U.S. patent application number 16/836471 was filed with the patent office on 2020-08-06 for distributed bragg reflector structures in multijunction solar cells.
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, Daniel McGlynn.
Application Number | 20200251603 16/836471 |
Document ID | / |
Family ID | 1000004769578 |
Filed Date | 2020-08-06 |
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United States Patent
Application |
20200251603 |
Kind Code |
A1 |
McGlynn; Daniel ; et
al. |
August 6, 2020 |
DISTRIBUTED BRAGG REFLECTOR STRUCTURES IN MULTIJUNCTION SOLAR
CELLS
Abstract
A multijunction solar cell and its method of fabrication, having
an upper first solar subcell composed of a semiconductor material
including aluminum and 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 first and second DBR structure
adjacent to the third solar subcell; and a fourth solar subcell
adjacent to the DBR structures and lattice matched with said third
solar subcell and composed of a semiconductor material having a
fourth band gap smaller than the third band gap; wherein the fourth
subcell has a direct bandgap of greater than 0.75 eV.
Inventors: |
McGlynn; Daniel;
(Albuquerque, NM) ; Derkacs; Daniel; (Albuquerque,
NM) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SolAero Technologies Corp. |
Albuquerque |
NM |
US |
|
|
Assignee: |
SolAero Technologies Corp.
Albuquerque
NM
|
Family ID: |
1000004769578 |
Appl. No.: |
16/836471 |
Filed: |
March 31, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15376195 |
Dec 12, 2016 |
10636926 |
|
|
16836471 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/0693 20130101;
H01L 31/0687 20130101; H01L 31/1844 20130101; H01L 31/03046
20130101; Y02E 10/50 20130101; H01L 31/0216 20130101; H01L 31/02168
20130101; H01L 31/0549 20141201; H01L 31/02167 20130101; H01L
31/06875 20130101; H01L 31/02327 20130101; Y02E 10/544
20130101 |
International
Class: |
H01L 31/054 20060101
H01L031/054; H01L 31/0687 20060101 H01L031/0687; H01L 31/0216
20060101 H01L031/0216; H01L 31/0232 20060101 H01L031/0232; H01L
31/18 20060101 H01L031/18; H01L 31/0304 20060101 H01L031/0304; H01L
31/0693 20060101 H01L031/0693 |
Claims
1. A multijunction solar cell comprising: a first solar subcell
comprising an emitter layer and a base layer composed of aluminum
gallium arsenide or indium gallium arsenide, the emitter layer and
the base layer forming a photoelectric junction; a second solar
subcell disposed below the first solar subcell and comprising an
emitter layer and a base layer forming a photoelectric junction;
and a combined DBR structure between the first solar subcell and
the second solar subcell with no intervening solar subcells, the
combined DBR structure comprising a first distributed Bragg
reflector (DBR) structure and a second DBR structure, wherein the
first DBR structure is disposed beneath the base layer of the first
solar subcell and composed of a plurality of alternating layers of
different semiconductor materials with discontinuities in their
respective indices of refraction and arranged so that light can
enter and pass through the first solar subcell and at least a first
portion of which is light in a first spectral wavelength range and
can be reflected back into the first solar subcell by the first DBR
structure, and a second portion of which is light in a second
spectral wavelength range and can be transmitted through the first
DBR structure to the layers disposed beneath the first DBR
structure, where the second spectral wavelength range is greater in
wavelength than the first spectral wavelength range, and wherein
the second DBR structure is disposed beneath and adjacent the first
DBR structure, such that there are not any active subcell layers
between said first DBR structure and the second DBR structure,
wherein the second DBR structure is compositionally different from
the first DBR structure and composed of a plurality of alternating
layers of different semiconductor materials with discontinuities in
their respective indices of refraction and arranged so that light
can enter and pass through the first DBR structure and at least a
portion of which is light in the second spectral wavelength range
and can be reflected back into the first solar subcell by the
second DBR structure, and a second portion of which is light in a
third spectral wavelength range and can be transmitted through the
second DBR structure to the second solar subcell disposed beneath
the second DBR structure.
2. The multijunction solar cell of claim 1 further comprising a
metamorphic layer between the combined DBR structure and the second
solar subcell.
3. The multijunction solar cell of claim 2 wherein for the second
solar subcell, the emitter layer is composed of germanium and the
base layer is composed of germanium.
4. The multijunction solar cell of claim 1 further comprising
tunnel diode layers between the combined DBR structure and the
second solar subcell.
5. The multijunction solar cell of claim 1 wherein the first solar
subcell further comprises a window layer, tunnel diode layers, and
a back surface field ("BSF") layer, wherein the base layer is
disposed on the BSF layer, the window layer is disposed on the
emitter layer, and the tunnel diode layers are disposed on the
window layer.
6. The multijunction solar cell of claim 1 further comprising a
first additional solar subcell above the first solar subcell
comprising an emitter layer and a base layer composed of indium
gallium aluminum phosphide, the emitter layer and the base layer
forming a photoelectric junction.
7. The multijunction solar cell of claim 6 further comprising a
first tunnel diode layer, a second tunnel diode layer, and a
nucleation layer between the combined DBR structure and the second
solar subcell, wherein the nucleation layer is composed of indium
gallium arsenide and is disposed on the emitter layer of the first
additional solar subcell and the first tunnel diode layer composed
of gallium arsenide is disposed on the nucleation layer and the
second tunnel diode layer composed of aluminum gallium arsenide is
disposed on the first tunnel diode layer, wherein the emitter layer
of the first solar subcell is composed of indium gallium phosphide
or aluminum gallium arsenide.
8. The multijunction solar cell of claim 6 further comprising: a
second additional solar subcell disposed between the first solar
subcell and the first additional solar subcell, the second
additional solar subcell comprising an emitter layer composed of
indium gallium arsenide or aluminum gallium arsenide and a base
layer composed of aluminum gallium arsenide, wherein the base layer
of the first solar subcell comprises InGaAs.
9. The multijunction solar cell according to claim 6 further
comprising a back surface field ("BSF") layer composed of p-type
aluminum gallium arsenide disposed on the combined DBR structure
and a window layer composed of n-type indium gallium aluminum
phosphide disposed on the emitter layer of the second solar
subcell, wherein the base layer is disposed on the BSF layer.
10. The multijunction solar cell of claim 1 wherein one or more of
the solar subcells have a gradation in doping in the base layer
that increases approximately exponentially from approximately
1.times.10.sup.15 free carriers per cubic centimeter in a region
adjacent the photoelectric junction to approximately
4.times.10.sup.18 free carriers per cubic centimeter in a region
adjacent an adjoining layer and a gradation in doping in the
emitter layer that increases from approximately 5.times.10.sup.17
free carriers per cubic centimeter in a region adjacent the
photoelectric junction to approximately 5.times.10.sup.18 free
carriers per cubic centimeter in a region immediately adjacent an
adjoining layer.
11. The multijunction solar cell of claim 1 wherein the emitter
layer of the first solar subcell comprises highly doped n-type
indium gallium phosphide ("InGaP").
12. The multijunction solar cell of claim 1 wherein the half width
value of reflection of the first DBR structure and the second DBR
structure is in a range between 250 and 350 nm.
13. The multijunction solar cell of claim 1 wherein the combined
DBR structure includes alternating layers of lattice mismatched
materials, the combined DBR structure includes a first DBR layer
composed of a plurality of n type or p type Al.sub.xGa.sub.1-xAs
layers, and a second DBR layer disposed over the first DBR layer
and composed of a plurality of n or p type Al.sub.yGa.sub.1-yAs
layers, where 0<x<1, 0<y<1, and y is greater than
x.
14. The multijunction solar cell of claim 1 wherein the combined
DBR structure comprises a sequence of alternating first and second
different semiconductor layers, and wherein an average lattice
constant of the sequence of alternating first and second
semiconductor layers is approximately equal to a lattice constant
of a substrate.
15. The multijunction solar cell of claim 1 wherein the first solar
subcell comprises a highly doped n-type indium gallium arsenide
emitter layer and a highly doped n-type indium gallium aluminum
phosphide window layer.
16. The multijunction solar cell of claim 1 wherein the first solar
subcell has a BSF layer comprising highly doped p-type aluminum
gallium arsenide ("AlGaAs").
17. The multijunction solar cell of claim 1 wherein first spectral
wavelength range--of approximately 780 to 860 nm.
18. The multijunction solar cell of claim 1 wherein the second
solar subcell has a band gap in the range of approximately 1.41 eV,
and the first solar subcell has a band gap in the range of 1.65 eV
to 1.8 eV.
19. The multijunction solar cell of claim 1 wherein the first
spectral wavelength range overlaps the second spectral wavelength
range by less than 10 nm.
20. The multijunction solar cell of claim 1 wherein the first
spectral wavelength range and the second spectral wavelength range
correspond to the spectral absorption band of the first solar
subcell.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 15/376,195 filed Dec. 12, 2016.
[0002] This application is related to co-pending U.S. patent
application Ser. No. 14/660,092 filed Mar. 17, 2015, which is a
division of U.S. patent application Ser. No. 12/716,814 filed Mar.
3, 2010, now U.S. Pat. No. 9,018,521; which was a continuation in
part of U.S. patent application Ser. No. 12/337,043 filed Dec. 17,
2008.
[0003] This application is also related to co-pending U.S. patent
application Ser. No. 13/872,663 filed Apr. 29, 2013, 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.
[0004] This application is also related to U.S. patent application
Ser. No. 14/828,197, filed Aug. 17, 2015.
[0005] All of the above related applications are incorporated
herein by reference in their entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0006] The present disclosure relates to solar cells and the
fabrication of solar cells, and more particularly the design and
specification of a lattice matched multijunction solar cells
adapted for specific space missions.
Description of the Related Art
[0007] 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.
[0008] In satellite and other space related applications, the size,
mass and cost of a satellite power system are dependent on the
power and energy conversion efficiency of the solar cells used.
Putting it another way, the size of the payload and the
availability of on-board services are proportional to the amount of
power provided. Thus, as payloads become more sophisticated, and
applications anticipated for five, ten, twenty or more years, the
power-to-weight ratio and lifetime efficiency of a solar cell
becomes increasingly more important, and there is increasing
interest not only the amount of power provided at initial
deployment, but over the entire service life of the satellite
system, or in terms of a design specification, the amount of power
provided at the "end of life" (EOL).
[0009] 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, with each subcell being designed
for photons in a specific wavelength band. After passing through a
subcell, the photons that are not absorbed and converted to
electrical energy propagate to the next subcells, where such
photons are intended to be captured and converted to electrical
energy.
[0010] 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 the effect of its exposure to radiation in the ambient
environment over time. The identification and specification of such
design parameters is a non-trivial engineering undertaking, and
would vary depending upon the specific space mission and customer
design requirements. Since the power output is a function of both
the voltage and the current produced by a subcell, a simplistic
view may seek to maximize both parameters in a subcell by
increasing a constituent element, or the doping level, to achieve
that effect. However, in reality, changing a material parameter
that increases the voltage may result in a decrease in current, and
therefore a lower power output. Such material design parameters are
interdependent and interact in complex and often unpredictable
ways, and for that reason are not "result effective" variables that
those skilled in the art confronted with complex design
specifications and practical operational considerations can easily
adjust to optimize performance. Electrical properties such as the
short circuit current density (J.sub.sc), the open circuit voltage
(V.sub.oc), and the fill factor (FF), which determine the
efficiency and power output of the solar cell, are 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, in a non-uniform manner, over time (i.e.
during the operational life of the system).
[0011] Another important mechanical or structural consideration 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.
SUMMARY OF THE DISCLOSURE
Objects of the Disclosure
[0012] It is an object of the present disclosure to provide
increased photoconversion efficiency in a multijunction solar cell
for space applications by incorporating a plurality of distributed
Bragg reflector structures between two adjacent subcells in the
multijunction solar cell.
[0013] It is another object of the present disclosure to provide a
multijunction solar cell in which the distributed Bragg reflector
structures have different wavelength bands of reflectivity.
[0014] It is another object of the present disclosure to provide a
multijunction solar cell in which the DBR structure or structures
enables a transition in lattice constant between two subcells.
[0015] It is another object of the present invention to provide a
lattice matched four junction solar cell incorporating a plurality
of different DBR structures.
[0016] 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
[0017] All ranges of numerical parameters set forth in this
disclosure are to be understood to encompass any and all subranges
or "intermediate generalizations" subsumed herein. For example, a
stated range of "1.0 to 2.0 eV" for a band gap value should be
considered to include any and all subranges beginning with a
minimum value of 1.0 eV or more and ending with a maximum value of
2.0 eV or less, e.g., 1.0 to 2.0, or 1.3 to 1.4, or 1.5 to 1.9
eV.
[0018] Briefly, and in general terms, the present disclosure
provides a multijunction solar cell comprising: a first and a
second solar subcell each having an emitter layer and a base layer
forming a photoelectric junction; a first distributed Bragg
reflector (DBR) structure disposed beneath the base layer of the
first solar subcell and composed of a plurality of alternating
layers of different semiconductor materials with discontinuities in
their respective indices of refraction and arranged so that light
can enter and pass through the first solar subcell and at least a
first portion of which in a first spectral wavelength range [of 840
to 890 nm] can be reflected back into the first solar subcell by
the DBR structure, and a second portion of which in a second
spectral wavelength range [of 790 to 840 nm] can be transmitted
through the DBR structure to the second solar subcell disposed
beneath the DBR structure, where the second wavelength range is
greater in wavelength than the first wavelength range [wherein the
half width value of reflection of the DBR structure being in a
range between 250 nm to 350 nm]; and wherein the alternating first
and second sublayer have a different lattice constant.
[0019] In some embodiments, the upper first subcell is composed of
indium gallium aluminum phosphide, with the amount of aluminum
being at least 20% by mole fraction.
[0020] In another aspect, the present disclosure provides a method
of manufacturing a multijunction solar cell comprising providing a
germanium substrate; growing on the germanium substrate a lattice
matched sequence of layers of semiconductor material using a metal
organic chemical vapor disposition process to form a plurality of
subcells including one or more middle subcells and a DBR structure
disposed over and lattice mismatched to the germanium substrate and
an upper or top subcell disposed over and lattice matched to the
last middle subcell and having a band gap in the range of 2.0 to
2.15 eV.
[0021] In another aspect, the present disclosure provides a method
of fabricating a four junction solar cell comprising an upper first
solar subcell composed of indium gallium aluminum phosphide and
having a first band gap, a second solar subcell adjacent to said
first solar subcell including an emitter layer composed of indium
gallium phosphide or aluminum gallium arsenide, and a base layer
composed of aluminum gallium arsenide and 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 indium gallium arsenide and
having a third band gap smaller than the second band gap and being
lattice matched with the second solar subcell; a DBR structure
adjacent to the third solar subcell; and a fourth solar subcell
adjacent to said DBR structure and having a fourth band gap smaller
than the third band gap.
[0022] In some embodiments, the fourth subcell is germanium.
[0023] In some embodiments, the fourth subcell is InGaAs, GaAsSb,
InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi,
InGaAsNSbBi, InGaSbN, InGaBiN, InGaSbBiN.
[0024] In some embodiments, the fourth subcell has a band gap of
approximately 0.67 eV, the third subcell has a band gap of
approximately 1.41 eV, the second subcell has a band gap in the
range of approximately 1.65 to 1.8 eV and the upper first subcell
has a band gap in the range of 2.0 to 2.2 eV.
[0025] In some embodiments, 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.
[0026] 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; and the fourth subcell is composed of
germanium.
[0027] In some embodiments, the first and second distributed Bragg
reflector (DBR) structures are disposed adjacent to and between the
middle and bottom solar subcells and arranged so that light can
enter and pass through the middle solar subcell and at least a
portion of which can be reflected back into the middle solar
subcell by the DBR structures.
[0028] In some embodiments, the first and second distributed Bragg
reflector (DBR) structures are disposed adjacent to and between the
second and the third solar subcells and arranged so that light can
enter and pass through the through the third solar subcell and at
least a portion of which can be reflected back into the third solar
subcell by the DBR structures.
[0029] In some embodiments, each of the distributed Bragg reflector
structures are composed of a plurality of alternating layers of
lattice matched materials with discontinuities in their respective
indices of refraction.
[0030] In some embodiments, at least some of the layers of at least
one of the distributed Bragg reflector structures is composed of a
plurality of alternating layers of different lattice constant.
[0031] In some embodiments, at least some of the layers of the
distributed Bragg reflector structures are composed of a plurality
of alternating layers having different doping levels and/or
different dopant materials.
[0032] In some embodiments, at least some of the layers of the
distributed Bragg reflector structures are composed of a plurality
of alternating layers of different thicknesses.
[0033] In some embodiments, the width of the first spectral
wavelength range is between 50 and 100 nm.
[0034] In some embodiments, the first spectral wavelength range
extends between 840 and 890 nm.
[0035] In some embodiments, the first spectral wavelength range
overlaps the second wavelength range by less than 10 nm.
[0036] In some embodiments, the first and second spectral
wavelength ranges correspond to the spectral absorption band of the
first solar subcell.
[0037] 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.
[0038] In some embodiments, each of the distributed Bragg reflector
structures are composed of a plurality of alternating layers that
includes a first DBR layer composed of an n type or p type
Al.sub.xGa.sub.1-xAs layer, and a second adjacent DBR layer
disposed over the first DBR layer and composed of an n or p type
Al.sub.yGa.sub.1-yAs layer, 0<x<1, 0<y<1, and where y
is greater than x.
[0039] In some embodiments, additional layer(s) may be added or
deleted in the cell structure without departing from the scope of
the present disclosure.
[0040] Some implementations of the present disclosure may
incorporate or implement fewer of the aspects and features noted in
the foregoing summaries.
[0041] 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
[0042] 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:
[0043] FIG. 1A is a graph representing the band gap of certain
binary materials and their lattice constants;
[0044] FIG. 1B is a graph representing the efficiency of two tandem
subcells as a function of the band gap of the two subcells;
[0045] FIG. 2A is a graph of the band gap versus lattice constant
of certain binary and ternary III-V semiconductors;
[0046] FIG. 2B is a graph of the band gap versus lattice constant
of certain binary and ternary III-V semiconductors;
[0047] FIG. 3A is a cross-sectional view of a three junction solar
cell after several stages of fabrication including the deposition
of certain semiconductor layers on the growth substrate, according
to a first embodiment of the present disclosure;
[0048] FIG. 3B is a cross-sectional view of a four junction solar
cell after several stages of fabrication including the deposition
of certain semiconductor layers on the growth substrate, according
to a second embodiment of the present disclosure;
[0049] FIG. 3C is a cross-sectional view of the solar cell of a
four junction solar cell after several stages of fabrication
including the deposition of certain semiconductor layers on the
growth substrate, according to a third embodiment of the present
disclosure;
[0050] FIG. 4A is a graph of the current density per unit energy
versus the photon energy of the incoming light in a solar cell;
[0051] FIG. 4B is a schematic representation of photons of
different wavelengths being absorbed by, or being transmitted
through, different subcells in a three junction tandem solar cell
with a single DBR structure;
[0052] FIG. 5A is a graph of the reflectance of a single
distributed Bragg reflector (DBR) structure as a function of
wavelength;
[0053] FIG. 5B is a graph of the reflectance of a first distributed
Bragg reflector (DBR) structure according to the present disclosure
compared with that of the structure of FIG. 5A;
[0054] FIG. 5C is a graph of the reflectance of a first distributed
Bragg reflector (DBR) structure according to the present disclosure
compared with that of the structure of FIG. 5A;
[0055] FIG. 6 is a schematic representation of photons of different
wavelengths being absorbed by, or being transmitted through,
different subcells in a solar cell that includes two distributed
Bragg reflector (DBR) structures according to the present
disclosure;
[0056] FIG. 7A is a graph of the quantum efficiency versus
wavelength in a three junction solar cell;
[0057] FIG. 7B is a graph of the quantum efficiency versus
wavelength in a three junction solar cell after incorporation of a
structure in the solar cell according to the present disclosure;
and
[0058] FIG. 8 is a graph of the doping profile in the base and
emitter layers of a subcell in the solar cell according to the
present disclosure.
GLOSSARY OF TERMS
[0059] "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).
[0060] "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.
[0061] "Beginning of Life (BOL)" refers to the time at which a
photovoltaic power system is initially deployed in operation.
[0062] "Bottom subcell" refers to the subcell in a multijunction
solar cell which is furthest from the primary light source for the
solar cell.
[0063] "Compound semiconductor" refers to a semiconductor formed
using two or more chemical elements.
[0064] "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.
[0065] "Deposited", with respect to a layer of semiconductor
material, refers to a layer of material which is epitaxially grown
over another semiconductor layer.
[0066] "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.
[0067] "Graded interlayer" (or "grading interlayer")--see
"metamorphic layer".
[0068] "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.
[0069] "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.
[0070] "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).
[0071] "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.
[0072] "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).
[0073] "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.
[0074] "Short circuit current density"--see "current density".
[0075] "Solar cell" refers to an electronic device operable to
convert the energy of light directly into electricity by the
photovoltaic effect.
[0076] "Solar cell assembly" refers to two or more solar cell
subassemblies interconnected electrically with one another.
[0077] "Solar cell subassembly" refers to a stacked sequence of
layers including one or more solar subcells.
[0078] "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.
[0079] "Substantially current matched" refers to the short circuit
current through adjacent solar subcells being substantially
identical (i.e. within plus or minus 1%).
[0080] "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.
[0081] "ZTJ" refers to the product designation of a commercially
available SolAero Technologies Corp. triple junction solar
cell.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0082] 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.
[0083] 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 lattice matched 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 with specific DBR
structures grown between subcells.
[0084] 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 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.
[0085] 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).
[0086] 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, 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 direction and the ultimate solar cell design proposed
by the Applicants.
[0087] In view of the foregoing, it is further evident that the
identification of one particular constituent element (e.g. indium,
or aluminum) in a particular subcell, or the thickness, band gap,
doping, or other characteristic of the incorporation of that
material in a particular subcell, is not a single "result effective
variable" that one skilled in the art can simply specify and
incrementally adjust to a particular level and thereby increase the
power output and efficiency of a solar cell.
[0088] 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, 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.
[0089] 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".
[0090] The efficiency of a solar cell is not a simple linear
algebraic equation as a function of 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 a 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.
[0091] 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.
[0092] 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, and are within the scope of the present
disclosure.
[0093] The present disclosure is in one embodiment directed to a
growth process using a metal organic chemical vapor deposition
(MOCVD) process in a standard, commercially available reactor
suitable for high volume production. Other embodiments may use
other growth technique, such as MBE. More particularly, regardless
of the growth technique, the present disclosure is directed to the
materials and fabrication steps that are particularly suitable for
producing commercially viable multijunction solar cells or inverted
metamorphic multijunction solar cells using commercially available
equipment and established high-volume fabrication processes, as
contrasted with merely academic expositions of laboratory or
experimental results.
[0094] Some comments about MOCVD processes used in one embodiment
are in order here.
[0095] It should be noted that the layers of a certain target
composition in a semiconductor structure grown in an MOCVD process
are inherently physically different than the layers of an identical
target composition grown by another process, e.g. Molecular Beam
Epitaxy (MBE). The material quality (i.e., morphology,
stoichiometry, number and location of lattice traps, impurities,
and other lattice defects) of an epitaxial layer in a semiconductor
structure is different depending upon the process used to grow the
layer, as well as the process parameters associated with the
growth. MOCVD is inherently a chemical reaction process, while MBE
is a physical deposition process. The chemicals used in the MOCVD
process are present in the MOCVD reactor and interact with the
wafers in the reactor, and affect the composition, doping, and
other physical, optical and electrical characteristics of the
material. For example, the precursor gases used in an MOCVD reactor
(e.g. hydrogen) are incorporated into the resulting processed wafer
material, and have certain identifiable electro-optical
consequences which are more advantageous in certain specific
applications of the semiconductor structure, such as in
photoelectric conversion in structures designed as solar cells.
Such high order effects of processing technology do result in
relatively minute but actually observable differences in the
material quality grown or deposited according to one process
technique compared to another. Thus, devices fabricated at least in
part using an MOCVD reactor or using a MOCVD process have inherent
different physical material characteristics, which may have an
advantageous effect over the identical target material deposited
using alternative processes.
[0096] 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.
[0097] FIG. 1A is a graph representing the band gap of certain
binary materials and their lattice constants. The band gap and
lattice constants of ternary materials are located on the lines
drawn between typical associated binary materials (such as the
ternary material AlGaAs being located between the GaAs and AlAs
points on the graph, with the band gap of the ternary material
lying between 1.42 eV for GaAs and 2.16 eV for AlAs depending upon
the relative amount of the individual constituents). Thus,
depending upon the desired band gap, the material constituents of
ternary materials can be appropriately selected for growth.
[0098] FIG. 1B is a graph representing the efficiency of two tandem
subcells as a function of the band gap of the two subcells. In
particular, it is depicted to demonstrate that the maximum
efficiency of tandem combination of subcells is not a simple linear
function of the band gap of either the top or high band gap
subcell, or the lower or low band gap subcell.
[0099] FIG. 2A is an enlargement of a portion of the graph of FIG.
1A illustrating different compounds of GalnAs and GaInP with
different proportions of gallium and indium, and the location of
specific compounds on the graph.
[0100] FIG. 2B is a representation of the theoretical efficiency of
a tandem solar cell in which the band gap of the top solar subcell
is plotted along the y-axis, and the band gap of the adjacent
middle solar subcell is plotted along the x-axis graph, with the
two ternary compounds Ga.sub.xIn.sub.1-xAs and Ga.sub.yIn.sub.1-yP
having identical lattice constants being plotted as a straight
line.
[0101] FIG. 3A illustrates a particular example of an embodiment of
a three junction solar cell 3000 after several stages of
fabrication including the growth of certain semiconductor layers on
the growth substrate up to the contact layer 322 as provided by the
present disclosure.
[0102] As shown in the illustrated example of FIG. 3A, the bottom
subcell C includes a substrate 300 formed of p-type germanium
("Ge") which also serves as a base layer. A back metal contact pad
350 formed on the bottom of base layer 300 provides electrical
contact to the multijunction solar cell 400. The bottom subcell C,
further includes, for example, a highly doped n-type Ge emitter
layer 301, and an n-type indium gallium arsenide ("InGaAs")
nucleation layer 302. The nucleation layer is deposited over the
base layer, and the emitter layer is formed in the substrate by
diffusion of deposits into the Ge substrate, thereby forming the
n-type Ge layer 301. Heavily doped p-type aluminum gallium arsenide
("AlGaAs") and heavily doped n-type gallium arsenide ("GaAs")
tunneling junction layers 303, 304 may be deposited over the
nucleation layer to provide a low resistance pathway between the
bottom and middle subcells.
[0103] A first Distributed Bragg reflector (DBR) structure (DBR-2)
consisting of layers 305 (specifically, individual layers 305a
through 305z) are then grown adjacent to and between the tunnel
diode 303, 304 of the bottom subcell C and the second solar subcell
B. The DBR layers 305 are arranged so that light can enter and pass
through the third solar subcell B and DBR structure 306 and at
least a portion of which can be reflected back into the second
solar subcell B by the DBR layers 305. In the embodiment depicted
in FIG. 3A, the distributed Bragg reflector (DBR) layers 305 are
specifically located between the second solar subcell B/DBR
structure 306 and tunnel diode layers 304, 303; in other
embodiments, the distributed Bragg reflector (DBR) layers may be
located between tunnel diode layers 304/303 and buffer layer
302.
[0104] A second Distributed Bragg reflector (DBR) structure (DBR-1)
consisting of layers 306 (specifically, 306a through 306z) being
compositionally and optically different from DBR structure DBR-1,
are then grown adjacent to and between the DBR-2 structure and the
second solar subcell B. The DBR layers 306 are arranged so that
light can enter and pass through the third solar subcell B and at
least a portion of which can be reflected back into the third solar
subcell B by the DBR layers 306. In the embodiment depicted in FIG.
3A, the distributed Bragg reflector (DBR) layers 306 are
specifically located between the second solar subcell B and tunnel
diode layers 304, 303; in other embodiments, the distributed Bragg
reflector (DBR) layers 306 may be located between tunnel diode
layers 304/303 and DBR-2 structure.
[0105] For some embodiments, distributed Bragg reflector (DBR)
layers 305 and 306 can be composed of a plurality of alternating
layers 305a through 305z and 306a through 306z, respectively, 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.
[0106] For some embodiments, distributed Bragg reflector (DBR)
layers 305a through 305z, and 306a through 306z includes a first
DBR layer composed of a plurality of p type Al.sub.xGa.sub.1-xAs
layers, and a second DBR layer disposed over the first DBR layer
and composed of a plurality of n or p type Al.sub.yGa.sub.1-yAs
layers, where 0<x<1, 0<y<1, and y is greater than
x.
[0107] The scope of the compositional and optical difference in the
structures 305 and 306 will be described and specified in more
detail subsequent to the discussion of other embodiments.
[0108] In the illustrated example of FIG. 3A, the subcell B
includes a highly doped p-type aluminum gallium arsenide ("AlGaAs")
back surface field ("BSF") layer 312, a p-type AlGaAs base layer
313, a highly doped n-type indium gallium phosphide ("InGaP") or
AlGaAs emitter layer 314 and a highly doped n-type indium gallium
aluminum phosphide ("InGaAlP") window layer 315. Other compositions
may be used as well. The base layer 313 is formed over the BSF
layer 312 after the BSF layer 312 is deposited over the DBR layers
306.
[0109] The window layer 315 helps reduce the recombination loss and
improves passivation of the cell surface of the underlying
junctions.
[0110] Before depositing the layers of the top cell A, heavily
doped n-type InGaP and p-type AlGaAs tunneling junction layers 316,
317 may be deposited over the subcell B.
[0111] In the illustrated example, the top subcell A includes a
highly doped p-type indium aluminum phosphide ("InAlP.sub.2") BSF
layer 318, a p-type InGaAlP base layer 319, a highly doped n-type
InGaAlP emitter layer 320 and a highly doped n-type InAlP.sub.2
window layer 321.
[0112] A cap or contact layer 322 of GaAs is deposited over the
window layer 321 and the grid lines are formed via evaporation and
lithographically patterned and deposited over the cap or contact
layer 322.
[0113] Turning to another embodiment of the multijunction solar
cell device of the present disclosure, FIG. 3B is a cross-sectional
view of an embodiment of a four junction solar cell 4000 after
several stages of fabrication including the growth of certain
semiconductor layers on the growth substrate up to the contact
layer 322, with various layers and subcells being similar to the
structure described and depicted in FIG. 3A.
[0114] The second embodiment depicted in FIG. 3B is similar to that
of the first embodiment depicted in FIG. 3A except that an
additional middle subcell, subcell C, including layers 307 through
311 is now included, and since the other layers in FIG. 3B are
substantially identical to that of layers in FIG. 3A, the
description of such layers will not be repeated here for
brevity.
[0115] In the illustrated example of FIG. 3B, the subcell C
includes a highly doped p-type aluminum gallium arsenide ("AlGaAs")
back surface field ("BSF") layer 307, a p-type InGaAs base layer
308a, a highly doped n-type indium gallium arsenide ("InGaAs")
emitter layer 308b and a highly doped n-type indium aluminum
phosphide ("AlInP.sub.2") or indium gallium phosphide ("GaInP")
window layer 309. The InGaAs base layer 308a of the subcell C can
include, for example, approximately 1.5% In. Other compositions may
be used as well. The base layer 308a is formed over the BSF layer
307 after the BSF layer 307 is deposited over the DBR layers
306.
[0116] The window layer 309 is deposited on the emitter layer 308
of the subcell C. The window layer 309 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 C, heavily doped n-type InGaP and p-type AlGaAs (or
other suitable compositions) tunneling junction layers 310, 311 may
be deposited over the subcell C.
[0117] Turning to another embodiment of the multijunction solar
cell device of the present disclosure, FIG. 3C is a cross-sectional
view of an embodiment of a four junction solar cell 600 after
several stages of fabrication including the growth of certain
semiconductor layers on the growth substrate up to the contact
layer 322, with various subcells being similar to the structure
described and depicted in FIG. 3B.
[0118] In a first embodiment of the present invention, shown in
FIG. 3C, an intrinsic layer constituted by a strain-balanced
multi-quantum well structure 500 is formed between base layer 410b
and emitter layer 411 of middle subcell C. The strain-balanced
quantum well structure 500 includes a sequence of quantum well
layers formed from alternating layers of compressively strained
InGaAs and tensionally strained gallium arsenide phosphide
("GaAsP"). Strain-balanced quantum well structures are known from
the paper of Chao-Gang Lou et al., Current-Enhanced Quantum Well
Solar Cells, Chinese Physics Letters, Vol. 23, No. 1 (2006), and M.
Mazzer et al., Progress in Quantum Well Solar Cells, Thin Solid
Films, Volumes 511-512 (26 Jul. 2006).
[0119] In an alternative example, the strain-balanced quantum well
structure 500, comprising compressively strained InGaAs and
tensionally strained gallium arsenide, may be provided as either
the base layer 410b or the emitter layer 411.
[0120] In the illustrated example, the strain-balanced quantum well
structure 500 is formed in the depletion region of the middle
subcell C and has a total thickness of about 3 microns (mm).
Different thicknesses may be used as well. Alternatively, as noted
above, the middle subcell C can incorporate the strain-balanced
quantum well structure 500 as either the base layer 410 or the
emitter layer 411 without an intervening layer between the base
layer 410b and emitter layer 411. A strain-balanced quantum well
structure can include one or more quantum wells. The quantum wells
may be formed from alternating layers of compressively strained
InGaAs and tensionally strained GaAsP. An individual quantum well
within the structure includes a well layer of InGaAs provided
between two barrier layers of GaAsP, each having a wider energy
band gap than InGaAs. The InGaAs layer is compressively strained
due to its larger lattice constant with respect to the lattice
constant of the substrate 400. The GaAsP layer is tensionally
strained due to its smaller lattice constant with respect to the
substrate 400. The "strain-balanced" condition occurs when the
average strain of the quantum well structure is approximately equal
to zero. Strain-balancing ensures that there is almost no stress in
the quantum well structure when the multijunction solar cell layers
are grown epitaxially. The absence of stress between layers can
help prevent the formation of dislocations in the crystal
structure, which would otherwise negatively affect device
performance. For example, the compressively strained InGaAs well
layers of the quantum well structure 500 may be strain-balanced by
the tensile strained GaAsP barrier layers.
[0121] The quantum well structure 500 may also be lattice matched
to the substrate 400. In other words, the quantum well structure
may possess an average lattice constant that is approximately equal
to a lattice constant of the substrate 400. In other embodiments,
lattice matching the quantum well structure 500 to the substrate
400 may further reduce the formation of dislocations and improve
device performance. Alternatively, the average lattice constant of
the quantum well structure 500 may be designed so that it maintains
the lattice constant of the parent material in the middle subcell
C. For example, the quantum well structure 500 may be fabricated to
have an average lattice constant that maintains the lattice
constant of the AlGaAs BSF layer 410a. In this way, dislocations
are not introduced relative to the middle cell C. However, the
overall device 600 is lattice mismatched if the lattice constant of
the middle cell C is not matched to the substrate 400. The
thickness and composition of each individual InGaAs or GaAsP layer
within the quantum well structure 500 may be adjusted to achieve
strain-balance and minimize the formation of crystal dislocations.
For example, the InGaAs and GaAsP layers may be formed having
respective thicknesses about 100 to 300 angstroms. Between 100 and
300 total InGaAs/GaAsP quantum wells may be formed in the
strain-balanced quantum well structure 500. More or fewer quantum
wells may be used as well. Additionally, the concentration of
indium in the InGaAs layers may vary between 10 and 30%.
[0122] Furthermore, the quantum well structure 500 can extend the
range of wavelengths absorbed by the middle subcell C. An example
of approximate quantum efficiency curves for the multijunction
solar cell of FIG. 3C is illustrated in FIG. 7A. As shown in the
example of FIG. 7A, the absorption spectrum for the bottom subcell
603, subcell D, extends between 890-1600 nm; the absorption
spectrum of the middle subcell 602 extends between 660-1000 nm,
overlapping the absorption spectrum of the bottom subcell; and the
absorption spectrum of the top subcell 601, subcell A, extends
between 300-660 nm. Incident photons having wavelengths located
within the overlapping portion of the middle and bottom subcell
absorption spectrums may be absorbed by the middle subcell 602
prior to reaching the bottom subcell 603. As a result, the
photocurrent produced by middle subcell 602 may increase by taking
some of the current that would otherwise be excess current in the
bottom subcell 603. In other words, the photo-generated current
density produced by the middle subcell 602 may increase. Depending
on the total number of layers and thickness of each layer within
the quantum well structure 500, the photo-generated current density
of the middle subcell 602 may be increased to match the
photo-generated current density of the bottom subcell 603.
[0123] The overall current produced by the multijunction cell solar
cell then may be raised by increasing the current produced by top
subcell 601. Additional current can be produced by top subcell 601
by increasing the thickness of the p-type InGaAlP2 base layer 422
in that cell. The increase in thickness allows additional photons
to be absorbed, which results in additional current generation.
Preferably, for space or AM0 applications, the increase in
thickness of the top subcell 601 maintains the approximately 4 to
5% difference in current generation between the top subcell A and
middle subcell C. For AM1 or terrestrial applications, the current
generation of the top cell and the middle cell may be chosen to be
equalized.
[0124] As a result, both the introduction of strain-balanced
quantum wells in the middle subcell 602 and the increase in
thickness of top subcell A provide an increase in overall
multijunction solar cell current generation and enable an
improvement in overall photon conversion efficiency. Furthermore,
the increase in current may be achieved without significantly
reducing the voltage across the multijunction solar cell.
[0125] In some embodiments, the sequence of first 501A and second
501B different semiconductor layers forms the base layer of the
second subcell.
[0126] In some embodiments, the sequence of first and second
different semiconductor layers comprises compressively strained and
tensionally strained layers, respectively.
[0127] In some embodiments, an average strain of the sequence of
first and second different semiconductor layers is approximately
equal to zero.
[0128] In some embodiments, each of the first and second
semiconductor layers is approximately 100 nm to 300 angstroms
thick.
[0129] In some embodiments, the first semiconductor layer comprises
InGaAs and the second semiconductor layer comprises GaAsP.
[0130] In some embodiments, a percentage of indium in each InGaAs
layer is in the range of 10 to 30%.
[0131] In some embodiments, the top subcell comprises InGaP and has
a thickness so that it generates approximately 4-5% less current
than said first current.
[0132] FIG. 4A is a graph of the current density per unit energy
versus the photon energy and the wavelength of the incoming light
in a solar cell under the AM0 spectral environment.
[0133] FIG. 4B is a schematic representation of photons of
different wavelengths being absorbed by, or being transmitted
through, different subcells in a three junction tandem solar cell
with a single DBR structure (DBR-1). In this particular
representation the light wavelengths
.lamda..sub.C>.lamda..sub.B>.lamda..sub.A. The top subcell A
is designed to absorb light of wavelength 4, the middle subcell is
designed to absorb light of wavelength .lamda..sub.B, and the
bottom subcell is designed to absorb light of wavelength
.lamda..sub.C.
[0134] FIG. 5A is a graph 500 of the reflectance of a single
distributed Bragg reflector structure as a function of wavelength
such as known in the prior art. In this particular example, the
wavelengths of light which are most strongly reflected extend over
the range 790 to 910 nm. Assuming that subcell B would absorb light
in the wavelength range of 790 to 910 nm, employment of this DBR
structure below subcell B will reflect any light entering the DBR
structure of that wavelength back into subcell B, to allow photons
of such wavelength to make a second pass through subcell B,
increasing the photocurrent of subcell B.
[0135] FIG. 5B is a graph 501 of the reflectance of a first
distributed Bragg reflector (DBR-2) structure according to the
present disclosure compared with that of the graph 500 of FIG. 5A.
In this particular example, the wavelengths of light which are most
strongly reflected by DBR-2 extend over the range 850 to 920 nm.
Assuming that subcell B would absorb light in the wavelength range
of 790 to 910 nm, employment of DBR-1 below subcell B will reflect
any light entering DBR-1 of wavelength range 850 to 920 nm back
into subcell B to allow photons of such wavelength to make a second
pass through subcell B, increasing the photocurrent of subcell
B.
[0136] FIG. 5C is a graph 502 of the reflectance of a second
distributed Bragg reflector (DBR-1) structure according to the
present disclosure compared with that of graph 500 of FIG. 5A. In
this particular example, the wavelengths of light which are most
strongly reflected extend over the range 780 to 860 nm. Assuming
that subcell B would absorb light in the wavelength range of 780 to
860 nm, employment of DBR-1 below subcell B will reflect any light
entering DBR-1 of that wavelength range back into subcell B to
allow photons of such wavelength to make a second pass through
subcell B, increasing the photocurrent of subcell B.
[0137] The present disclosure contemplates the use of both DBR-2
and DBR-1 to more finely tune or accurately cover the wavelength
range to be reflected back into subcell B compared with the single
DBR structure of FIG. 5A.
[0138] FIG. 6 is a schematic representation of photons of different
wavelengths (.lamda..sub.A, .lamda..sub.B1, .lamda..sub.B2,
.lamda..sub.C) being absorbed by, or being transmitted through,
different subcells in a solar cell that includes two distributed
Bragg reflector (DBR) structures DBR-1 and DBR-2 according to the
present disclosure.
[0139] FIG. 7A is a graph of the quantum efficiency versus
wavelength in a three junction solar cell, represented by the top
cell graph 601, the middle cell graph 602, and the bottom cell
graph 603.
[0140] FIG. 7B is a graph of the quantum efficiency versus
wavelength in a three junction solar cell after incorporation of a
structure in the solar cell according to the present
disclosure.
[0141] FIG. 8 is a graph of the doping profile in the base and
emitter layers of a subcell in the solar cell according to the
present disclosure. In some embodiments, at least the base of at
least one of the first A, second B or third C solar subcells has a
graded doping, i.e., the level of doping varies from one surface to
the other throughout the thickness of the base layer. In some
embodiments, the gradation in doping is exponential. In some
embodiments, the gradation in doping is incremental and
monotonic.
[0142] In some embodiments, the emitter of at least one of the
first A, second B or third C solar subcells also has a graded
doping, i.e., the level of doping varies from one surface to the
other throughout the thickness of the emitter layer. In some
embodiments, the gradation in doping is linear or monotonically
decreasing.
[0143] As a specific example, the doping profile of the emitter and
base layers may be illustrated in FIG. 8, which depicts the amount
of doping in the emitter region and the base region of a subcell.
N-type dopants include silicon, selenium, sulfur, germanium or tin.
P-type dopants include silicon, zinc, chromium, or germanium.
[0144] In the example of FIG. 8, the emitter doping decreases from
anywhere in the range of approximately 5.times.10.sup.18 to
1.times.10.sup.17 free carriers per cubic centimeter in the region
immediately adjacent the adjoining layer to anywhere in the range
of 1.times.10.sup.18 to 1.times.10.sup.15 free carriers per cubic
centimeter in the region adjacent to the p-n junction which is
shown by the dotted line in the referenced Figure.
[0145] The base doping increases from anywhere in the range of
1.times.10.sup.15 to 1.times.10.sup.18 free carriers per cubic
centimeter adjacent the p-n junction to anywhere in the range of
1.times.10.sup.16 to 1.times.10.sup.19 free carriers per cubic
centimeter adjacent to the adjoining layer at the rear of the
base.
[0146] In some embodiments, the doping level throughout the
thickness of the base layer may be exponentially graded from the
range of 1.times.10.sup.16 free carriers per cubic centimeter to
1.times.10.sup.18 free carriers per cubic centimeter, as
represented by the curve 603 depicted in the Figure.
[0147] In some embodiments, the doping level throughout the
thickness of the emitter layer may decline linearly from
5.times.10.sup.18 free carriers per cubic centimeter to
5.times.10.sup.17 free carriers per cubic centimeter as represented
by the curve 602 depicted in the Figure.
[0148] The absolute value of the collection field generated by an
exponential doping gradient exp [-x/.lamda.] is given by the
constant electric field of magnitude
E=kT/q(1/.lamda.))(exp[-x.sub.b/.lamda.]), where k is the Boltzman
constant, T is the absolute temperature in degrees Kelvin, q is the
absolute value of electronic change, and .lamda. is a parameter
characteristic of the doping decay.
[0149] The efficacy of an embodiment of the present disclosure has
been demonstrated in a test solar cell which incorporated an
exponential doping profile in the three micron thick base layer of
a subcell, according to one embodiment of the present disclosure.
Following measurements of the electrical parameters of the test
cell, there was observed a 6.7% increase in current collection. The
measurements indicated an open circuit voltage (V.sub.oc) equal to
at least 3.014 volts, a short circuit current density (J.sub.sc) of
at least 16.55 mA/cm.sup.2, and a fill factor (FF) of at least 0.86
at AM0.
[0150] The exponential doping profile taught by the present
disclosure produces a constant field in the doped region. In the
particular multijunction solar cell materials and structure of the
present disclosure, the bottom subcell has the smallest short
circuit current among all the subcells. Since in a multijunction
solar cell, the individual subcells are stacked and form a series
circuit, the total current flow in the entire solar cell is
therefore limited by the smallest current produced in any of the
subcells. Thus, by increasing the short circuit current in the
bottom cell, the current more closely approximates that of the
higher subcells, and the overall efficiency of the solar cell is
increased as well. In addition to an increase in efficiency, the
collection field created by the exponential doping profile will
enhance the radiation hardness of the solar cell, which is
important for spacecraft applications.
[0151] Although the exponentially doped profile is the doping
design which has been implemented and verified, other doping
profiles may give rise to a linear varying collection field which
may offer yet other advantages, including for both minority carrier
collection and for radiation hardness at the end-of-life (EOL) of
the solar cell. Such other doping profiles in one or more base
layers are within the scope of the present disclosure.
[0152] The doping profile 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.
[0153] In some embodiments, the composition of the window layer is
linearly graded so that the concentration of Al in the window layer
linearly increases from the bottom surface of the window layer to
the top surface of the window layer.
[0154] In some embodiments, the window layer is composed of InAlP
or InGaP and the Al content at the bottom surface of the window
layer is between 40.0 and 48.5% by mole fraction.
[0155] In some embodiments, the composition of the window layer is
graded so that the lattice constant in the window layer is in
compression at the bottom surface of the window layer, and
increases to the top surface of the window layer so that the
lattice constant in the window layer is in compression at the top
surface.
[0156] 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.
[0157] Although described embodiments of the present disclosure
utilizes a vertical stack of three subcells, various aspects and
features of the present disclosure can apply to stacks with fewer
or greater number of subcells, i.e. two junction cells, three
junction cells, five, six, seven junction cells, etc.
[0158] 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.
[0159] 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 309, with p-type and n-type InGaP is one example
of a homojunction subcell.
[0160] 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.
[0161] 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, AlGaInP,
AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GalnAs, GaInPAs, AlGaAs,
AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GalnSb,
AlGaInSb, AIN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe,
CdSSe, and similar materials, and still fall within the spirit of
the present invention.
[0162] 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.
[0163] 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.
[0164] 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.
* * * * *