U.S. patent application number 17/579420 was filed with the patent office on 2022-05-12 for multijunction solar cell assembly.
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 | 20220149211 17/579420 |
Document ID | / |
Family ID | |
Filed Date | 2022-05-12 |
United States Patent
Application |
20220149211 |
Kind Code |
A1 |
DERKACS; DANIEL |
May 12, 2022 |
MULTIJUNCTION SOLAR CELL ASSEMBLY
Abstract
A multijunction solar cell assembly and its method of
manufacture including interconnected first and second discreate
semiconductor body subassemblies disposed adjacent and parallel to
each other, in the sense of the incoming illumination, each
semiconductor body subassembly including first top subcell, and
possibly third middle subcells and a bottom solar subcell; wherein
the interconnected subassemblies form at least a Three junction
solar cell by a series connection being formed between the bottom
solar subcell in the first semiconductor body with its at least
least two junctions and the bottom solar subcell in the second
semiconductor body representing the additional junction.
Inventors: |
DERKACS; DANIEL;
(Albuquerque, NM) |
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Applicant: |
Name |
City |
State |
Country |
Type |
SolAero Technologies Corp. |
Albuquerque |
NM |
US |
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Assignee: |
SolAero Technologies Corp.
Albuquerque
NM
|
Appl. No.: |
17/579420 |
Filed: |
January 19, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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17119115 |
Dec 11, 2020 |
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17579420 |
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16803519 |
Feb 27, 2020 |
10896982 |
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17119115 |
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15980983 |
May 16, 2018 |
10714636 |
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16803519 |
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15249204 |
Aug 26, 2016 |
9935209 |
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15980983 |
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62288181 |
Jan 28, 2016 |
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International
Class: |
H01L 31/02 20060101
H01L031/02; H01L 31/078 20060101 H01L031/078; H01L 31/0693 20060101
H01L031/0693; H01L 31/0224 20060101 H01L031/0224; H01L 31/0687
20060101 H01L031/0687; H01L 31/18 20060101 H01L031/18; H01L 31/054
20060101 H01L031/054; H01L 31/05 20060101 H01L031/05; H01L 31/0725
20060101 H01L031/0725; H01L 31/0735 20060101 H01L031/0735 |
Claims
1. A multijunction solar cell assembly comprising: a first
semiconductor body, including a first tandem vertical stack of
semiconductor layers forming at least an upper solar subcell having
a top surface facing the incoming illumination, and a bottom solar
subcell having an emitter region and a base region disposed below
the emitter region, and a back metal layer disposed below the base
region, a first cut-out in the first semiconductor body on a first
edge of the first semiconductor body, the first cut-out extending
through the thickness of the semiconductor body from the top
surface of the upper solar subcell in the first semiconductor body
to at least one of the layers in the first semiconductor body and
terminating at and forming a first ledge on the one layer in the
first semiconductor body; a second semiconductor body mounted
adjacent to and, with respect to the incoming illumination,
parallel to, the first semiconductor body, including a second
tandem vertical stack of semiconductor layers forming at least an
upper solar subcell and a bottom solar subcell having an emitter
region and a base region disposed below the emitter region; and a
second cut-out in the second semiconductor body extending through
the thickness of the semiconductor body from the top surface of the
upper solar subcell in the second semiconductor body to one of the
layers in the second semiconductor body and terminating at and
forming a second ledge on the one layer in the second semiconductor
body.
2. A multijunction solar cell as defined in claim 1, wherein
comprising: the bottom solar subcell includes a back metal layer
disposed below the base region of the bottom solar subcell.
3. A multijunction solar cell as defined in claim 1, further
comprising: a first cut-out in the first semiconductor body on a
first edge of the first semiconductor body, the first cut-out
extending through the thickness of the first semiconductor body
from the top surface of the upper solar subcell in the first
semiconductor body to at least one of the layers in the first
semiconductor body and terminating at and forming a first ledge on
the one layer in the first semiconductor body.
4. A multijunction solar cell as defined in claim 3, further
comprising: a second cut-out in the second semiconductor body
extending through the thickness of the semiconductor body from the
top surface of the upper solar subcell in the second semiconductor
body to one of the layers in the second semiconductor body and
terminating at and forming a second ledge on the one layer in the
second semiconductor body.
5. A multijunction solar cell assembly as defined in claim 4,
wherein the first cut-out lies along a first edge of the first
semiconductor body, the second cut-out lies along a second edge of
the second semiconductor body said first edge being directly
adjacent to a second edge of the second semiconductor body.
6. A multijunction solar cell assembly as defined in claim 3,
further comprising providing a first metal contact on the first
ledge in the first semiconductor body.
7. A multijunction solar cell assembly as defined in claim 4,
further comprising providing a second metal contact on the second
ledge in the second semiconductor body.
8. A multijunction solar cell assembly as defined in claim 7,
further comprising a first electrical interconnect coupling the
first metal contact and the second metal contact.
9. A multijunction solar cell assembly as defined in claim 8,
wherein the first electrical interconnect makes an electrical
connection between the base region of the bottom solar subcell in
the first semiconductor body with the emitter region of the bottom
solar subcell in the second semiconductor body.
10. A multijunction solar cell assembly as defined in claim 3,
wherein the first ledge in the first semiconductor body is disposed
on the back metal layer in the first semiconductor body.
11. A multijunction solar cell assembly as defined in claim 4,
wherein the tandem vertical stack in the first semiconductor body
includes a middle solar subcell disposed below the upper solar
subcell and above the bottom solar subcell, and further comprising
a third cut-out in the first semiconductor body extending through
the thickness of the first semiconductor body from the top surface
of the upper solar subcell in the first semiconductor body to a
semiconductor layer in the first semiconductor body disposed below
the middle solar subcell and above the bottom solar subcell in the
first semiconductor body.
12. A multijunction solar cell assembly as defined in claim 11,
wherein the semiconductor layer in the first semiconductor body at
which the third cut-out terminates is a first highly doped lateral
conduction layer, and further comprising a third ledge on the first
highly doped lateral conduction layer
13. A multijunction solar cell assembly as defined in claim 12,
further comprising a third metal contact on the third ledge in the
first semiconductor body.
14. A multijunction solar cell assembly as defined in claim 12,
wherein in the tandem vertical stack in the second semiconductor
body includes a middle solar subcell disposed below the upper solar
subcell and above the bottom solar subcell, and further comprising
a third cut-out in the second semiconductor body extending through
the thickness of the second semiconductor body from the top surface
of the upper solar subcell in the second semiconductor body to a
semiconductor layer in the second semiconductor body disposed below
the middle solar subcell and above the bottom solar subcell in the
second semiconductor body.
15. A multijunction solar cell assembly as defined in claim 14,
wherein the semiconductor layer in the second semiconductor body at
which the fourth cut-out terminates is a third highly doped lateral
conduction layer and further comprising forming a fourth ledge on
the third highly doped lateral conduction layer.
16. A multijunction solar cell assembly as defined in claim 15,
further comprising a fourth metal contact pad disposed on the
fourth ledge of the second semiconductor body, and a second
electrical interconnect coupling the third metal contact and the
second metal contact.
17. A multijunction solar cell assembly as defined by claim 16,
wherein the second electrical interconnect makes an electrical
connection between the emitter region of the bottom solar subcell
in the first semiconductor body with the base region of the middle
solar subcell in the second semiconductor body.
18. A multijunction solar cell assembly as defined in claim 15,
further comprising a first blocking p-n diode or insulating layer
disposed adjacent to and above the second highly doped lateral
conduction layer, and below the third highly doped lateral
conduction layer in the second semiconductor body.
19. A multijunction solar cell assembly as defined in claim 12,
further comprising a fifth cut-out in the first semiconductor body
on a first edge of the first semiconductor body, the fifth cut-out
extending through the thickness of the semiconductor body to at
least one the layers in the first semiconductor body and
terminating at a fourth highly doped lateral conduction layer in
the first semiconductor body and forming a fifth ledge on the
fourth highly doped lateral conduction layer in the first
semiconductor body,
20. A multijunction solar cell assembly as defined in claim 19,
further comprising a second blocking p-n diode or insulating layer
disposed adjacent to and above the first highly doped lateral
conduction layer, and below the fourth highly doped lateral
conduction layer in the first semiconductor body.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 17/119,115 filed Dec. 11, 2020, which was a divisional of
U.S. patent application Ser. No. 16/803,519 filed Feb. 27, 2020,
now U.S. Pat. No. 10,896,982, which was a divisional of U.S. patent
application Ser. No. 15/980,983 filed May 16, 2018, now U.S. Pat.
No. 10,714,636, which was a divisional of U.S. patent application
Ser. No. 15/249,204 filed Aug. 26, 2016, now U.S. Pat. No.
9,935,209, which claims the benefit of U.S. Provisional Application
No. 62/288,181 filed Jan. 28, 2016.
[0002] The present application is related to U.S. patent
application Ser. No. 15/249,185 filed Aug. 26, 2016, now U.S. Pat.
No. 9,985,161.
[0003] This application is also related to 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.
[0004] This application is also related to U.S. patent application
Ser. No. 13/872,663 filed Apr. 29, 2012, now U.S. Pat. No.
10,541,349, which was also a continuation-in-part of application
Ser. No. 12/337,043, filed Dec. 17, 2008.
[0005] This application is also related to U.S. patent application
Ser. Nos. 14/828,197 and 14/828,206 filed Aug. 17, 2015.
[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 bodies 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 a specific future time, often referred to as the "end
of life" (EOL).
[0010] 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.
[0011] 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.
[0012] 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.
[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. The designation and specification of such parameters is
a non-trivial engineering undertaking, and would vary depending
upon the specific space mission and customer design requirements.
Such design variables are interdependent and interact in complex
and often unpredictable ways. In the terminology of some, such
parameters are NOT simple "result effective" variables to those
skilled in the art confronted with complex design specifications
and practical operational considerations.
[0014] As a result of the selection of such design variables,
electrical properties such as the short circuit current density
(J.sub.sc), the open circuit voltage (V.sub.oc), and the fill
factor, which determine the efficiency and power output of the
solar cell, are thereby affected by the slightest change in such
design variables, and as noted above, to further complicate the
calculus, such variables and resulting properties also vary over
time (i.e. during the operational life of the system).
[0015] One electrical 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. Again, the value of such parameter 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. 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
bodies based on II-V semiconductor compounds.
SUMMARY OF THE DISCLOSURE
Objects of the Disclosure
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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 so that the total current provided by
the two subassemblies matches the total current handling capability
of the bottom subcell of the assembly.
[0021] 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.
[0022] 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
[0023] Briefly, and in general terms, the present disclosure
describes solar cells that include a solar cell assembly of two or
more solar cell subassemblies, each of which includes a respective
monolithic semiconductor body composed of a tandem stack of solar
subcells, where the subassemblies are interconnected electrically
to one another.
[0024] As described in greater detail, the inventors of the present
application have discovered that interconnecting two or more
spatially split multijunction solar cell 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
noted in the Reference to Related Applications).
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] Briefly, and in general terms, the present disclosure
provides a five junction solar cell comprising:
(a) a first semiconductor body including:
[0031] 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;
[0032] 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;
[0033] 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;
[0034] 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
[0035] 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 body disposed adjacent and parallel to
the first semiconductor body and including:
[0036] 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;
[0037] 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;
[0038] 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;
[0039] an interlayer adjacent to said third solar subcell, said
interlayer having a fourth band gap greater than said third band
gap; and
[0040] 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 body is
electrically coupled with the top contact of the second
semiconductor body and to a terminal of first polarity;
[0041] wherein the first contact on the top surface of the fourth
solar subcell of the first semiconductor body is electrically
coupled with the bottom contact of the third solar subcell of the
second semiconductor body; and
[0042] the second contact on the bottom surface of the fourth solar
subcell of the first semiconductor body is electrically coupled
with the first contact on the top surface of the fourth solar
subcell of the second semiconductor body thereof so as to form a
five junction solar cell.
[0043] In some embodiments, the interlayer in each of the first and
second semiconductor bodies 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.
[0044] In some embodiments, the interlayer in each of the first and
second semiconductor bodies is compositionally graded to
substantially lattice match the upper solar subcell on one side and
the adjacent lower fourth 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 approximately equal to that of the
third solar subcell on the first surface and greater than or
approximately equal to that of the lower fourth solar subcell on
the 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 first and second semiconductor
bodies are separate semiconductor bodies that are disposed adjacent
and parallel to each other. The semiconductor bodies being
"parallel" to each other is meant that the bodies are disposed so
that the incoming light illuminates both the upper first solar
subcell of the first semiconductor body and the first solar subcell
of the second semiconductor body, and that parallel light beams
traverses the stack of subcells of each semiconductor body.
[0050] In some embodiments, the first and second semiconductor
bodies constitute a single semiconductor body that has been etched
to form two spatially separated and electrically interconnected
bodies.
[0051] In some embodiments, the selection of the composition of the
subcells, band gaps and short circuit current maximizes the
efficiency 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), such time being in the range of one to twenty-five
years.
[0052] In some embodiments, the first and second semiconductor
bodies further comprises a first highly doped lateral conduction
layer disposed adjacent to and beneath the second solar
subcell.
[0053] In some embodiments, the first and second semiconductor
bodies further comprises a blocking p-n diode or insulating layer
disposed adjacent to and beneath the first highly doped lateral
conduction layer.
[0054] In some embodiments, the first and second semiconductor
bodies further comprises a second highly doped lateral conduction
layer disposed adjacent to and beneath the blocking p-n diode or
insulating layer.
[0055] In some embodiments, the short circuit density (J.sub.sc) of
the first, second and third middle subcells are approximately 12
mA/cm.sup.2. The short circuit density (J.sub.sc/cm.sup.2) of the
subcells may have another value for different implementations.
[0056] In some embodiments, the short circuit current density
(J.sub.sc) of the bottom subcell is approximately 34
mA/cm.sup.2.
[0057] 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, and at least the base of at least one of
the first, second or third solar subcells has a graded doping.
[0058] In some embodiments, the solar cell assembly further
comprises a first opening in the first semiconductor body extending
from a top surface of the semiconductor body to the first lateral
conduction layer, a second opening in the first semiconductor body
extending from the top 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 first
semiconductor body to the p-type semiconductor material of the
bottom subcell of the first semiconductor body.
[0059] 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 first and second
semiconductor bodies; and a second metallic contact pad disposed on
the second lateral conduction layer of the first semiconductor
body; and an electrical interconnect connecting the first and
second contact pads.
[0060] In some embodiments, the solar cell assembly further
comprises a third metallic contact pad disposed on the second
lateral conduction layer of the second semiconductor body; a fourth
metallic contact pad disposed on the p-type semiconductor material
of the bottom subcell of the first semiconductor body; and an
electrical interconnect connecting the third and fourth contact
pads.
[0061] In another aspect, the present disclosure provides a
multijunction solar cell assembly including a terminal of first
polarity and a terminal of second polarity comprising first and
second semiconductor bodies 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 body is mounted adjacent
and parallel to the first semiconductor body; a bottom contact on
the bottom second subcell of the second semiconductor body
connected to the terminal of second polarity; a top electric
contact on both the upper first subcells of the first and second
semiconductor bodies 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 body in a series electrical circuit with the bottom
second subcell of the second semiconductor body so that at least a
multijunction solar cell is formed by the electrically
interconnected semiconductor bodies. The series connection of the
bottom subcell of the second semiconductor body 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 assembly.
[0062] In another aspect, the present disclosure provides a method
of forming a multijunction solar cell assembly including a terminal
of first polarity and a terminal of second polarity comprising
forming a semiconductor body including a tandem vertical stack of
at least an upper first and a bottom second solar subcell lattice
mismatched to the upper first solar subcell; separating the
semiconductor body into first and second discrete semiconductor
devices, each including the tandem vertical stack of at least an
upper first and a bottom second solar subcells; mounting the second
semiconductor body adjacent and parallel to the first semiconductor
body, providing a bottom contact on the bottom second subcell of
the second semiconductor body; connecting the bottom contact on the
bottom second subcell of the second semiconductor body to the
terminal of second polarity; connecting the bottom second subcell
of the first semiconductor body in a series electrical circuit with
the bottom second subcell of the second semiconductor body so that
at least a multijunction solar cell is formed by the electrically
interconnected semiconductor bodies; and providing a top electric
contact on both the upper first subcells of the first and second
semiconductor bodies and electrically connecting the top electrical
contacts to the terminal of first polarity.
[0063] In another aspect, the present disclosure provides a
multijunction solar cell subassembly including an upper first solar
subcell composed of a semiconductor material having a first band
gap; 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 third solar subcell and being
lattice mismatched with the third solar subcell and composed of a
semiconductor material having a fifth band gap smaller than the
fourth band gap; 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, and wherein the
average band gap of all four subcells (i.e., the sum of the four
lowest direct or indirect band gaps of the materials of each
subcell divided by four) is greater than 1.44 eV.
[0064] The solar cell subassembly can further include a plurality
of openings in the first semiconductor body, each of the openings
extending from a top surface of the first semiconductor body to a
different respective contact layer in the first semiconductor body.
Thus, for example, a first opening in the first semiconductor body
can extend from the top surface of the semiconductor body to the
first lateral conduction layer. A metallic contact pad can be
disposed on the lateral conduction layer. A second opening in the
first semiconductor body can extend from the top surface of the
semiconductor body to the contact back metal layer of the bottom
subcell.
[0065] In another aspect, the present disclosure provides a solar
cell module including a terminal of first polarity and a terminal
of second polarity comprising a first semiconductor body including
a tandem vertical stack of at least a first upper, a second, third
and fourth solar subcells which are current matched, the first
upper subcell having a top contact connected to the terminal of
first polarity and a bottom fourth solar subcell that is current
mismatched from the first, second and third solar subcells; a
second semiconductor body disposed adjacent to the first
semiconductor body and including a tandem vertical stack of at
least a first upper, second and third subcells, and a bottom fourth
solar subcell that is current mismatched from the first, second and
third solar subcells; wherein the top contact of the first upper
subcells of the first and second semiconductor bodies are
connected; and wherein the fourth subcell of the first
semiconductor body is connected in a series electrical circuit with
the fourth subcell of the second semiconductor body.
[0066] In another aspect, the present disclosure provides a method
of forming a solar cell module including a terminal of first
polarity and a terminal of second polarity comprising forming a
first semiconductor body including a tandem vertical stack of at
least a first upper, a second, third and fourth solar subcells
which are current matched, the first upper subcell having a top
contact connected to the terminal of first polarity and a bottom
fourth solar subcell that is current mismatched from the first,
second and third solar subcells; forming a second semiconductor
body including a tandem vertical stack of at least a first upper,
second and third subcells, and a bottom fourth solar subcell that
is current mismatched from the first, second and third solar
subcells; disposing the first semiconductor body adjacent to the
second semiconductor body and connecting the top contact of the
first upper subcells of the first and second semiconductor bodies;
and connecting the fourth subcell of the first semiconductor body
in a series electrical circuit with the fourth subcell of the
second semiconductor body.
[0067] 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 each 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.
[0068] 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.
[0069] In some implementations, the first semiconductor body
further includes one or more of the following features. For
example, there may be a first highly doped lateral conduction layer
disposed adjacent to the fourth solar subcell. The first
semiconductor body also can include a blocking p-n diode or
insulating layer disposed adjacent to and above the highly doped
lateral conduction layer. The first semiconductor body may further
include a second highly doped lateral conduction layer disposed
adjacent to and above the blocking p-n diode or insulating layer. A
metamorphic layer can be disposed adjacent to and above the second
highly doped lateral conduction layer.
[0070] In another aspect, a solar cell assembly includes a terminal
of first polarity and a terminal of second polarity. The solar cell
assembly includes a first semiconductor body including a tandem
vertical stack of at least a first upper, a second, a third and a
lattice mismatched fourth solar subcell, the first upper subcell
having a top contact connected to the terminal of first polarity.
The solar cell assembly further includes a second semiconductor
body disposed adjacent and parallel to the first semiconductor body
and including a tandem vertical stack of at least a first upper, a
second, third and a lattice mismatched fourth bottom solar
subcells, the fourth bottom subcell having a bottom contact
connected to the terminal of second polarity. The fourth subcell of
the first semiconductor body is connected in a series electrical
circuit with the fourth subcell of the second semiconductor
body.
[0071] 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.
[0072] 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.
[0073] In some embodiments, the band gap of the interlayer is in
the range of 1.41 to 1.6 eV throughout its thickness, and may be
either constant or may be a graded interlayer and vary throughout
the thickness of the interlayer.
[0074] 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.
[0075] 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.
[0076] 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.
[0077] In some embodiments, 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.
[0078] In another aspect, the present disclosure provides a five
junction solar cell comprising a pair of adjacently disposed
semiconductor bodies, each body including an upper first solar
subcell composed of a semiconductor material having a first band
gap; 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; and a fourth solar
subcell adjacent to and lattice mismatched to said third solar
subcell and composed of a semiconductor material having a fourth
band gap smaller than the third band gap; wherein the average band
gap of all four subcells (i.e., the sum of the four band gaps of
each subcell divided by four) is greater than 1.44 eV.
[0079] In another aspect, the present disclosure provides a four
junction solar cell subassembly comprising an upper first solar
subcell composed of a semiconductor material having a first band
gap; 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 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; 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 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. In other instances, the fourth subcell may have a direct
band gap of less than 0.90 eV.
[0080] 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.
[0081] 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.
[0082] 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
II-V or II-VI compound semiconductor materials.
[0083] In some embodiments, additional layer(s) may be added or
deleted in the cell structure without departing from the scope of
the present disclosure.
[0084] Some implementations can include additional solar subcells
in one or more of the semiconductor bodies.
[0085] Some implementations of the present disclosure may
incorporate or implement fewer of the aspects and features noted in
the foregoing summaries.
[0086] 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
[0087] 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:
[0088] 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;
[0089] FIG. 2A is a cross-sectional view of a first embodiment of a
first semiconductor body including a four junction solar cell after
several stages of fabrication including the growth of certain
semiconductor layers on the growth substrate up to the contact
layer and etching contact steps on lower levels according to the
present disclosure;
[0090] FIG. 2B is a cross-sectional view of a first embodiment of a
first semiconductor body of FIG. 2A after the next step of
depositing a metal contact on the top surface;
[0091] FIG. 2C is a cross-sectional view of a second embodiment of
a first semiconductor body of FIG. 2A;
[0092] FIG. 3 is a cross-sectional view of a first embodiment of a
second semiconductor body including a four junction solar cell
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;
[0093] FIG. 4 is a cross-sectional view of an embodiment of a five
junction solar cell after following electrical connection of the
first and second semiconductor bodies according to the present
disclosure; and
[0094] FIG. 5 is a schematic diagram of the five junction solar
cell of FIG. 4.
GLOSSARY OF TERMS
[0095] "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).
[0096] "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.
[0097] "Beginning of Life (BOL)" refers to the time at which a
photovoltaic power system is initially deployed in operation.
[0098] "Bottom subcell" refers to the subcell in a multijunction
solar cell which is furthest from the primary light source for the
solar cell.
[0099] "Compound semiconductor" refers to a semiconductor formed
using two or more chemical elements.
[0100] "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.
[0101] "Deposited", with respect to a layer of semiconductor
material, refers to a layer of material which is epitaxially grown
over another semiconductor layer.
[0102] "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.
[0103] "Graded interlayer" (or "grading interlayer")--see
"metamorphic layer".
[0104] "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.
[0105] "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.
[0106] "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).
[0107] "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.
[0108] "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).
[0109] "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.
[0110] "Short circuit current density"--see "current density".
[0111] "Solar cell" refers to an electronic device operable to
convert the energy of light directly into electricity by the
photovoltaic effect.
[0112] "Solar cell assembly" refers to two or more solar cell
subassemblies interconnected electrically with one another.
[0113] "Solar cell subassembly" refers to a stacked sequence of
layers including one or more solar subcells.
[0114] "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.
[0115] "Substantially current matched" refers to the short circuit
current through adjacent solar subcells being substantially
identical (i.e. within plus or minus 1%).
[0116] "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.
[0117] "ZTJ" refers to the product designation of a commercially
available SolAero Technologies Corp. triple junction solar
cell.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0118] 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.
[0119] 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 forms a solar cell
subassembly. More specifically, however, in some embodiments, the
present disclosure relates to multijunction solar cell
subassemblies 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, respectively, and the
connection of two or more such subassemblies to form a solar cell
assembly.
[0120] 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").
[0121] 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).
[0122] 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).
[0123] The present disclosure provides 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.
[0124] 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.
Alternatively, the solar cell subassemblies can be fabricated as
separate semiconductor chips that can be coupled together
electrically.
[0125] 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.
[0126] 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.
[0127] Another aspect of the present disclosure is that to provide
a five junction solar cell assembly composed of two interconnected
spatially separated four junction solar cell subassemblies, 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.
[0128] 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.
[0129] 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.
[0130] 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.
[0131] 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.
[0132] 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.
[0133] 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.
[0134] 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.
[0135] 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.
[0136] 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.
[0137] 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).
[0138] 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.
[0139] 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 specification of the
thickness, band gap, doping, or other characteristic of the
incorporation of that material in a particular subcell, is
interdependent on many other factors, and thus 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.
[0140] 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.
[0141] 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.
[0142] 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.
[0143] 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.
[0144] 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. (The charge q is introduced in the
denominator for normalization purposes, since the band gap of a
semiconductor material is measured in electron volts which is
dimensioned in units of energy, and thus to compare the terms with
the open circuit voltage, measured in volts, the band gap is
divided by the charge in coulombs, which is 1.6.times.10.sup.-19
coulombs, to produce a parameter also measured in volts). 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.
[0145] 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.
[0146] Thus, contrary to the conventional wisdom as indicated
above, the present application utilizes a substantial amount of
aluminum, i.e., between 10% and 40% aluminum by mole fraction in at
least the top subcell, and in some embodiments in one or more of
the middle subcells as well. In some embodiments the amount of
aluminum in each of the top three subcells is in excess of 20% by
mole fraction.
[0147] 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 four junction solar cell
subassembly 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
semiconductor body.
[0148] As shown in the illustrated example of FIG. 2A, the bottom
subcell D.sub.1 includes a substrate 600 formed of p-type germanium
("Ge") in some embodiments, which also serves as a base layer. A
back metal contact pad 650 formed on the bottom of base layer 600
provides electrical contact 651 to the multijunction solar cell
subassembly 500.
[0149] In some embodiments, the contact 651 may be in the interior
of the region 600 and not on the bottom surface of the region
600.
[0150] In some embodiments, the bottom subcell D.sub.1 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.
[0151] The bottom subcell D.sub.1, further includes, for example, a
highly doped n-type Ge emitter layer 601, and an n-type indium
gallium arsenide ("InGaAs") nucleation layer 602a. The nucleation
layer 602a is deposited over the base layer, and the emitter layer
601 is formed in the substrate by diffusion of atoms from the
nucleation layer 602a into the Ge substrate, thereby forming the
n-type Ge layer 601.
[0152] In the first solar cell subassembly 500 of FIG. 2A, a highly
doped lateral conduction layer 602b is deposited over layer 602a,
and a blocking p-n diode or insulating layer 602c is deposited over
the layer 602b. A second highly doped lateral conduction layer 602d
is then deposited over layer 602c.
[0153] In the embodiment of FIG. 2A, a first alpha layer 603,
composed of n-type (Al)GaIn(As)P, is deposited over the lateral
conduction layer 602d, 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.sub.1, 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.).
[0154] A metamorphic layer (or graded interlayer) 604 is deposited
over the alpha layer 603 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 604 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.
[0155] In the surfactant assisted growth of the metamorphic layer
604, a suitable chemical element is introduced into the reactor
during the growth of layer 604 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.
[0156] 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.
[0157] 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 606.
[0158] In one embodiment of the present disclosure, the layer 604
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.
[0159] 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.
[0160] Although the preferred embodiment of the present disclosure
utilizes a plurality of layers of (Al)InGaAs for the metamorphic
layer 604 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.sub.1. 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.
[0161] A second alpha layer 605, composed of n+ type GaInP, is
deposited over metamorphic buffer layer 604, 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.sub.1, 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.).
[0162] 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 alpha
layer 605 to provide a low resistance pathway between the bottom
and middle subcells D.sub.1 and C.sub.1.
[0163] 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.
[0164] 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.
[0165] 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.
[0166] On top of the DBR layers 608 the subcell C.sub.1 is
grown.
[0167] 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, 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.
[0168] 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 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.
[0169] 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.
[0170] 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.
[0171] 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 Bt. 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.
[0172] In some embodiments, the amount of aluminum in the top
subcell A, is 20% or more by mole fraction.
[0173] 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. 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.
[0174] 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.
[0175] 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.
[0176] 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.
[0177] Following deposition of the semiconductor layers, the
semiconductor body 500 is etched so that several ledges or
platforms are formed on intermediate layers so that electrical
contact may be made thereto, in particular, in one embodiment,
ledges 651, 661, and 662.
[0178] To this end, the solar cell subassembly can include a
plurality of openings in the first semiconductor body, each of the
openings extending from a top surface of the first semiconductor
body to a different respective contact layer in the first
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 rear
of the substrate and have all the openings come from the back side.
This approach may be more efficient as it does not shadow the top
two or top three solar subcells, but it results in a solar
epitaxial structure of only a few tens of microns in thickness.
[0179] FIG. 2B 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. 2A.
[0180] A metal contact pad 602e is deposited on the surface of the
ledge of 662 which exposes a portion of the top surface of the
lateral conduction layer 602d. This pad 602e allows electrical
contact to be made to the bottom of the stack of subcells A.sub.1
through C.sub.1.
[0181] A metal contact pad 602f is deposited on the surface of the
ledge of 661 which exposes a portion of the top surface of the
lateral conduction layer 602b. This pad 602f allows electrical
contact to be made to the top of the subcell D.sub.1.
[0182] A metal contact 651 is further provided to a ledge 650 in
the p-region of the substitute 600, or as part of the back metal
layer 660 which allows electrical contact to be made to the
p-terminal of subcell D.sub.1.
[0183] The solar cell subassemblies, 150 and 250 are presented in a
highly simplified form, but each represent the structure 500 of
FIG. 2.
[0184] A second solar cell subassembly 700, which is similar to the
solar cell subassembly 500 or 600 of FIG. 2B or 2C, respectively,
may be formed with substantially the same sequence of semiconductor
layers with the same compositions and band gaps as the
corresponding layers in the first solar cell subassembly 500 or
600. Thus, the solar cell subassembly 700 also includes multiple
subcells in a tandem stack. In the illustrated example of FIG. 3,
the second solar cell subassembly 700 includes an upper first
subcell (Subcell A.sub.2), a second and third solar subcells
(Subcell B.sub.2 and C.sub.2) disposed adjacent to and lattice
matched to the upper first subcell A.sub.2, and a bottom subcell
(Subcell D.sub.2) lattice mismatched to the third subcell
C.sub.2.
[0185] 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.
[0186] Since the semiconductor layers 700 through 725 in
subassembly 700 are substantially identical to layers 600 through
625 in subassembly 500, a detailed description of them will not be
illustrated or provided here for brevity.
[0187] In order to provide access to the various layers in the
second solar cell subassembly 700, various ones of the layers can
be exposed partially. Thus, as shown in the example of FIG. 3,
various surfaces are partially exposed on the left side of the
subassembly 700, for example, using standard photolithographic
etching techniques to etch from the top surface of the
semiconductor body 700 to the particular contact layer 762, 761 and
751 of interest.
[0188] A metal contact pad 703e is deposited on the surface of the
ledge of 762 which exposes a portion of the top surface of the
lateral conduction layer 702d. This pad 702e allows electrical
contact to be made to the bottom of the stack of subcells A.sub.2
through C.sub.2.
[0189] A metal contact pad 702f is deposited on the surface of the
ledge of 761 which exposes a portion of the top surface of the
lateral conduction layer 702b. This pad 702f allows electrical
contact to be made to the top of the subcell E.
[0190] A metal contact 751 is further provided as part of the back
metal layer 750 which allows electrical contact to be made to the
p-terminal of subcell E.
[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] The foregoing multijunction solar cell subassemblies 500,
600, or 700 can be fabricated, for example, in wafer-level
processes and then singulated or diced into individual
semiconductor chips. The various semiconductor layers can be grown,
one atop another, using known growth techniques (e.g., MOCVD) as
discussed above.
[0193] Each solar cell subassembly 500, 600 and 700 also can
include grid lines, interconnecting bus lines, and contact pads.
The geometry and number of the grid lines, bus lines and/or
contacts may vary in other implementations.
[0194] As previously mentioned, two (or more) solar cell
subassemblies (e.g., 500 and 700) or chips can be disposed adjacent
and parallel to one another and connected together electrically.
For example, as shown in FIG. 4, conductive (e.g., metal)
interconnections 801, 802, 803, and 804 can be made between
different layers of the solar cell subassemblies 500 and 700. Some
of the interconnections are made between different layers of a
single one of the solar cell subassemblies, whereas others of the
interconnections are made between the two different solar cell
subassemblies. Thus, for example, the interconnection 801
electrically connects together the metal contacts 133 and 233 of
the first and second solar cell subassemblies 150 and 250
respectively. In particular, interconnection 803 connects together
a contact on the lateral conduction layer 104B of the first solar
cell subassembly 150 to a contact on the lateral conduction layer
204b of the second solar cell subassembly 250. Similarly, the
interconnection 804 connects together a contact 130 on the p-region
102 of the first solar cell subassembly 150 to a contact 231 on the
lateral conduction layer 204a of the second solar cell subassembly
250. Likewise, the interconnection 802 connects together a contact
132 on the lateral conduction layer 104b of the first solar cell
subassembly 150 to a contact 131 on the lateral conduction layer
104a of the first solar cell subassembly 150.
[0195] In some instances, multiple electrically conductive (e.g.,
metal) contacts can be provided for each of the respective contacts
of the solar cell subassemblies 150, 250. This allows each of the
interconnections 801-804 to be implemented by multiple
interconnections between the solar cell subassembly layers rather
than just a single interconnection.
[0196] 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 807 is connected to the metal contact 160 on the
first solar cell subassembly 150 by an interconnection 805, and the
second electrical contact 808 is connected to the back metal
contact 217 of the second solar cell subassembly 250.
[0197] As illustrated in FIG. 4, two or more solar cell
subassemblies can be connected electrically as described above to
obtain a multijunction (e.g. a four-, five- or six-junction) solar
cell assembly. In FIG. 4, the top side (n-polarity) conductivity
contact 807 and bottom side (p-polarity) conductive contact 808 for
the solar cell assembly are schematically depicted respectively, at
the left and right-hand sides of the assembly.
[0198] In the example of FIG. 4, one solar cell subassembly 150
includes an upper subcell A.sub.1, two middle subcells B.sub.1,
C.sub.1 and a bottom subcell D.sub.1. The other solar cell
subassembly includes an upper subcell A.sub.2, two middle subcells
B.sub.2, C.sub.2 and a bottom subcell D.sub.2. In some
implementations, each solar cell subassembly 500, 700 has band gaps
in the range of 2.0 to 2.20 eV (or higher) for the top subcell, and
(i) 1.65 to 1.8, and (ii) 1.41 eV for the middle subcell, and 0.6
to 0.9 eV, for the bottom subcell, respectively, Further, 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 a
given solar cell subassembly 500 or 700 is greater than 1.44 eV.
Other band gap ranges may be appropriate for some
implementations.
[0199] In some instances, the fourth (i.e., bottom)subcell 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. For example, the fourth subcell may be composed of InGaAs,
GaAsSb, InAsP, InAlAs, or SiGeSn, or other III-V or II-VI compound
semiconductor materials.
[0200] FIG. 5 is a schematic diagram of the five junction solar
cell assembly of FIG. 4.
[0201] 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 12 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 24 mA/cm.sup.2 or greater. Other implementations may have
different values.
[0202] FIG. 6 is a graph of a doping profile in the emitter and
base layers in one or more subcells of the multijunction solar cell
of the present invention. The various doping profiles within the
scope of the present invention, and the advantages of such doping
profiles are more particularly described in U.S. patent application
Ser. No. 11/956,069 filed Dec. 13, 2007, herein incorporated by
reference. The doping profiles depicted herein are merely
illustrative, and other more complex profiles may be utilized as
would be apparent to those skilled in the art without departing
from the scope of the present invention.
[0203] The present disclosure like that of the parallel
applications noted above 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.
[0204] 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
1.50V at 28.degree. C. produces 1.50-42*(0.0023)=1.403V at
70.degree. C. which is a 6.4% voltage loss, A cell that produces
0.25V at 28.degree. C. produces 0.25-42*(0.0018)=0.174V at
70.degree. which is a 30.2% voltage loss.
[0205] In view of different satellite and space vehicle
requirements in terms of temperature, radiation exposure, and
operational life, a range of subcell designs using the design
principles of the present disclosure may be provided satisfying
typical customer and mission requirements, and several embodiments
are set forth hereunder, along with the computation of their
efficiency at the end-of-life. The radiation exposure is
experimentally measured using 1 MeV electron fluence per square
centimeter (abbreviated in the text that follows as e/cm.sup.2), so
that a comparison can be made between the current commercial
devices and embodiments of solar cells discussed in the present
disclosure.
[0206] As an example, 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.
[0207] 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
[0208] 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
[0209] 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.
[0210] 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.
[0211] 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.
[0212] 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.
[0213] For completeness and clarification in the recitation of
various claimed elements with corresponding elements depicted in
the Figure, an annotated set of claims with reference numbers is
set forth in the following clauses:
[0214] Clause 1 . . . A multijunction solar cell assembly having a
terminal of a first polarity and a terminal of a second polarity
(808) comprising:
(a) a first semiconductor body (500) including:
[0215] an upper first solar subcell (A1) composed of a
semiconductor material having a first band gap, and including a top
contact (627) on the top surface thereof;
[0216] a second solar subcell (B1) adjacent to said first solar
subcell (Al) 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 (A1);
[0217] a third solar subcell (C1) adjacent to said second solar
subcell (B1) 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 (B1);
[0218] an interlayer adjacent to said third solar subcell (C1),
said interlayer having a fourth band gap or band gaps greater than
said third band gap; and
[0219] a fourth solar subcell (D1) 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 (C1), and including a first contact (602f)
on the top surface thereof, and a second contact (650) on the
bottom surface thereof;
(b) a second semiconductor body (700) disposed adjacent and
parallel to the first semiconductor body (500) and including:
[0220] an upper first solar subcell (A2) composed of a
semiconductor material having a first band gap, and including a top
contact (727) on the top surface thereof;
[0221] a second solar subcell (B2) adjacent to said first solar
subcell (A2) 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 (A2);
[0222] a third solar subcell (C2) adjacent to said second solar
subcell (B2) 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 (B2) and having a bottom
contact (702e);
[0223] an interlayer adjacent to said third solar subcell (C2),
said interlayer having a fourth band gap greater than said third
band gap; and
[0224] a fourth solar subcell (D2) 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 (C2), and including a first contact (702f)
on the top surface thereof, and a second contact (750) on the
bottom surface thereof connected to the terminal of a second
polarity (808);
(c) wherein the top contact (625) of the first semiconductor body
(500) is electrically coupled with the top contact (725) of the
second semiconductor body (700) and to a terminal of first polarity
(807);
[0225] wherein the first contact (602f) on the top surface of the
fourth solar subcell (D1) of the first semiconductor body (500) is
electrically coupled with the bottom contact (702e) of the third
solar subcell (C2) of the second semiconductor body (700);
[0226] the second contact (650) on the bottom surface of the fourth
solar subcell (D1) of the first semiconductor body (500) is
electrically coupled with the first contact (702f) on the top
surface of the fourth solar subcell (D.sub.2) of the second
semiconductor body (700) thereof so as to form a five junction
solar cell;
[0227] and wherein the interlayer in each of the first and second
semiconductor bodies (500, 700) is compositionally graded to
substantially lattice match the third solar subcell (C1, C2) on one
side and the lower fourth solar subcell (D1, D2) 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 (C1, C2) and greater than or equal to that of the lower
fourth solar subcell (D1, D2).
[0228] Clause 2 . . . A multijunction solar cell as defined in
clause 1, wherein the short circuit density of each fourth subcell
(D1, D2) being at least twice that of the solar subcells in each
semiconductor body which are connected in a series with the fourth
subcell (D1, D2).
[0229] Clause 3 . . . A multijunction solar cell as defined in
clause 1, wherein the fourth subcell (D1, D2) has a band gap of
approximately 0.67 eV, the third subcell (C1, C2) has a band gap in
the range of approximately 1.41 eV and 1.31 eV, the second subcell
(B1, B2) has a band gap in the range of approximately 1.65 to 1.8
eV and the upper first subcell (A1, A2) has a band gap in the range
of 2.0 to 2.20 eV.
[0230] Clause 4 . . . A multijunction solar cell as defined in
clause 1, wherein the upper first subcell (A1, A2) is composed of
indium gallium aluminum phosphide;
[0231] the second solar subcell (B1, B2) includes an emitter layer
composed of indium gallium phosphide or aluminum gallium arsenide
or indium aluminum gallium arsenide, and a base layer composed of
aluminum gallium arsenide, indium gallium arsenide phosphide or
indium aluminum gallium arsenide;
[0232] the third solar subcell (C1, C2) is composed of indium
gallium arsenide;
[0233] the fourth subcell (D1, D2) is composed of germanium.
[0234] Clause 5 . . . A multijunction solar cell as defined in
clause 1, wherein the interlayer is composed of p type
(Al)In.sub.xGa1.sub.-xAs or In.sub.xGa.sub.1-xP with 0<x<1,
and (Al) designates that aluminum is an optional constituent.
[0235] Clause 6 . . . A multijunction solar cell as defined in
clause 1, wherein the first (500) and second (700) semiconductor
bodies further comprise a first highly doped lateral conduction
layer (602b) disposed adjacent to and above the fourth solar
subcell (D1) and a blocking p-n diode or insulating layer (602c)
disposed adjacent to and above the first highly doped lateral
conduction layer (602b), and a second highly doped lateral
conduction layer disposed (602d) adjacent to and above the blocking
p-n diode or insulating layer (602c).
[0236] Clause 7 . . . A multijunction solar cell assembly as
defined in clause 3, wherein the third subcell (C1, C2) has a band
gap of approximately 1.37 eV, the second subcell (B1, B2) has a
band gap of approximately 1.73 eV and the upper first subcell (A1,
A2) has a band gap of approximately 2.10 eV.
[0237] Clause 8 . . . A multijunction solar cell assembly as
defined in clause 1, wherein the band gap of the interlayer is in
the range of 1.41 eV to 1.6 eV throughout its thickness.
[0238] Clause 9 . . . A multijunction solar cell assembly as
defined in clause 1, further comprising: a distributed Bragg
reflector layer (608) adjacent to and between the third (C1, C2)
and the fourth (D1, D2) solar subcells and arranged so that light
can enter and pass through the third solar subcell (C1, C2) and at
least a portion of which can be reflected back into the third solar
subcell (C1, C2) by the distributed Bragg reflector layer (608),
and the distributed Bragg reflector layer (608) 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 distributed Bragg reflector layer (608)
includes a first distributed Bragg reflector layer (608) composed
of a plurality of p type Al.sub.xGa.sub.1-x(In)As layers, and a
second distributed Bragg reflector layer (608) disposed over the
first distributed Bragg reflector layer (608) 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.
[0239] Clause 10 . . . A multijunction solar cell assembly as
defined in clause 2, wherein the short circuit current density
(J.sub.sc) of the first (A1, A2), second (B1, B2) and third middle
(C1, C2) subcells are approximately 11 mA/cm.sup.2, and the short
circuit current density (J.sub.sc) of the bottom subcell (D1, D2)
is approximately 34 mA/cm.sup.2.
[0240] Clause 11 . . . A multijunction solar cell assembly as
defined in clause 1, wherein at least the base of at least one of
the first (A1, A2), second (B1, B2) or third (C1, C2) solar
subcells has a graded doping.
[0241] Clause 12 . . . A multijunction solar cell assembly as
defined in clause 6 comprising:
[0242] a first opening in the first semiconductor body (500)
extending from a top surface of the semiconductor body to the first
lateral conduction layer (602b);
[0243] a second opening in the first semiconductor body (500)
extending from the top surface of the semiconductor body (500) to
the second lateral conduction layer (602d); and
[0244] a third opening in the first semiconductor body (500)
extending from a surface of the first semiconductor body (500) to
the p-type semiconductor material of the bottom subcell (D1), a
first metallic contact pad (602f, 702f) disposed on the first
lateral conduction layer (602b) of each of the first (500) and
second (700) semiconductor bodies;
[0245] a second metallic contact pad (602e) disposed on the second
lateral conduction layer (602d) of the first semiconductor body
(500); and
[0246] an electrical interconnect connecting the first and second
contact pads, a third metallic contact pad (702e) disposed on the
second lateral conduction layer (702d) of the second semiconductor
body (700);
[0247] a fourth metallic contact pad (651) disposed on the p-type
semiconductor material of the bottom subcell (D1) of the first
semiconductor body (500); and
[0248] an electrical interconnect (804) connecting the third and
fourth (651) contact pads.
[0249] Clause 13 . . . A multijunction solar cell assembly having a
terminal of a first polarity (807) and a terminal of a second
polarity (808) comprising:
(a) a first semiconductor body (500) including:
[0250] an upper first solar subcell (Al) composed of a
semiconductor material having a first band gap, and including a top
contact (627) on the top surface thereof;
[0251] a second solar subcell (B1) adjacent to said first solar
subcell (Al) 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 (Al);
[0252] a third solar subcell (C1) adjacent to said second solar
subcell (B1) 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 (B1);
[0253] a fourth solar subcell (D1) adjacent to said interlayer and
composed of a semiconductor material having a fifth band gap
smaller than the fourth band gap, and including a first contact
(602f) on the top surface thereof, and a second contact (650) on
the bottom surface thereof;
(b) a second semiconductor body (700) disposed adjacent and
parallel to the first semiconductor body (500) and including:
[0254] an upper first solar subcell (A2) composed of a
semiconductor material having a first band gap, and including a top
contact (727) on the top surface thereof;
[0255] a second solar subcell (B2) adjacent to said first solar
subcell (A2) 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 (A2);
[0256] a third solar subcell (C2) adjacent to said second solar
subcell (B2) 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 (B2) and having a bottom
contact (702e);
[0257] a fourth solar subcell (D2) adjacent to said interlayer and
composed of a semiconductor material having a fifth band gap
smaller than the fourth band gap, and including a first contact
(702e) on the top surface thereof, and a second contact (750) on
the bottom surface thereof connected to the terminal of a second
polarity (808);
[0258] (c) wherein the top contact (625) of the first semiconductor
body (500) is electrically coupled with the top contact (725) of
the second semiconductor body (700) and to a terminal of first
polarity (807);
[0259] wherein the first contact (602f) on the top surface of the
fourth solar subcell (D1) of the first semiconductor body (500) is
electrically coupled with the bottom contact (702e) of the third
solar subcell (C2) of the second semiconductor body (700);
[0260] the second contact (650) on the bottom surface of the fourth
solar subcell (D1) of the first semiconductor body (500) is
electrically coupled with the first contact (702f) on the top
surface of the fourth solar subcell (D2) of the second
semiconductor body (700) thereof so as to form a five junction
solar cell;
[0261] wherein the first and second semiconductor bodies further
comprise a first highly doped lateral conduction layer disposed
adjacent to and above the fourth solar subcell (D1) and a blocking
p-n diode or insulating layer disposed adjacent to and above the
first highly doped lateral conduction layer, and a second highly
doped lateral conduction layer disposed adjacent to and above the
blocking p-n diode or insulating layer.
[0262] Clause 14 . . . A multijunction solar cell as defined in
clause 1, wherein the fourth subcell (D1, D2) has a band gap of
approximately 0.67 eV, the third subcell (C1, C2) has a band gap in
the range of approximately 1.41 eV and 1.31 eV, the second subcell
(B1, B2) has a band gap in the range of approximately 1.65 to 1.8
eV and the upper first subcell (A1, A2) has a band gap in the range
of 2.0 to 2.20 eV.
[0263] Clause 15 . . . A multijunction solar cell as defined in
clause 13, wherein the upper first subcell (A1, A2) is composed of
indium gallium aluminum phosphide;
[0264] the second solar subcell (B1, B2) includes an emitter layer
composed of indium gallium phosphide or aluminum gallium arsenide
or indium aluminum gallium arsenide, and a base layer composed of
aluminum gallium arsenide, indium gallium arsenide phosphide or
indium aluminum gallium arsenide;
[0265] the third solar subcell (C1, C2) is composed of indium
gallium arsenide; and
[0266] the fourth subcell (D1, D2) is composed of germanium.
[0267] Clause 16 . . . A multijunction solar cell assembly as
defined in clause 13, further comprising:
[0268] a distributed Bragg reflector layer adjacent to and between
the third (C1, C2) and the fourth (D1, D2) solar subcells are
arranged so that light can enter and pass through the third solar
subcell (C1, C2) and at least a portion of which can be reflected
back into the third solar subcell (C1, C2) by the distributed Bragg
reflector layer (608), and the distributed Bragg reflector layer
(608) 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 distributed Bragg
reflector layer (608) includes a first distributed Bragg reflector
layer (608) disposed over the first distributed Bragg reflector
layer (608) 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.
[0269] Clause 17 . . . A multijunction solar cell assembly as
defined in clause 13, wherein the short circuit current density
(J.sub.sc) of the first (A1, A2), second (B1, B2) and third middle
(C1, C2) subcells are approximately 11 mA/cm.sup.2, and the short
circuit current density (J.sub.sc) of the bottom subcell (D1, D2)
is approximately 34 mA/cm.sup.2.
[0270] Clause 18 . . . A multijunction solar cell assembly as
defined in clause 13, wherein at least the base of at least one of
the first (A1, A2), second (B1, B2) or third (C1, C2) solar
subcells has a graded doping.
[0271] Clause 19 . . . A multijunction solar cell assembly as
defined in clause 13, comprising:
[0272] a first opening in the first semiconductor body (500)
extending from a top surface of the semiconductor body (500) to the
first lateral conduction layer (602b);
[0273] a second opening in the first semiconductor body (500)
extending from the top surface of the semiconductor body (500) to
the second lateral conduction layer (602d);
[0274] a third opening in the first semiconductor body (500)
extending from a surface of the first semiconductor body (500) to
the p-type semiconductor material of the bottom subcell (D1), a
first metallic contact pad (602f, 702f) disposed on the first
lateral conduction layer (602b) of each of the first (500) and
second (700) semiconductor bodies;
[0275] a second metallic contact pad (602e) disposed on the second
lateral conduction layer (602d) of the first semiconductor body
(500); and
[0276] an electrical interconnect connecting the first and second
contact pads, a third metallic contact pad (702e) disposed on the
second lateral conduction layer (702d) of the second semiconductor
body (700);
[0277] a fourth metallic contact pad (651) disposed on the p-type
semiconductor material of the bottom subcell (D1) of the first
semiconductor body (500); and an electrical interconnect (804)
connecting the third and fourth (651) contact pads.
[0278] Clause 20 . . . A multijunction solar cell assembly
including a terminal of first polarity and a terminal of second
polarity comprising:
[0279] first and second semiconductor bodies 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 body is
mounted adjacent and parallel to the first semiconductor body;
[0280] a bottom contact on the bottom second subcell of the second
semiconductor body connected to the terminal of second
polarity;
[0281] a top electric contact on both the upper first subcells of
the first and second semiconductor bodies electrically connected to
the top electrical contacts to the terminal of first polarity;
and
[0282] an electrical interconnect connecting the bottom second
subcell of the first semiconductor body in a series electrical
circuit with the bottom second subcell of the second semiconductor
body so that at least a three junction solar cell is formed by the
electrically interconnected semiconductor bodies;
[0283] wherein the first and second semiconductor bodies further
comprise a first highly doped lateral conduction layer disposed
adjacent to and above the bottom second solar subcell and a
blocking p-n diode or insulating layer disposed adjacent to and
above the first highly doped lateral conduction layer, and a second
highly doped lateral conduction layer disposed adjacent to and
above the blocking p-n diode or insulating layer.
[0284] 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.
[0285] 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.
[0286] 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.
[0287] 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.
[0288] 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.
[0289] 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, (In)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.
[0290] 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.
[0291] 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.
[0292] 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.
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