 and the subcell D 600/601 is divided
into two semiconductor regions. One portion 602a of the buffer
layer on the left hand side of the Figure and one portion 602b of
the buffer layer on the right hand side. One portion 600a/601a of
the solar subcell D (which we now designate as solar subcell
D.sub.1) on the left hand side of the Figure and one portion
600a/601a of the solar cell D (which we now designate as solar
subcell D.sub.2) the right hand side. A ledge 668 is formed on the
left portion 600a of the subcell D.sub.1.
[0199] After the cap or contact layer 625 is deposited, the grid
lines 626 are formed via evaporation and lithographically patterned
and deposited over the cap or contact layer 625.
[0200] A contact pad 627 connected to the grid 626 is formed on one
edge of the subassembly 500 to allow an electrical interconnection
to be made, inter alia, to an adjacent subassembly.
[0201] As with the first solar cell subassembly 500, the subcells
A.sub.2, B.sub.2, C.sub.2 of the second solar cell subassembly 700
can be configured so that the short circuit current densities of
the three subcells A.sub.2, B.sub.2, C.sub.2 have a substantially
equal predetermined first value (J1<=J2<=J3), and the short
circuit current density (J4) of the bottom subcell D.sub.2 is at
least twice that of the predetermined first value.
[0202] FIG. 2E is a cross-sectional view of the multijunction solar
cell subassembly 500 of FIG. 2A after additional stages of
fabrication including the deposition of metal contact pads on the
ledges depicted in FIG. 2D.
[0203] A metal contact pad 680 is deposited on the surface of the
ledge of 667 which exposes a portion of the bottom surface of the
lateral conduction layer 605b. This pad 680 allows electrical
contact to be made to the bottom of the stack of subcells A.sub.1
through C.sub.1.
[0204] A metal contact pad 681 is deposited on the surface of the
ledge of 666 which exposes a portion of the bottom surface of the
lateral conduction layer 603a. This pad 681 allows electrical
contact to be made to the n-polarity terminal of subcell
D.sub.1.
[0205] A metal contact pad 682 is deposited on the surface of the
ledge of 669 which exposes a portion of the bottom surface of the
lateral conduction layer 603b. This pad 682 allows electrical
contact to be made to the n-polarity terminal of subcell
D.sub.2.
[0206] A metal contact pad 683 is deposited on the surface of the
ledge of 668 which exposes a portion of the surface of the
p-polarity region of subcell D.sub.1. Alternatively, contact may be
made to a part of the back metal layer 684, which allows electrical
contact to be made to the p-terminal of subcell D.sub.1.
[0207] For example, as shown in the bottom plan view depicted in
FIG. 3, conductive (e.g., metal) interconnections 690 (i.e., 690a
and 690b), and 691 (i.e., 691a and 691b) can be made between
different layers of the solar cell subregions. Similarly, the
interconnection 691 connects together a contact 683 on the p-region
600a of the solar subcell D.sub.1 to a contact 682 on the lateral
conduction layer 603b associated with the solar subcell D.sub.2.
Likewise, the interconnection 690 connects together a contact 681
on the lateral conduction layer 603a associated with the solar
subcell D.sub.1 to a contact 680 on the lateral conduction layer
605a and 605b.
[0208] As noted above, the solar cell assembly includes a first
electrical contact of a first polarity and a second electrical
contact of a second polarity. In some embodiments, the first
electrical contact 695 is connected to the metal contact 627 on the
solar cell subassembly 500 by an interconnection 694, and the
second electrical contact 693 is connected to the back metal
contact of the solar subcell D.sub.2 by interconnection 692.
[0209] As illustrated in FIG. 3, two or more solar cell subregions
684 and 685 can be connected electrically as described above to
obtain a multijunction (e.g. a four-, five- or six-junction) solar
cell assembly.
[0210] Some implementations provide that at least the base of at
least one of the first, second or third solar subcells has a graded
doping, i.e., the level of doping varies from one surface to the
other throughout the thickness of the base layer. In some
embodiments, the gradation in doping is exponential. In some
embodiments, the gradation in doping is incremental and
monotonic.
[0211] In some embodiments, the emitter of at least one of the
first, second or third solar subcells also has a graded doping,
i.e., the level of doping varies from one surface to the other
throughout the thickness of the emitter layer. In some embodiments,
the gradation in doping is linear or monotonically decreasing.
[0212] As a specific example, the doping profile of the emitter and
base layers may be illustrated in FIG. 4, which depicts the amount
of doping in the emitter region and the base region of a subcell.
N-type dopants include silicon, selenium, sulfur, germanium or tin.
P-type dopants include silicon, zinc, chromium, or germanium.
[0213] In the example of FIG. 4, in some embodiments, one or more
of the subcells have a base region having a gradation in doping
that increases from a value in the range of 1.times.10.sup.15 to
1.times.10.sup.18 free carriers per cubic centimeter adjacent the
p-n junction to a value in the range of 1.times.10.sup.16 to
4.times.10.sup.18 free carriers per cubic centimeter adjacent to
the adjoining layer at the rear of the base, and an emitter region
having a gradation in doping that decreases from a value in the
range of approximately 5.times.10.sup.18 to 1.times.10.sup.17 free
carriers per cubic centimeter in the region immediately adjacent
the adjoining layer to a value in the range of 5.times.10.sup.15 to
1.times.10.sup.18 free carriers per cubic centimeter in the region
adjacent to the p-n junction.
[0214] FIG. 5 is a schematic diagram of the five junction solar
cell assembly of FIG. 2E that includes two solar cell semiconductor
regions, each of which includes four subcells. The bottom (i.e.,
fourth) subcell D.sub.1 of the left region 684 is connected in a
series electrical circuit with the bottom (i.e., fourth) subcell
D.sub.2 of the right region 685. On the other hand, the upper and
middle subcells are connected in parallel with one another (i.e.,
subcells A.sub.1, B.sub.1, C.sub.1 are connected in parallel with
subcells A.sub.2, B.sub.2, C.sub.2).
[0215] In some implementations of a five-junction solar cell
assembly, such as in the example of FIG. 5, the short circuit
density (J.sub.sc) of the upper first subcells (A.sub.1 and
A.sub.2) and the middle subcells (B.sub.1, B.sub.2, C.sub.1,
C.sub.2) is about 11 mA/cm.sup.2, and the short circuit current
density (J.sub.sc) of the bottom subcells (D.sub.1 and D.sub.2) is
about 22 mA/cm.sup.2 or greater. Other implementations may have
different values.
[0216] The present disclosure like that of the parallel
applications, U.S. patent application Ser. Nos. 14/828,206 and
15/213,594, provides a multijunction solar cell that follows a
design rule that one should incorporate as many high band gap
subcells as possible to achieve the goal to increase the efficiency
at high temperature EOL. For example, high band gap subcells may
retain a greater percentage of cell voltage as temperature
increases, thereby offering lower power loss as temperature
increases. As a result, both high temperature beginning-of-life
(HT-BOL) and HT-EOL performance of the exemplary multijunction
solar cell, according to the present disclosure, may be expected to
be greater than traditional cells.
[0217] The open circuit voltage (V.sub.oc) of a compound
semiconductor subcell loses approximately 2 mV per degree C. as the
temperature rises, so the design rule taught by the present
disclosure takes advantage of the fact that a higher band gap (and
therefore higher voltage) subcell loses a lower percentage of its
V.sub.oc with temperature. For example, a subcell that produces a
1.50 volts at 28.degree. C. produces 1.50-42*(0.0023)=1.403 volts
at 70.degree. C. which is a 6.4% voltage loss, A cell that produces
0.25 volts at 28.degree. C. produces 0.25-42*(0.0018)=0.174 volts
at 70.degree. which is a 30.2% voltage loss.
[0218] For example, the cell efficiency (%) measured at room
temperature (RT) 28.degree. C. and high temperature (HT) 70.degree.
C., at beginning of life (BOL) and end of life (EOL), for a
standard three junction commercial solar cell (e.g. a SolAero
Technologies Corp. Model ZTJ), such as depicted in FIG. 2 of U.S.
patent application Ser. No. 14/828,206, is as follows:
TABLE-US-00001 Condition Efficiency BOL 28.degree. C. 29.1% BOL
70.degree. C. 26.4% EOL 70.degree. C. 23.4% After 5E14 e/cm.sup.2
radiation EOL 70.degree. C. 22.0% After 1E15 e/cm.sup.2
radiation
[0219] For the 5J solar cell assembly (comprising two
interconnected four-junction subassemblies) described in the
present disclosure, the corresponding data is as follows:
TABLE-US-00002 Condition Efficiency BOL 28.degree. C. 30.6% BOL
70.degree. C. 27.8% EOL 70.degree. C. 26.6% After 5E14 e/cm.sup.2
radiation EOL 70.degree. C. 26.1% After 1E15 e/cm.sup.2
radiation
[0220] The new solar cell of the present disclosure has a slightly
higher cell efficiency than the standard commercial solar cell
(ZTJ) at BOL at 70.degree. C. However, more importantly, the solar
cell described in the present disclosure exhibits substantially
improved cell efficiency (%) over the standard commercial solar
cell (ZTJ) at 1 MeV electron equivalent fluence of
5.times.10.sup.14 e/cm.sup.2, and dramatically improved cell
efficiency (%) over the standard commercial solar cell (ZTJ) at 1
MeV electron equivalent fluence of 1.times.10.sup.15
e/cm.sup.2.
[0221] The selected radiation exposure levels noted above are meant
to simulate the environmental conditions of typical satellites in
earth orbit. A low earth orbit (LEO) satellite will typically
experience radiation equivalent to 5.times.10.sup.14 e/cm.sup.2
over a five year lifetime. A geosynchronous earth orbit (GEO)
satellite will typically experience radiation in the range of
5.times.10.sup.14 e/cm.sup.2 to 1.times.10 e/cm.sup.2 over a
fifteen year lifetime.
[0222] The wide range of electron and proton energies present in
the space environment necessitates a method of describing the
effects of various types of radiation in terms of a radiation
environment which can be produced under laboratory conditions. The
methods for estimating solar cell degradation in space are based on
the techniques described by Brown et al. [Brown, W. L., J. D.
Gabbe, and W. Rosenzweig, Results of the Telstar Radiation
Experiments, Bell System Technical J., 42, 1505, 1963] and Tada
[Tada, H. Y., J. R. Carter, Jr., B. E. Anspaugh, and R. G. Downing,
Solar Cell Radiation Handbook, Third Edition, JPL Publication
82-69, 1982]. In summary, the omnidirectional space radiation is
converted to a damage equivalent unidirectional fluence at a
normalised energy and in terms of a specific radiation particle.
This equivalent fluence will produce the same damage as that
produced by omnidirectional space radiation considered when the
relative damage coefficient (RDC) is properly defined to allow the
conversion. The relative damage coefficients (RDCs) of a particular
solar cell structure are measured a priori under many energy and
fluence levels. When the equivalent fluence is determined for a
given space environment, the parameter degradation can be evaluated
in the laboratory by irradiating the solar cell with the calculated
fluence level of unidirectional normally incident flux. The
equivalent fluence is normally expressed in terms of 1 MeV
electrons or 10 MeV protons.
[0223] The software package Spenvis (www.spenvis.oma.be) is used to
calculate the specific electron and proton fluence that a solar
cell is exposed to during a specific satellite mission as defined
by the duration, altitude, azimuth, etc. Spenvis employs the EQFLUX
program, developed by the Jet Propulsion Laboratory (JPL) to
calculate 1 MeV and 10 MeV damage equivalent electron and proton
fluences, respectively, for exposure to the fluences predicted by
the trapped radiation and solar proton models for a specified
mission environment duration. The conversion to damage equivalent
fluences is based on the relative damage coefficients determined
for multijunction cells [Marvin, D. C., Assessment of Multijunction
Solar Cell Performance in Radiation Environments, Aerospace Report
No. TOR-2000 (1210)-1, 2000]. A widely accepted total mission
equivalent fluence for a geosynchronous satellite mission of 15
year duration is 1 MeV 1.times.10.sup.15 electrons/cm.sup.2.
[0224] The exemplary solar cell described herein may require the
use of aluminum in the semiconductor composition of each of the top
two subcells. Aluminum incorporation is widely known in the III-V
compound semiconductor industry to degrade BOL subcell performance
due to deep level donor defects, higher doping compensation,
shorter minority carrier lifetimes, and lower cell voltage and an
increased BOL E.sub.g/q-V.sub.oc metric. However, consideration of
the EOL E.sub.g/q-V.sub.oc metric, or more generally, total power
output over a defined predetermined operational life, in the
present disclosure presents a different approach. In short,
increased BOL E.sub.g/q-V.sub.oc may be the most problematic
shortcoming of aluminum containing subcells; the other limitations
can be mitigated by modifying the doping schedule or thinning base
thicknesses.
[0225] 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 structures or constructions differing from the
types of structures or constructions described above.
[0226] Although described embodiments of the present disclosure
utilizes a vertical tandem stack of four subcells, various aspects
and features of the present disclosure can apply to tandem stacks
with fewer or greater number of subcells, i.e. two junction cells,
three junction cells, five junction cells, etc.
[0227] In addition, although the disclosed embodiments are
configured with top and bottom electrical contacts, the subcells
may alternatively be contacted by means of metal contacts to
laterally conductive semiconductor layers between the subcells.
Such arrangements may be used to form 3-terminal, 4-terminal, and
in general, n-terminal devices. The subcells can be interconnected
in circuits using these additional terminals such that most of the
available photogenerated current density in each subcell can be
used effectively, leading to high efficiency for the multijunction
cell, notwithstanding that the photogenerated current densities are
typically different in the various subcells.
[0228] As noted above, the solar cell described in the present
disclosure may utilize an arrangement of one or more, or all,
homojunction cells or subcells, i.e., a cell or subcell in which
the p-n junction is formed between a p-type semiconductor and an
n-type semiconductor both of which have the same chemical
composition and the same band gap, differing only in the dopant
species and types, and one or more heterojunction cells or
subcells. Subcell A, with p-type and n+ type InGaAlP is one example
of a homojunction subcell.
[0229] In some cells, a thin so-called "intrinsic layer" may be
placed between the emitter layer and base layer, with the same or
different composition from either the emitter or the base layer.
The intrinsic layer may function to suppress minority-carrier
recombination in the space-charge region. Similarly, either the
base layer or the emitter layer may also be intrinsic or
not-intentionally-doped ("NID") over part or all of its
thickness.
[0230] The composition of the window or BSF layers may utilize
other semiconductor compounds, subject to lattice constant and band
gap requirements, and may include AlInP, AlAs, AlP, GaP, InP, GaSb,
AlSb, InAs, InSb, ZnSe, AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs,
GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb,
GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AlN, GaN, InN, GaInN, AlGaInN,
GaInNAs, GaInNAsSb, GaNAsSb, GaNAsInBi, GaNAsSbBi, GaNAsInBiSb,
AlGaInNAs, ZnSSe, CdSSe, SiGe, SiGeSn, and similar materials, and
still fall within the spirit of the present invention.
[0231] While the solar cell described in the present disclosure has
been illustrated and described as embodied in a conventional
multijunction solar cell, it is not intended to be limited to the
details shown, since it is also applicable to inverted metamorphic
solar cells, and various modifications and structural changes may
be made without departing in any way from the spirit of the present
invention.
[0232] Thus, while the description of the semiconductor device
described in the present disclosure has focused primarily on solar
cells or photovoltaic devices, persons skilled in the art know that
other optoelectronic devices, such as thermophotovoltaic (TPV)
cells, photodetectors and light-emitting diodes (LEDS), are very
similar in structure, physics, and materials to photovoltaic
devices with some minor variations in doping and the minority
carrier lifetime. For example, photodetectors can be the same
materials and structures as the photovoltaic devices described
above, but perhaps more lightly-doped for sensitivity rather than
power production. On the other hand LEDs can also be made with
similar structures and materials, but perhaps more heavily-doped to
shorten recombination time, thus radiative lifetime to produce
light instead of power. Therefore, this invention also applies to
photodetectors and LEDs with structures, compositions of matter,
articles of manufacture, and improvements as described above for
photovoltaic cells.
[0233] Without further analysis, from the foregoing others can, by
applying current knowledge, readily adapt the present invention for
various applications. Such adaptations should and are intended to
be comprehended within the meaning and range of equivalence of the
following claims.
* * * * *