U.S. patent application number 16/818258 was filed with the patent office on 2020-08-27 for multijunction solar cells on bulk gesi substrate.
This patent application is currently assigned to SolAero Technologies Corp.. The applicant listed for this patent is SolAero Technologies Corp.. Invention is credited to Zachary Bittner, Daniel Derkacs, John Hart, Nathaniel Miller, Paul Sharps, Samantha Whipple.
Application Number | 20200274016 16/818258 |
Document ID | / |
Family ID | 1000004814783 |
Filed Date | 2020-08-27 |
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United States Patent
Application |
20200274016 |
Kind Code |
A1 |
Hart; John ; et al. |
August 27, 2020 |
MULTIJUNCTION SOLAR CELLS ON BULK GeSi SUBSTRATE
Abstract
A solar cell comprising a bulk germanium silicon growth
substrate; a diffused photoactive junction in the germanium silicon
substrate; and a sequence of subcells grown over the substrate,
with the first grown subcell either being lattice matched or
lattice mis-matched to the growth substrate.
Inventors: |
Hart; John; (Albuquerque,
NM) ; Bittner; Zachary; (Albuquerque, NM) ;
Whipple; Samantha; (Albuquerque, NM) ; Miller;
Nathaniel; (Albuquerque, NM) ; Derkacs; Daniel;
(Albuquerque, NM) ; Sharps; Paul; (Albuquerque,
NM) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SolAero Technologies Corp. |
Albuquerque |
NM |
US |
|
|
Assignee: |
SolAero Technologies Corp.
Albuquerque
NM
|
Family ID: |
1000004814783 |
Appl. No.: |
16/818258 |
Filed: |
March 13, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15938246 |
Mar 28, 2018 |
10707366 |
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16818258 |
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15873135 |
Jan 17, 2018 |
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15938246 |
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14828206 |
Aug 17, 2015 |
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15873135 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/044 20141201;
H01L 31/1816 20130101; Y02E 10/52 20130101; Y02E 10/544 20130101;
H01L 21/28255 20130101; H01L 27/302 20130101; H01L 31/06875
20130101; H01L 2924/10271 20130101; H01L 31/0735 20130101; H01L
31/1852 20130101; H01L 31/0693 20130101; H01L 31/074 20130101; H01L
31/076 20130101; H01L 31/0725 20130101; H01L 31/073 20130101; H01L
31/0687 20130101; H01L 31/0745 20130101; H01L 31/03765 20130101;
H01L 31/03687 20130101; H01L 31/078 20130101; H01L 31/204 20130101;
Y02P 70/50 20151101; H01L 29/7378 20130101; H01L 31/1812 20130101;
H01L 31/0547 20141201; H01L 31/035254 20130101 |
International
Class: |
H01L 31/0725 20060101
H01L031/0725; H01L 31/0735 20060101 H01L031/0735; H01L 31/18
20060101 H01L031/18; H01L 31/054 20060101 H01L031/054; H01L 31/044
20060101 H01L031/044; H01L 31/074 20060101 H01L031/074; H01L 31/078
20060101 H01L031/078; H01L 31/0687 20060101 H01L031/0687 |
Claims
1. A method of manufacturing a multijunction solar cell comprising:
providing a growth substrate; forming a first solar subcell in the
growth substrate; growing a sequence of layers of semiconductor
material using a disposition process to form a solar cell
comprising a plurality of subcells including a first middle subcell
disposed over the growth substrate and having a band gap in the
range of 0.9 to 1.6 eV, at least a second middle subcell disposed
over the first middle subcell and having a band gap in the range of
approximately 1.55 to 1.8 eV and an upper subcell disposed over the
last middle subcell and a band gap in the range of 2.0 to 2.20 eV;
wherein the growth substrate is composed of GeSi with the Ge
content in the GeSi substrate in the range of 85% to 87%.
2. A method as defined in claim 1, wherein the first solar subcell
has a band gap of less than 2.15 eV, the second middle solar
subcell has a band gap of less than 1.73 eV; and the first middle
solar subcell has a band gap in the range of 1.15 to 1.2 eV.
3. A method as defined in claim 1, wherein the first solar subcell
has a band gap of 2.05 eV, and the first solar subcell has an
indirect band gap of 0.7 to 1.1 eV, or 0.85 to 1.65 eV.
4. A method as defined in claim 1, wherein the band gap of the
first middle solar subcell is less than 1.41 eV, and greater than
that of the first solar subcell.
5. A method as defined in claim 1, further comprising: providing a
distributed Bragg reflector (DBR) layer adjacent to and disposed
between the first middle and the first solar subcells and arranged
so that light can enter and pass through the first middle solar
subcell and at least a portion of which can be reflected back into
the first middle solar subcell by the DBR layer, and is composed of
a plurality of alternating sublayers of lattice matched materials
with discontinuities in their respective indices of refraction; and
wherein the difference in refractive indices between alternating
sublayers 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.
6. A method as defined in claim 5, wherein the DBR layer includes a
first DBR layer composed of a plurality of p type
In.sub.zAl.sub.xGa.sub.1-x-zAs sublayers, and a second DBR layer
disposed over and adjacent to the first DBR layer and composed of a
plurality of p type In.sub.wAl.sub.yGa.sub.1-y-wAs sublayers, where
0<w<1, 0<y<1, 0<z<1 and y is greater than x,
thereby increasing the reflection bandwidth of the DBR layer.
7. A method as defined in claim 1, wherein the growth substrate is
lattice mismatched with respect to the first middle subcell, and
has a band gap between 0.83 and 0.88 eV as measured at 300 degrees
Kelvin, corresponding to a percentage of Si in the GeSi substrate
ranging between 13.0 and 15.0 percent by mole fraction.
8. A method as defined in claim 1, wherein the first subcell is
composed of a base layer of (In.sub.xGa.sub.1-x).sub.1-yAl.sub.yP
where x is 0.505, and y is 0.142, corresponding to a band gap of
2.10 eV, and an emitter layer of
(In.sub.xGa.sub.1-x).sub.1-yAl.sub.yP where x is 0.505, and y is
0.107, corresponding to a band gap of 2.05 eV.
9. A method as defined in claim 1, further comprising a tunnel
diode disposed over the growth substrate, and an intermediate layer
disposed between the first middle subcell and the tunnel diode,
wherein the intermediate layer is compositionally graded to lattice
match the first middle solar subcell on one side and the tunnel
diode 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 greater than or equal to that
of the first middle solar subcell and different from that of the
tunnel diode, and having a band gap energy greater than that of the
growth substrate.
10. A method as defined in claim 1, further comprising an
intermediate layer disposed between the first middle subcell and
the growth substrate wherein the intermediate layer is
compositionally step-graded with between one and four steps to
lattice match the growth substrate on one side and composed of
In.sub.xGa.sub.1-xAs or (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs with
0<x<1, 0<y<1, and x and y selected such that the band
gap is in the range of 1.15 to 1.41 eV throughout its
thickness.
11. A method as defined in claim 10, wherein the intermediate layer
has a graded band gap in the range of 1.15 to 1.41 eV, or 1.2 to
1.35 eV, or 1.25 to 1.30 eV.
12. A method as defined in claim 1, wherein either (i) the emitter
layer; or (ii) the base layer and emitter layer, or the upper
subcell have different lattice constants from the lattice constant
of the second middle subcell.
13. A method as defined in claim 1, further comprising: providing a
distributed Bragg reflector (DBR) layer adjacent to and beneath the
first middle solar subcell and arranged so that light can enter and
pass through the first middle solar subcell and at least a portion
of which can be reflected back into the first middle solar subcell
by the DBR layer, wherein 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, wherein the difference in refractive indices between
alternating layers is maximized in order to minimize the number of
periods required to achieve a given reflectivity, and the thickness
and refractive index of each period determines the stop band and
its limiting wavelength, and wherein the DBR layer includes a first
DBR layer composed of a plurality of p type
In.sub.zAl.sub.xGa.sub.1-x-zAs layers, and a second DBR layer
disposed over the first DBR layer and composed of a plurality of p
type In.sub.wAl.sub.yGa.sub.1-y-wAs layers, where 0<w<1,
0<x<1, 0<y<1, 0<z<1 and y is greater than x; and
providing an intermediate layer disposed between the DBR layer and
the first solar subcell, wherein the intermediate layer is
compositionally step-graded to lattice match the DBR layer on one
side and the first 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
greater than or equal to that of the DBR layer and less than or
equal to that of the first solar subcell, and having a band gap
energy greater than that of the first solar subcell.
14. A method as defined in claim 1, wherein each subcell includes
an emitter region and a base region, and one or more of the
subcells have a base region having a gradation in doping that
increases exponentially from 1.times.10.sup.15 atoms per cubic
centimeter adjacent the p-n junction to 4.times.10.sup.18 atoms per
cubic centimeter adjacent to the adjoining layer at the rear of the
base, and an emitter region having a gradation in doping that
decreases from approximately 5.times.10.sup.18 atoms per cubic
centimeter in the region immediately adjacent the adjoining layer
to 5.times.10.sup.17 atoms per cubic centimeter in the region
adjacent to the p-n junction.
15. A method as defined in claim 9, wherein at least one of the
upper sublayers of the intermediate layer has a larger lattice
constant than the adjacent layers of the upper sublayer disposed
directly above the intermediate layer.
16. A method as defined in claim 1, wherein the difference in
lattice constant between the adjacent first middle subcell and the
first subcell is in the range of 0.1 to 0.2 Angstroms.
17. A method as defined in claim 1, further comprising an inactive
majority carrier layer (i.e., a window, BSF, or tunnel diode layer)
disposed over the first middle subcell or second middle solar
subcell, and having a lattice constant that is greater than that of
the first middle subcell and the first subcell so that the tunnel
diode layers are strained in tension.
18. A method as defined in claim 1, further comprising a first
threading dislocation inhibition layer having a thickness in the
range of 0.10 to 1.0 microns disposed over said second middle solar
subcell.
19. A method as defined in claim 18, further comprising a second
threading dislocation inhibition layer having a thickness in the
range of 0.10 to 1.0 micron and composed of InGa(Al)P, the second
threading dislocation inhibition layer being disposed over and
directly adjacent to said grading interlayer for reducing the
propagation of threading dislocations, said second threading
dislocation inhibition layer having a composition different from a
composition of the first threading dislocation inhibition
layer.
20. A method as defined in claim 1, including providing an
intermediate layer disposed between the first middle solar subcell
and the first solar subcell so as to provide a gradual transition
in lattice constant in semiconductor structure from the first
middle solar subcell to the first solar subcell, wherein the
intermediate layer has a band gap that in constant throughout its
thickness, and wherein the multijunction solar cell is a four
junction solar cell in which the numerical sum of the band gaps of
the four solar subcells, divided by four, is equal to 1.35 eV.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 15/938,246 filed Mar. 28, 2018, which is a
continuation-in-part of U.S. patent application Ser. No.
15/873,135, filed Jan. 17, 2018, which in turn is a
continuation-in-part of U.S. patent application Ser. No.
14/828,206, filed Aug. 17, 2015.
[0002] This application is also related to U.S. patent application
Ser. No. 15/938,266, filed Mar. 28, 2018, which is also a
continuation-in-part of U.S. patent application Ser. No.
15/873,135, filed Jan. 17, 2018, which in turn is a
continuation-in-part of U.S. patent application Ser. No.
14/828,206, filed Aug. 17, 2015.
[0003] 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.
[0004] This application is also related to co-pending U.S. patent
application Ser. No. 13/872,663 filed Apr. 29, 2013, 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. No. 14/828,197, filed Aug. 17, 2015.
[0006] All of the above related applications are incorporated
herein by reference in their entireties.
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 based on III-V
semiconductor compounds grown on a bulk GeSi substrate.
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. Compared to silicon, III-V compound semiconductor
multijunction devices have greater energy conversion efficiencies
and are 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 29.5% 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 series
connected photovoltaic regions with different band gap energies,
and accumulating the voltage at a given 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 use increasing amounts of power
as they become more sophisticated, and missions and applications
anticipated for five, ten, twenty or more years, the
power-to-weight (W/kg) and power-to-area (W/m.sup.2) ratios of the
solar cell array and the lifetime efficiency of a solar cell
becomes increasingly more important. There is increasing interest
not only the amount of power provided per gram of weight and
spatial area of the solar cell, not only at initial deployment but
over the entire service life of the satellite system, or in terms
of a design specification, the amount of residual power provided at
the specified "end of life" (EOL), which is affected by the
radiation exposure of the solar cell over time in the particular
space environment of the solar cell array, the period of the EOL
being different for different missions and applications.
[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, 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.
[0011] The individual solar cells or wafers are then disposed in
horizontal arrays, 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 needed by the payload or subcomponents of the payload, the
amount of electrical storage capacity (batteries) on the
spacecraft, and the power demands of the payloads during different
orbital configurations.
[0012] A solar cell designed for use in a space vehicle (such as a
satellite, space station, or an interplanetary mission vehicle),
has a sequence of subcells with compositions and band gaps which
have been optimized to achieve maximum energy conversion efficiency
for the AM0 solar spectrum in space. The AM0 solar spectrum in
space is notably different from the AM1.5 solar spectrum at the
surface of the earth, and accordingly terrestrial solar cells are
designed with subcell band gaps optimized for the AM1.5 solar
spectrum.
[0013] There are substantially more rigorous qualification and
acceptance testing protocols used in the manufacture of space solar
cells compared to terrestrial cells, to ensure that space solar
cells can operate satisfactorily at the wide range of temperatures
and temperature cycles encountered in space. These testing
protocols include (i) high-temperature thermal vacuum bake-out;
(ii) thermal cycling in vacuum (TVAC) or ambient pressure nitrogen
atmosphere (APTC); and in some applications (iii) exposure to
radiation equivalent to that which would be experienced in the
space mission, and measuring the current and voltage produced by
the cell and deriving cell performance data.
[0014] As used in this disclosure and claims, the term
"space-qualified" shall mean that the electronic component (i.e.,
in this disclosure, the solar cell) provides satisfactory operation
under the high temperature and thermal cycling test protocols. The
exemplary conditions for vacuum bake-out testing include exposure
to a temperature of +100.degree. C. to +135.degree. C. (e.g., about
+100.degree. C., +110.degree. C., +120.degree. C., +125.degree. C.,
+135.degree. C.) for 2 hours to 24 hours, 48 hours, 72 hours, or 96
hours; and exemplary conditions for TVAC and/or APTC testing that
include cycling between temperature extremes of -180.degree. C.
(e.g., about -180.degree. C., -175.degree. C., -170.degree. C.,
-165.degree. C., -150.degree. C., -140.degree. C., -128.degree. C.,
-110.degree. C., -100.degree. C., -75.degree. C., or -70.degree.
C.) to +145.degree. C. (e.g., about +70.degree. C., +80.degree. C.,
+90.degree. C., +100.degree. C., +110.degree. C., +120.degree. C.,
+130.degree. C., +135.degree. C., or +145.degree. C.) for 600 to
32,000 cycles (e.g., about 600, 700, 1500, 2000, 4000, 5000, 7500,
22000, 25000, or 32000 cycles), and in some space missions up to
+180.degree. C. See, for example, Fatemi et al., "Qualification and
Production of Emcore ZTJ Solar Panels for Space Missions,"
Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th (DOI:
10. 1109/PVSC 2013 6745052). Such rigorous testing and
qualifications are not generally applicable to terrestrial solar
cells and solar cell arrays.
[0015] Conventionally, such measurements are made for the AM0
spectrum for "one-sun" illumination, but for PV systems which use
optical concentration elements, such measurements may be made under
concentrations such as 2.times., 100.times., or 1000.times. or
more.
[0016] The space solar cells and arrays experience a variety of
complex environments in space missions, including the vastly
different illumination levels and temperatures seen during normal
earth orbiting missions, as well as even more challenging
environments for deep space missions, operating at different
distances from the sun, such as at 0.7, 1.0 and 3.0 AU (AU meaning
astronomical units). The photovoltaic arrays also endure anomalous
events from space environmental conditions, and unforeseen
environmental interactions during exploration missions. Hence,
electron and proton radiation exposure, collisions with space
debris, and/or normal aging in the photovoltaic array and other
systems could cause suboptimal operating conditions that degrade
the overall power system performance, and may result in failures of
one or more solar cells or array strings and consequent loss of
power.
[0017] A further distinctive difference between space solar cell
arrays and terrestrial solar cell arrays is that a space solar cell
array utilizes welding and not soldering to provide robust
electrical interconnections between the solar cells, while
terrestrial solar cell arrays typically utilize solder for
electrical interconnections. Welding is required in space solar
cell arrays to provide the very robust electrical connections that
can withstand the wide temperature ranges and temperature cycles
encountered in space such as from -175.degree. C. to +180.degree.
C. In contrast, solder joints are typically sufficient to survive
the rather narrow temperature ranges (e.g., about -40.degree. C. to
about +50.degree. C.) encountered with terrestrial solar cell
arrays.
[0018] A further distinctive difference between space solar cell
arrays and terrestrial solar cell arrays is that a space solar cell
array utilizes silver-plated metal material for interconnection
members, while terrestrial solar cells typically utilize copper
wire for interconnects. In some embodiments, the interconnection
member can be, for example, a metal plate. Useful metals include,
for example, molybdenum; a nickel-cobalt ferrous alloy material
designed to be compatible with the thermal expansion
characteristics of borosilicate glass such as that available under
the trade designation KOVAR from Carpenter Technology Corporation;
a nickel iron alloy material having a uniquely low coefficient of
thermal expansion available under the trade designation Invar,
FeNi36, or 64FeNi; or the like.
[0019] An additional distinctive difference between space solar
cell arrays and terrestrial solar cell arrays is that space solar
cell arrays typically utilize an aluminum honeycomb panel for a
substrate or mounting platform. In some embodiments, the aluminum
honeycomb panel may include a carbon composite face sheet adjoining
the solar cell array. In some embodiments, the face sheet may have
a coefficient of thermal expansion (CTE) that substantially matches
the CTE of the bottom germanium (Ge) layer of the solar cell that
is attached to the face sheet. Substantially matching the CTE of
the face sheet with the CTE of the Ge layer of the solar cell can
enable the array to withstand the wide temperature ranges
encountered in space without the solar cells cracking,
delaminating, or experiencing other defects. Such precautions are
generally unnecessary in terrestrial applications.
[0020] Thus, a further distinctive difference of a space solar cell
from a terrestrial solar cell is that the space solar cell must
include a cover glass over the semiconductor device to provide
radiation resistant shielding from particles in the space
environment which could damage the semiconductor material. The
cover glass is typically a ceria doped borosilicate glass which is
typically from three to six mils in thickness and attached by a
transparent adhesive to the solar cell.
[0021] In summary, it is evident that the differences in design,
materials, and configurations between a space-qualified III-V
compound semiconductor solar cell and subassemblies and arrays of
such solar cells, on the one hand, and silicon solar cells or other
photovoltaic devices used in terrestrial applications, on the other
hand, are so substantial that prior teachings associated with
silicon or other terrestrial photovoltaic system are simply
unsuitable and have no applicability to the design configuration of
space-qualified solar cells and arrays. Indeed, the design and
configuration of components adapted for terrestrial use with its
modest temperature ranges and cycle times often teach away from the
highly demanding design requirements for space-qualified solar
cells and arrays and their associated components.
[0022] The assembly of individual solar cells together with
electrical interconnects and the cover glass form a so-called "CIC"
(Cell-Interconnected-Cover glass) assembly, which are then
typically electrically connected to form an array of
series-connected solar cells. The solar cells used in many arrays
often have a substantial size; for example, in the case of the
single standard substantially "square" solar cell trimmed from a
100 mm wafer with cropped corners, the solar cell can have a side
length of seven cm or more.
[0023] The radiation hardness of a solar cell is defined as how
well the cell performs after exposure to the electron or proton
particle radiation which is a characteristic of the space
environment. A standard metric is the ratio of the end of life
performance (or efficiency) divided by the beginning of life
performance (EOL/BOL) of the solar cell. The EOL performance is the
cell performance parameter after exposure of that test solar cell
to a given fluence of electrons or protons (which may be different
for different space missions or orbits). The BOL performance is the
performance parameter prior to exposure to the particle
radiation.
[0024] Charged particles in space could lead to damage to solar
cell structures, and in some cases, dangerously high voltage being
established across individual devices or conductors in the solar
array. These large voltages can lead to catastrophic electrostatic
discharging (ESD) events. Traditionally for ESD protection the
backside of a solar array may be painted with a conductive coating
layer to ground the array to the space plasma, or one may use a
honeycomb patterned metal panel which mounts the solar cells and
incidentally protects the solar cells from backside radiation.
Furthermore, the front side of the solar array may provide a
conductive coating or adhesive to the periphery of the cover glass
to ground the top surface of the cover glass.
[0025] The radiation hardness of the semiconductor material of the
solar cell itself is primarily dependent on a solar cell's minority
carrier diffusion length (L.sub.min) in the base region of the
solar cell (the term "base" region referring to the p-type base
semiconductor region disposed directly adjacent to an n-type
"emitter" semiconductor region, the boundary of which establishes
the p-n photovoltaic junction). The less degraded the parameter
L.sub.min is after exposure to particle radiation, the less the
solar cell performance will be reduced. A number of strategies have
been used to either improve L.sub.min, or make the solar cell less
sensitive to L.sub.min reductions. Improving L.sub.min has largely
involved including a gradation in dopant elements in the
semiconductor base layer of the subcells so as to create an
electric field to direct minority carriers to the junction of the
subcell, thereby effectively increasing L.sub.min. The effectively
longer L.sub.min will improve the cell performance, even after the
particle radiation exposure. Making the cell less sensitive to
L.sub.min reductions has involved increasing the optical absorption
of the base layer such that thinner layers of the base can be used
to absorb the same amount of incoming optical radiation.
[0026] Another consideration in connection with the manufacture of
space solar cell arrays is that conventionally, solar cells have
been arranged on a support and interconnected using a substantial
amount of manual labor. For example, first individual CICs are
produced with each interconnect individually welded to the solar
cell, and each cover glass individually mounted. Then, these CICs
are connected in series to form strings, generally in a
substantially manual manner, including the welding steps from CIC
to CIC. Then, these strings are applied to a panel substrate and
electrically interconnected in a process that includes the
application of adhesive, wiring, etc. All of this has traditionally
been carried out in a manual and substantially artisanal
manner.
[0027] 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.
[0028] Moreover, the current (or more precisely, the short circuit
current density J.sub.sc) and the voltage (or more precisely, the
open circuit voltage V.sub.oc) are not the only factors that
determine the power output of a solar cell. In addition to the
power being a function of the short circuit density (J.sub.sc), and
the open circuit voltage (V.sub.oc), the output power is actually
computed as the product of V.sub.oc and J.sub.sc, and a Fill Factor
(FF). As might be anticipated, the Fill Factor parameter is not a
constant, but in fact may vary at a value between 0.5 and somewhat
over 0.85 for different arrangements of elemental compositions,
subcell thickness, and the dopant level and profile. Although the
various electrical contributions to the Fill Factor such as series
resistance, shunt resistance, and ideality (a measure of how
closely the semiconductor diode follows the ideal diode equation)
may be theoretically understood, from a practical perspective the
actual Fill Factor of a given subcell cannot always be predicted,
and the effect of making an incremental change in composition or
band gap of a layer may have unanticipated consequences and effects
on the solar subcell semiconductor material, and therefore an
unrecognized or unappreciated effect on the Fill Factor. Stated
another way, an attempt to maximize power by varying a composition
of a subcell layer to increase the V.sub.oc or J.sub.sc or both of
that subcell, may in fact not result in high power, since although
the product V.sub.oc and J.sub.sc may increase, the FF may decrease
and the resulting power also decrease. Thus, the V.sub.oc and
J.sub.sc parameters, either alone or in combination, are not
necessarily "result effective" variables that those skilled in the
art confronted with complex design specifications and practical
operational considerations can easily adjust to optimize
performance.
[0029] Furthermore, the fact that the short circuit current density
(J.sub.sc), the open circuit voltage (V.sub.oc), and the fill
factor (FF), are affected by the slightest change in such design
variables, the purity or quality of the chemical pre-cursors, or
the specific process flow and fabrication equipment used, and such
considerations further complicates the proper specification of
design parameters and predicting the efficiency of a proposed
design which may appear "on paper" to be advantageous.
[0030] It must be further emphasized that in addition to process
and equipment variability, the "fine tuning" of minute changes in
the composition, band gaps, thickness, and doping of every layer in
the arrangement has critical effect on electrical properties such
as the open circuit voltage (V.sub.oc) and ultimately on the power
output and efficiency of the solar cell.
[0031] To illustrate the practical effect, consider a design change
that results in a small change in the V.sub.oc of an active layer
in the amount of 0.01 volts, for example changing the V.sub.oc from
2.72 to 2.73 volts. Assuming all else is equal and does not change,
such a relatively small incremental increase in voltage would
typically result in an increase of solar cell efficiency from
29.73% to 29.84% for a triple junction solar cell, which would be
regarded as a substantial and significant improvement that would
justify implementation of such design change.
[0032] For a single junction GaAs subcell in a triple junction
device, a change in V.sub.oc from 1.00 to 1.01 volts (everything
else being the same) would increase the efficiency of that junction
from 10.29% to 10.39%, about a 1% relative increase. If it were a
single junction stand-alone solar cell, the efficiency would go
from 20.58% to 20.78%, still about a 1% relative improvement in
efficiency.
[0033] Present day commercial production processes are able to
define and establish band gap values of epitaxially deposited
layers as precisely as 0.01 eV, so such "fine tuning" of
compositions and consequential open circuit voltage results are
well within the range of operational production specifications for
commercial products.
[0034] 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 or substantially similar crystal lattice constants or
parameters.
[0035] Here again there are trade-offs between including specific
elements in the composition of a layer which may result in improved
voltage associated with such subcell and therefore potentially a
greater power output, and deviation from exact crystal lattice
matching with adjoining layers as a consequence of including such
elements in the layer which may result in a higher probability of
defects, and therefore lower manufacturing yield.
[0036] In that connection, it should be noted that there is no
strict definition of what is understood to mean two adjacent layers
are "lattice matched" or "lattice mismatched". For purposes in this
disclosure, "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 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 as defining "lattice mismatched" layers).
[0037] The conventional wisdom for many years has been that in a
monolithic 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").
[0038] 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).
[0039] As discussed in Applicant's U.S. Pat. No. 7,339,109, the
current state-of-the-art triple junction solar cell is a device
that uses layers of indium gallium phosphide (InGaP), gallium
arsenide (GaAs), and germanium (Ge). The contribution of a
germanium (Ge) junction improves the energy conversion efficiency
of a solar cell by adding open-circuit voltage to the structure.
More recently, there is interest in the design of alternative
semiconductor structures with higher band gaps than germanium and
greater open-circuit voltages than that of germanium for use in the
bottom subcell.
[0040] In addition to the use of a different growth substrate, the
present disclosure further proposes design features for metamorphic
multijunction solar cells which departs from such conventional
wisdom for increasing the efficiency of the multijunction solar
cell in converting solar energy (or photons) to electrical energy
and optimizing such efficiency at the "end-of-life" period.
SUMMARY OF THE DISCLOSURE
Objects of the Disclosure
[0041] It is an object of the present disclosure to provide in a
multijunction solar cell grown on a bulk GeSi substrate.
[0042] It is another object of the present invention to provide an
upright metamorphic four junction solar cell in which the average
band gap of all four subcells is greater than 1.35 eV grown on a
GeSi substrate.
[0043] It is another object of the present invention to increase
the band gap of the bottom subcell in a multijunction solar cell by
using a GeSi substrate in lieu of germanium.
[0044] 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
[0045] All ranges of numerical parameters set forth in this
disclosure are to be understood to encompass any and all subranges
or "intermediate generalizations" subsumed therein. 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 1.2, or 1.3 to 1.4, or 1.5 to 1.9
eV.
[0046] Briefly, and in general terms, the present disclosure
provides a four-junction space-qualified solar cell assembly
designed for operation at AM0 and at a 1 MeV electron equivalent
fluence of at least 1.times.10.sup.14 e/cm.sup.2, the solar cell
comprising subcells, wherein a combination of compositions and band
gaps of the subcells is designed to maximize efficiency of the
solar cell at a predetermined time, after initial deployment, when
the solar cell is deployed in space at AM0 and at an operational
temperature in the range of 40 to 70 degrees Centigrade, the
predetermined time being at least five years and referred to as the
end-of-life (EOL), the solar cell comprising: providing an upper
first solar subcell composed of indium gallium aluminum phosphide
and having a first band gap in the range of 2.0 to 2.2 eV;
providing a second solar subcell adjacent to said upper first solar
subcell and including an emitter layer composed of indium gallium
phosphide or aluminum indium gallium arsenide, and a base layer
composed of aluminum indium gallium arsenide and having a second
band gap in the range of approximately 1.55 to 1.8 eV and being
lattice matched with the upper first solar subcell, wherein the
emitter and base layers of the second solar subcell form a
photoelectric junction; providing a third solar subcell adjacent to
said second solar subcell and composed of indium gallium arsenide
and having a third band gap less than that of the second solar
subcell and being lattice matched with the second solar subcell;
and providing a fourth solar subcell adjacent to said third solar
subcell and composed of germanium silicon and has an indirect band
gap in the range of 0.7 to 1.1 eV, or 0.85 to 1.05 eV; wherein each
of the upper first solar subcell, the second solar subcell and the
third solar subcell is lattice-mismatched to the fourth solar
subcell, and a numerical sum of the band gaps of the four solar
subcells, divided by four) is equal to 1.35 eV.
[0047] In another aspect, the present disclosure provides a method
for forming a four-junction space-qualified solar cell assembly
designed for operation at AM0 and at a 1 MeV electron equivalent
fluence of at least 1.times.10.sup.14 e/cm.sup.2, the solar cell
comprising subcells, wherein a combination of compositions and band
gaps of the subcells is designed to maximize efficiency of the
solar cell at a predetermined time, after initial deployment, when
the solar cell is deployed in space at AM0 and at an operational
temperature in the range of 40 to 70 degrees Centigrade, the
predetermined time being at least five years and referred to as the
end-of-life (EOL), the solar cell comprising: providing an upper
first solar subcell composed of indium gallium aluminum phosphide
and having a first band gap in the range of 2.0 to 2.2 eV;
providing a second solar subcell adjacent to said upper first solar
subcell and including an emitter layer composed of indium gallium
phosphide or aluminum indium gallium arsenide, and a base layer
composed of aluminum indium gallium arsenide and having a second
band gap in the range of approximately 1.55 to 1.8 eV and being
lattice matched with the upper first solar subcell, wherein the
emitter and base layers of the second solar subcell form a
photoelectric junction; providing a third solar subcell adjacent to
said second solar subcell and composed of indium gallium arsenide
and having a third band gap less than that of the second solar
subcell and being lattice matched with the second solar subcell;
and providing a fourth solar subcell adjacent to said third solar
subcell and composed of germanium silicon and has an indirect band
gap in the range of 0.7 to 1.1 eV, or 0.85 to 1.05 eV; wherein each
of the upper first solar subcell, the second solar subcell and the
third solar subcell is lattice-mismatched to the fourth solar
subcell, and wherein a numerical sum of the band gaps of the four
solar subcells, divided by four, is equal to 1.35 eV.
[0048] In another aspect, the present disclosure provides a method
of manufacturing a multijunction solar cell comprising providing a
growth substrate, forming a first solar subcell in the growth
substrate having a band gap in the range of 0.83 to 0.88 eV,
growing a sequence of layers of semiconductor material using a
disposition process to form a solar cell comprising a plurality of
subcells including a first middle subcell disposed over the growth
substrate and having a band gap in the range of 0.9 to 1.6 eV, at
least a second middle subcell disposed over the first middle
subcell and having a band gap in the range of approximately 1.55 to
1.8 eV and an upper subcell disposed over the last middle subcell
and a band gap in the range of 2.0 to 2.20 eV; wherein the growth
substrate is composed of SiGe with the Ge content in the SiGe in
the range of 85% to 97%.
[0049] In another aspect, the present disclosure provides a method
of manufacturing a multijunction solar cell comprising: providing a
growth substrate, forming a first solar subcell in the growth
substrate, growing a sequence of layers of semiconductor material
using a disposition process to form a solar cell comprising a
plurality of subcells including a first middle subcell disposed
over and lattice matched with respect to the growth substrate and
having a band gap in the range of 0.9 to 1.6 eV, at least a second
middle subcell disposed over the first middle subcell; and an upper
subcell disposed over the last middle subcell and a band gap in the
range of 2.0 to 2.20 eV; wherein the growth substrate is composed
of SiGe with the Ge content in the SiGe in the range of 85% to
97%.
[0050] In another aspect, the present disclosure provides a method
of manufacturing a multijunction solar cell comprising: providing a
growth substrate, forming a first solar subcell in the growth
substrate, and growing a sequence of layers of semiconductor
material using a disposition process to form a solar cell
comprising a plurality of subcells including a first middle subcell
disposed over and lattice mis-matched with respect to the growth
substrate and having a band gap in the range of 0.9 to 1.6 eV, at
least a second middle subcell disposed over the first middle
subcell and having a band gap in the range of approximately 1.55 to
1.8 eV and an upper subcell disposed over the last middle subcell
and a band gap in the range of 2.0 to 2.20 eV; wherein the growth
substrate is composed of SiGe with the Ge content in the SiGe
substrate in the range of 85% to 97%.
[0051] In some embodiments, the bulk germanium silicon substrate is
grown by the Czochralski method, and the Ge content in the SiGe
substrate in the range of 85% to 87%.
[0052] In another aspect, the present disclosure provides a method
of manufacturing a multijunction solar cell comprising: growing a
growth substrate by the Czochralski method; forming a first solar
subcell in the growth substrate; and growing a sequence of layers
of semiconductor material using a deposition process to form a
solar cell comprising a plurality of subcells over the growth
substrate; wherein the growth substrate is composed of SiGe with
the Ge content by mole fraction in the SiGe substrate being in the
range of 85% to 97%.
[0053] In some embodiments, the germanium silicon growth substrate
has an indirect band gap in the range of 0.7 to 1.1 eV, or 0.85 to
1.05 eV.
[0054] In some embodiments, the germanium silicon substrate has a
thickness in the range of 50 to 600 .mu.m, or 100 to 200 .mu.m.
[0055] In some embodiments, there further comprises a buffer layer
and/or nucleation layer disposed directly over the growth substrate
and composed of a material that has a similar lattice parameter as
the growth substrate.
[0056] In some embodiments, the nucleation layer comprises
InGaP.
[0057] In some embodiments, the buffer layer comprises GaAs.
[0058] In some embodiments, a graded interlayer is provided above
the growth substrate and the buffer layer which is compositionally
graded to lattice match the growth substrate on one side and the
directly adjacent middle solar subcell on the other side, and is
composed of any of the A, S, P, N, Sb based III-V compound
semiconductors subject to the constraints of having its in-plane
lattice parameter throughout its thickness being greater than or
equal to that of the growth substrate.
[0059] In some embodiments, the upper first solar subcell has a
band gap of approximately 2.05 eV, the second solar subcell has a
band gap of approximately 1.55 eV; and the third solar subcell has
a band gap in the range of 0.9 to 1.55 eV.
[0060] In some embodiments, the third solar subcell has a band gap
of 1.41 eV or less.
[0061] In some embodiments, the third solar subcell has a band gap
in the range of 1.15 to 1.35 eV.
[0062] In some embodiments, the third solar subcell has a band gap
in the range of 1.1 to 1.2 eV.
[0063] In some embodiments, the third solar subcell has a band gap
of approximately 1.2 eV.
[0064] In some embodiments, the fourth solar subcell has a band gap
in the range of 0.83 to 0.88 eV as measured at 300 degrees Kelvin,
corresponding to a percentage of Si in the GeSi substrate ranging
from 13% to 15%.
[0065] In some embodiments, the third solar subcell is lattice
mis-matched with respect to the fourth solar subcell.
[0066] In some embodiments, the third solar subcell is lattice
matched with respect to the fourth solar subcell.
[0067] 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 indium gallium arsenide, and a base layer composed of
aluminum indium gallium arsenide; the third solar subcell is
composed of indium gallium arsenide; and the fourth subcell is
composed of germanium silicon.
[0068] In some embodiments, there further comprises an intermediate
layer above the bottom subcell or growth substrate wherein the
intermediate layer is compositionally graded to lattice match the
third solar subcell on one side and the 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 greater than or equal to that of the
third solar subcell and less than or equal to that of the lower
fourth solar subcell, and having a band gap energy greater than
that of the fourth solar subcell.
[0069] In some embodiments, the intermediate layer is
compositionally step-graded with between one and four steps to
lattice match the fourth solar subcell on one side and composed of
In.sub.xGa.sub.1-xAs or (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs with
0<x<1, 0<y<1, and x and y selected such that the band
gap remains in the range of 1.15 to 1.41 eV throughout its
thickness.
[0070] In some embodiments, the intermediate layer has a constant
band gap in the range of 1.15 to 1.41 eV, or 1.2 to 1.35 eV, or
1.25 to 1.30 eV.
[0071] 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.
[0072] 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.
[0073] 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.
[0074] In some embodiments, the DBR layer includes a first DBR
layer composed of a plurality of p type
In.sub.zAl.sub.xGa.sub.1-x-zAs layers, and a second DBR layer
disposed over the first DBR layer and composed of a plurality of p
type In.sub.wAl.sub.yGa.sub.1-y-xAs layers, where 0<x<1,
0<y<1, 0<z<1 and y is greater than x.
[0075] In some embodiments, the selection of the composition of the
subcells and their band gaps maximizes the efficiency at high
temperature (in the range of 40 to 70 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) and
being at least five years, and the average band gap (i.e., the
numerical average of the lowest band gap material in each subcell)
of all four subcells is greater than 1.35 eV.
[0076] In some embodiments, at least one of the upper sublayers of
the graded interlayer has a larger lattice constant than the
adjacent layers to the upper sublayer disposed above the grading
interlayer.
[0077] In some embodiments, the difference in lattice constant
between the adjacent third and fourth subcells is in the range of
0.1 to 0.2 Angstroms.
[0078] In some embodiments, there further comprises at least a
first threading dislocation inhibition layer having a thickness in
the range of 0.10 to 1.0 microns and disposed over said second
solar subcell.
[0079] In some embodiments, there further comprises at least a
second threading dislocation inhibition layer having a thickness in
the range of 0.10 to 1.0 micron and composed of InGa(Al)P, the
second threading dislocation inhibition layer being disposed over
and directly adjacent to said grading interlayer for reducing the
propagation of threading dislocations, said second threading
dislocation inhibition layer having a composition different from a
composition of the first threading dislocation inhibition
layer.
[0080] In some embodiments, additional layer(s) may be added or
deleted in the cell structure without departing from the scope of
the present disclosure.
[0081] Some implementations of the present disclosure may
incorporate or implement fewer of the aspects and features noted in
the foregoing summaries.
[0082] 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 disclosure.
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.
[0083] 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
[0084] 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:
[0085] FIG. 1 is a graph representing the band gap of GeSi and
their lattice constants;
[0086] FIG. 2A is a cross-sectional view of a bulk GeSi
substrate;
[0087] FIG. 2B is a cross-sectional view of an embodiment of a GeSi
solar cell after several stages of fabrication including the
deposition of certain semiconductor layers on the growth substrate
of FIG. 2A, according to the present disclosure;
[0088] FIG. 2C is a cross-sectional view of an embodiment of a GeSi
solar cell of FIG. 2B after the diffusion of dopant elements into
the growth substrate, according to the present disclosure; and
[0089] FIG. 3 is a highly simplified cross-sectional view of a
portion of an embodiment of a multijunction solar cell grown on a
GeSi substrate according to the present disclosure.
GLOSSARY OF TERMS
[0090] "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).
[0091] "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. More
particularly, the expression "band gap" of a solar subcell, which
internally has layers of different band gaps shall be defined to
mean the band gap of the layer of the solar subcell in which the
majority of the charge carriers are generated (such sublayer
typically being the p-type base semiconductor layer of the
base/emitter photovoltaic junction of such subcell). In the event
such layer in turn has sublayers with different band gaps (such as
the case of a base layer having a graded composition and more
particularly a graded band gap), the sublayer of that solar subcell
with the lowest band gap shall be taken as defining the "band gap"
of such a subcell. Apart from a solar subcell, and more generally
in the case of a specifically designated semiconductor region (such
as a metamorphic layer), in which that semiconductor region has
sublayers or subregions with different band gaps (such as the case
of a semiconductor region having a graded composition and more
particularly a graded band gap), the sublayer or subregion of that
semiconductor region with the lowest band gap shall be taken as
defining the "band gap" of that semiconductor region.
[0092] "Beginning of Life (BOL)" refers to the time at which a
photovoltaic power system is initially deployed in operation.
[0093] "Bottom subcell" refers to the subcell in a multijunction
solar cell which is furthest from the primary light source for the
solar cell.
[0094] "Compound semiconductor" refers to a semiconductor formed
using two or more chemical elements.
[0095] "Current density" refers to the short circuit current
density Jsc through a solar subcell through a given planar area, or
volume, of semiconductor material constituting the solar
subcell.
[0096] "Deposited", with respect to a layer of semiconductor
material, refers to a layer of material which is epitaxially grown
over another semiconductor layer.
[0097] "Dopant" refers to a trace impurity element that is
contained within a semiconductor material to affect the electrical
or optical characteristics of that material. As used in the context
of the present disclosure, typical dopant levels in semiconductor
materials are in the 1016 to 1019 atoms per cubic centimeter range.
The standard notation or nomenclature, when a particular identified
dopant is proscribed, is to use, for example, the expression
"GaAs:Se" or "GaAs:C" for selenium or carbon doped gallium arsenide
respectively. Whenever a ternary or quaternary compound
semiconductor is expressed as "AlGaAs" or "GaInAsP", it is
understood that all three or four of the constituent elements are
much higher in mole concentration, say on the 1% level or above,
which is in the 1021 atoms/cm-3 or larger range. Such constituent
elements are not considered "dopants" by those skilled in the art
since the atoms of the constituent element form part of the crystal
structure of the compound semiconductor. In addition, a further
distinction is that a dopant has a different valence number than
the constituent component elements. In a commonly implemented III-V
compound semiconductor such as AlGaInAs, none of the individual
elements Al, Ga, In, or As are considered to be dopants since they
have the same valence as the component atoms that make up the
crystal lattice.
[0098] "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.
[0099] "Graded interlayer" (or "grading interlayer")--see
"metamorphic layer".
[0100] "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.
[0101] "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.
[0102] "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).
[0103] "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.
[0104] "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).
[0105] "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.
[0106] "Short circuit current density"--see "current density".
[0107] "Solar cell" refers to an electronic device operable to
convert the energy of light directly into electricity by the
photovoltaic effect.
[0108] "Solar cell assembly" refers to two or more solar cell
subassemblies interconnected electrically with one another.
[0109] "Solar cell subassembly" refers to a stacked sequence of
layers including one or more solar subcells.
[0110] "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.
[0111] "Space qualified" refers to an electronic component (e.g.,
as used in this disclosure, to a solar cell) provides satisfactory
operation under the high temperature and thermal cycling test
protocols that establish typical "qualification" requirements for
use by customers who utilize such components in the outer space
environment. The exemplary conditions for such qualifications
include (i) vacuum bake-out testing that includes exposure to a
temperature of +100.degree. C. to +135.degree. C. (e.g., about
+100.degree. C., +110.degree. C., +120.degree. C., +125.degree. C.,
+135.degree. C.) for 2 hours to 24 hours, 48 hours, 72 hours, or 96
hours; and (ii) TVAC and/or APTC test that includes cycling between
temperature extremes of -180.degree. C. (e.g., about -180.degree.
C., -175.degree. C., -170.degree. C., -165.degree. C., -150.degree.
C., -140.degree. C., -128.degree. C., -110.degree. C., -100.degree.
C., -75.degree. C., or -70.degree. C.) to +145.degree. C. (e.g.,
about +70.degree. C., +80.degree. C., +90.degree. C., +100.degree.
C., +110.degree. C., +120.degree. C., +130.degree. C., +135.degree.
C., or +145.degree. C.) for 600 to 32,000 cycles (e.g., about 600,
700, 1500, 2000, 4000, 5000, 7500, 22000, 25000, or 32000 cycles),
and in some space missions up to +180.degree. C. See, for example,
Fatemi et al., "Qualification and Production of Emcore ZTJ Solar
Panels for Space Missions," Photovoltaic Specialists Conference
(PVSC), 2013 IEEE 39th (DOI: 10.1109/PVSC 2013 6745052).
[0112] "Substantially current matched" refers to the short circuit
current through adjacent solar subcells being substantially
identical (i.e. within plus or minus 1%).
[0113] "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.
[0114] "Upright multijunction solar cell" refers to a solar cell in
which the subcells are deposited or grown on a substrate in a
sequence such that the lower band gap subcells are deposited or
grown on a growth substrate prior to depositing or growing the
higher band gap subcells.
[0115] "ZTJ" refers to the product designation of a commercially
available SolAero Technologies Corp. triple junction solar
cell.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0116] 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.
[0117] 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 "upright" solar cells of the present
disclosure. However, more particularly, some embodiments of the
present disclosure are directed to the fabrication of a
multijunction solar cell grown on a germanium silicon growth
substrate.
[0118] The novel proposal for the use of germanium silicon instead
of germanium as the growth substrate and bottom solar subcell is
the cornerstone of the present disclosure. Following on Applicant's
earlier advances as represented by U.S. Pat. No. 7,339,109 and
subsequent proposals and developments, the formation of a
photoelectric junction in such a germanium silicon substrate is an
improvement that extends the spectral band that can be captured by
the bottom subcell in a multijunction solar cell to be an indirect
band gap in the range of 0.7 to 1.1 eV. In addition to the
specification of a germanium silicon bottom subcell, the present
disclosure also provides for an embodiment of a multijunction solar
cell in which the two lower subcells (e.g., the third and fourth
subcells in a four junction solar cell) are lattice mismatched.
More specifically, in some embodiments, the present disclosure
relates to four junction solar cells 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 or less, for the middle subcells,
and (iii) 0.7 to 1.1 eV indirect bandgaps for the bottom subcell,
respectively.
[0119] The present disclosure, similar to the related applications
of Applicant, provides an unconventional four junction design (with
three grown lattice matched subcells, which are lattice mismatched
to the GeSi substrate) that leads to a surprising 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, thus specifically
addressing the problem of ensuring continues adequate efficiency
and power output at the "end-of-life".
[0120] In some embodiments, the fourth subcell is germanium
silicon, 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.
[0121] 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 0.67 eV, since it is lower than
the direct band gap value of 0.8 eV.
[0122] In the present disclosure, the indirect band gap of the
germanium silicon growth substrate would broadly be in the range of
0.7 to 1.1 eV, or for certain applications in the range of 0.85 to
1.05 eV.
[0123] More specifically, the present disclosure intends to provide
a relatively simple and reproducible technique for "upright"
processing of metamorphic multijunction solar cells, that 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 the physical damage to the quality of the
deposited layers, thereby simplifying wafer handling and ensuring a
relatively high yield of operable solar cells meeting
specifications at the conclusion of the fabrication processes.
[0124] As suggested above, incremental improvements in the design
of multijunction solar cells are made in view of a variety of new
space missions and application requirements. Moreover, although
such improvements may be relatively minute quantitative
modifications in the composition, lattice constant, or band gap of
certain subcells or adjoining layers, as we noted above, such
minute parametric changes can provide substantial improvements in
efficiency that specifically address the "problems" that have been
identified associated with the existing current commercial
multijunction solar cells, and provide a "solution" that represents
an "inventive step" in the design process.
[0125] 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 in 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.
[0126] 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).
[0127] 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 the ultimate solar cell design proposed by the
Applicants.
[0128] 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 "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. 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.
[0129] 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.
[0130] Furthermore, as in the case here, where multiple design
variables interact in unpredictable ways, the proper choice of the
combination of variables can produce new and unexpected results,
and constitute an "inventive step".
[0131] 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.
[0132] 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.
[0133] Some comments about MOCVD processes used in one embodiment
are in order here.
[0134] 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.
[0135] FIG. 1 is a graph representing the band gap of GeSi and
their lattice constants in the range of interest of the present
disclosure, i.e, a Ge concentration by mole fraction of over 85%,
and in some embodiments in the range of 85% to 87%.
[0136] FIG. 2A is a cross-sectional view of a GeSi growth substrate
100 which as bulk semiconductor material may be between 50 and 600
microns in thickness, or in some embodiments between 100 and 200
microns.
[0137] FIG. 2B depicts the epitaxial growth of a nucleation layer
102 and a buffer layer 103 on top of the GeSi growth substrate 100.
The buffer layer 103 may be composed of GaAs.
[0138] FIG. 2C depicts the result of diffusion of As and P into the
growth substrate 100, and the formation of a photovoltaic junction
depicted by the dashed line in the interior of the substrate 100.
As a result of the diffusion, the upper portion 101 of the
substrate 100 is converted into an n+ type semiconductor which
forms the emitter of the solar subcell formed in the growth
substrate 100, which in the embodiments discussed below will be the
"bottom" subcell D of a multijunction solar cell.
[0139] Turning to a multijunction solar cell device of the present
disclosure, FIG. 3 is a cross-sectional view of an embodiment of a
four junction solar cell 450 after several stages of fabrication
including the growth of certain semiconductor layers on the growth
substrate up to the contact layer 322 according to the present
disclosure.
[0140] As shown in the illustrated example of FIG. 4, the bottom or
fourth subcell D includes a growth substrate 300 formed of p-type
germanium silicon ("GeSi") 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 200.
The bottom subcell D, further includes, for example, an n+ type
nucleation and buffer layer 302. The nucleation layer is deposited
over the growth substrate, and the emitter layer is formed in the
substrate by diffusion of dopants into the GeSi substrate, thereby
forming the n+ type GeSi layer 301. Heavily doped p-type aluminum
indium gallium arsenide ("AlGaAs") and heavily doped n-type gallium
arsenide ("GaAs") tunneling junction layers 304, 303 may be
deposited over the buffer layer to provide a low resistance pathway
between the bottom and middle subcells.
[0141] In the embodiment depicted, an intermediate graded
interlayer 506, comprising in one embodiment step-graded sublayers
505a through 505zz, is disposed over the tunnel diode layer
303/304. In particular, the graded interlayer provides a transition
in the in-plane lattice constant from the lattice constant of the
substrate subcell D to the larger lattice constant of the middle
and upper subcells C, B and A.
[0142] The graph on the left side of FIG. 3 depicts the in-plane
lattice constant being incrementally monotonically increased from
sublayer 505a through sublayer 505zz, such sublayers being fully
relaxed.
[0143] At least a first "alpha" or threading dislocation inhibition
layer 504, preferably composed of p-type InGaP, is deposited over
the tunnel diode 303/304, to a thickness of from 0.10 to about 1.0
micron. Such an alpha layer is intended to prevent threading
dislocations from propagating, either opposite to the direction of
growth into the bottom subcell D, or in the direction of growth
into the subcell C, and is more particularly described in U.S.
Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.). More
generally, the alpha layer has a different composition than the
adjacent layers above and below it.
[0144] The metamorphic layer (or graded interlayer) 506 is
deposited over the alpha layer 504 using a surfactant. Layer 505 is
preferably a compositionally step-graded series of p-type InGaAs or
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 to subcell C while
minimizing threading dislocations from occurring. In one
embodiment, the band gap of layer 506 is constant throughout its
thickness, preferably approximately equal to 1.22 to 1.34 eV, or
otherwise consistent with a value slightly greater than the band
gap of the middle subcell C. In another embodiment, the band gap of
the sublayers of layer 506 vary in the range of 1.22 to 1.34 eV,
with the first layer having a relatively high band gap, and
subsequent layers incrementally lower band gaps. One embodiment of
the graded interlayer may also be expressed as being composed of
InxGal-xAs, with 0<x<1, 0<y<1, and x and y selected
such that the band gap of the interlayer remains constant at
approximately 1.22 to 1.34 eV or other appropriate band gap.
[0145] In one embodiment, aluminum is added to one sublayer to make
one particular sublayer harder than another, thereby forcing
dislocations in the softer material.
[0146] In the surfactant assisted growth of the metamorphic layer
506, a suitable chemical element is introduced into the reactor
during the growth of layer 506 to improve the surface
characteristics of the layer. In the preferred 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 506, 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.
[0147] 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.
[0148] 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 506.
[0149] In one embodiment of the present disclosure, the layer 506
is composed of a plurality of layers of InGaAs, with monotonically
changing lattice constant, each layer having a band gap in the
range of 1.22 to 1.34 eV. In some embodiments, the band gap is
constant in the range of 1.27 to 1.31 eV through the thickness of
layer 505. In some embodiments, the constant band gap is in the
range of 1.28 to 1.29 eV.
[0150] The advantage of utilizing a constant bandgap material such
as InGaAs is that arsenide-based semiconductor material is much
easier to process in standard commercial MOCVD reactors.
[0151] Although the described embodiment of the present disclosure
utilizes a plurality of layers of InGaAs for the metamorphic layer
506 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 to subcell D. Other embodiments of the present disclosure
may utilize continuously graded, as opposed to step graded,
materials. More generally, the graded interlayer may be composed of
any of the As, P, N, Sb based III-V compound semiconductors subject
to the constraints of having the in-plane lattice parameter less
than or equal to that of the third solar subcell C and greater than
or equal to that of the fourth solar subcell D. In some
embodiments, the layer 505 has a band gap energy greater than that
of the third solar subcell C, and in other embodiments has a band
gap energy level less than that of the third solar subcell C.
[0152] In some embodiments, a second "alpha" or threading
dislocation inhibition layer 507, preferably composed of p type
GaInP, is deposited over metamorphic buffer layer 506, to a
thickness of from 0.10 to about 1.0 micron. Such an alpha layer is
intended to prevent threading dislocations from propagating, either
opposite to the direction of growth into the fourth subcell D, or
in the direction of growth into the third subcell C, and is more
particularly described in U.S. Patent Application Pub. No.
2009/0078309 A1 (Cornfeld et al.).
[0153] In the specific embodiment depicted in FIG. 3, the top or
uppermost sublayer 505zz of the graded interlayer 506 is strained
or only partially relaxed, and has a lattice constant which is
greater than that of the layer above it, i.e., the alpha layer 507
(should there be a second alpha layer) or the BSF layer 306. In
short, in this embodiment, there is an "overshoot" of the last one
sublayer 505zz of the grading sublayers, as depicted on the left
hand side of FIG. 4B, which shows the step-grading of the lattice
constant becoming larger from layer 505a to 505zz, and then
decreasing back to the lattice constant of the upper layers 507
through 322.
[0154] In the illustrated example of FIG. 3, the third subcell C
includes a highly doped p-type aluminum indium gallium arsenide
("AlInGaAs") back surface field ("BSF") layer 306, a p-type InGaAs
base layer 307, a highly doped n-type indium gallium arsenide
("InGaAs") emitter layer 308 and a highly doped n-type indium
aluminum phosphide ("AlInP.sub.2") or indium gallium phosphide
("GaInP") window layer 309. The InGaAs base layer 307 of the
subcell C can include, for example, approximately 1.5% In. Other
compositions may be used as well. The base layer 307 is formed over
the BSF layer 306 after the BSF layer is deposited over the DBR
layers 305.
[0155] 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 B, 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.
[0156] The second subcell B includes a highly doped p-type aluminum
indium gallium arsenide ("AlInGaAs") back surface field ("BSF")
layer 312, a p-type AlInGaAs base layer 313, a highly doped n-type
indium gallium phosphide ("InGaP.sub.2") or AlInGaAs layer 314 and
a highly doped n-type indium gallium aluminum phosphide ("AlGaAlP")
window layer 315. The InGaP emitter layer 314 of the subcell B can
include, for example, approximately 50% In. Other compositions may
be used as well.
[0157] Before depositing the layers of the top or upper first cell
A, heavily doped n-type InGaP and p-type AlGaAs tunneling junction
layers 316, 317 may be deposited over the subcell B.
[0158] 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. The base layer 319 of the top subcell A is
deposited over the BSF layer 318 after the BSF layer 318 is
formed.
[0159] After the cap or contact layer 322 is deposited, the grid
lines are formed via evaporation and lithographically patterned and
deposited over the cap or contact layer 322.
[0160] 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.
[0161] Although described embodiments of the present disclosure
utilizes a vertical stack of four 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.
[0162] 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.
[0163] 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 C, with p-type and n-type InGaAs is one example
of a homojunction subcell.
[0164] 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.
[0165] 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, AlN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe,
CdSSe, and similar materials, and still fall within the spirit of
the present invention.
[0166] While the solar cell described in the present disclosure has
been illustrated and described as embodied in a conventional
upright multijunction solar cell, it is not intended to be limited
to the details shown, since various modifications and structural
changes may be made without departing in any way from the spirit of
the present invention.
[0167] 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.
[0168] 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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