U.S. patent application number 15/433641 was filed with the patent office on 2018-04-12 for inverted metamorphic multijunction solar cell with multiple metamorphic layers.
The applicant listed for this patent is SolAero Technologies Corp.. Invention is credited to Benjamin Cho, Arthur Cornfeld, Yong Lin, Pravin Patel, Paul R. Sharps, Mark A. Stan.
Application Number | 20180102454 15/433641 |
Document ID | / |
Family ID | 58799851 |
Filed Date | 2018-04-12 |
United States Patent
Application |
20180102454 |
Kind Code |
A9 |
Lin; Yong ; et al. |
April 12, 2018 |
INVERTED METAMORPHIC MULTIJUNCTION SOLAR CELL WITH MULTIPLE
METAMORPHIC LAYERS
Abstract
The disclosure describes multi-junction solar cell structures
that include two or more graded interlayers.
Inventors: |
Lin; Yong; (Albuquerque,
NM) ; Sharps; Paul R.; (Albuquerque, NM) ;
Cornfeld; Arthur; (Sandy Springs, GA) ; Patel;
Pravin; (Albuquerque, NM) ; Stan; Mark A.;
(Albuquerque, NM) ; Cho; Benjamin; (Albuquerque,
NM) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SolAero Technologies Corp. |
Albuquerque |
NM |
US |
|
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20170162739 A1 |
June 8, 2017 |
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|
Family ID: |
58799851 |
Appl. No.: |
15/433641 |
Filed: |
February 15, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14623883 |
Feb 17, 2015 |
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15433641 |
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13463069 |
May 3, 2012 |
8969712 |
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14623883 |
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12401157 |
Mar 10, 2009 |
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13463069 |
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14485121 |
Sep 12, 2014 |
9634172 |
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12401157 |
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13401181 |
Feb 21, 2012 |
9117966 |
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14485121 |
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12271192 |
Nov 14, 2008 |
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13401181 |
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12023772 |
Jan 31, 2008 |
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12271192 |
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11860142 |
Sep 24, 2007 |
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12023772 |
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11860183 |
Sep 24, 2007 |
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11860142 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/03046 20130101;
Y02P 70/521 20151101; H01L 31/1844 20130101; H01L 31/06875
20130101; H01L 31/0725 20130101; H01L 31/0687 20130101; H01L 31/078
20130101; H01L 31/1892 20130101; Y02E 10/544 20130101; H01L 31/0735
20130101; Y02P 70/50 20151101 |
International
Class: |
H01L 31/078 20060101
H01L031/078; H01L 31/0735 20060101 H01L031/0735; H01L 31/0687
20060101 H01L031/0687; H01L 31/0725 20060101 H01L031/0725; H01L
31/18 20060101 H01L031/18 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0032] Parts of this disclosure were made with government support
under Contract No. FA9453-06-C-0345 awarded by the U.S. Air Force.
The government may have certain rights in the invention.
Claims
1. A multijunction solar cell comprising: an upper portion
including at least one solar subcell; a first graded interlayer
composed of (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs with a step
graded changing lattice constant, wherein 0<x<1 and
0<y<1, and wherein the first graded interlayer has a band gap
in a range of 1.5 eV-1.65 eV that is constant throughout its
thickness; a middle portion including at least one solar subcell,
the first graded interlayer providing a transition in lattice
constant between a particular solar subcell in the upper portion
and a particular solar subcell in the middle portion that is
lattice mismatched to the particular solar subcell in the upper
portion; a second graded interlayer composed of
(In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs with step graded changing
lattice constant, wherein 0<x<1 and 0<y<1, and wherein
the second graded interlayer has a band gap in a range of 1.1 eV+3%
that is constant throughout its thickness; and a lower portion
including at least one solar subcell, the second graded interlayer
providing a transition in lattice constant between a specific solar
subcell in the middle portion and a specific solar subcell in the
lower portion that is lattice mismatched to the specific solar
subcell in the middle portion.
2. The multijunction solar cell of claim 1 wherein the upper
portion of the multijunction solar cell includes: a first upper
solar subcell; and a second solar subcell disposed between the
first upper solar subcell and the first graded interlayer.
3. The multijunction solar cell of claim 2 wherein the first upper
solar subcell includes a GaInP emitter layer and an (Al)GaInP base
layer.
4. The multijunction solar cell of claim 3 wherein an amount of
aluminum in the base layer of the first upper solar subcell is in a
range of 0% to 30%.
5. The multijunction solar cell of claim 1 including a respective
barrier layer adjacent at least one of the first graded interlayer
or the second graded interlayer, wherein the respective barrier
layer prevents propagation of threading dislocations into adjacent
ones of the solar subcells, wherein each barrier layer has a
composition that differs from a composition of an adjacent graded
interlayer or an adjacent solar subcell layer.
6. The multijunction solar cell of claim 1 including a first
barrier layer adjacent one side of the first graded interlayer and
a second barrier layer adjacent a second side of the first graded
interlayer, wherein the barrier layers prevent propagation of
threading dislocations into adjacent ones of the solar subcells,
the second barrier layer being composed of a material different
from the first barrier layer, wherein each barrier layer has a
composition that differs from a composition of an adjacent graded
interlayer or an adjacent solar subcell layer.
7. The multijunction solar cell of claim 1 wherein the middle
portion of the multijunction solar cell includes a homojunction
solar subcell including a GaInAs emitter layer and a GaInAs base
layer.
8. The multijunction solar cell of claim 1 wherein the middle
portion of the multijunction solar cell includes a heterojunction
solar subcell including a GaInP emitter layer and a GaInAs base
layer.
9. The multijunction solar cell of claim 1 wherein the lower
portion of the multijunction solar cell includes a solar subcell
including a GaInAs emitter layer and a GaInAs base layer.
10. The multijunction solar cell of claim 1 wherein the upper
portion of the multijunction solar cell includes: an upper solar
subcell; a middle solar subcell disposed below the upper solar
subcell; and a lower solar subcell disposed below the middle solar
subcell, wherein the lower solar subcell is closer to the first
graded interlayer than the upper solar subcell.
11. The multijunction solar cell of claim 10 wherein the upper
solar subcell of the upper portion includes an AlGaInP emitter and
an AlGaInP base.
12. The multijunction solar cell of claim 11 wherein the middle
solar subcell of the upper portion includes an AlGaAs emitter and
an AlGaAs base.
13. The multijunction solar cell of claim 12 wherein the lower
solar subcell of the upper portion includes a GaInP emitter and a
GaAs base.
14. The multijunction solar cell of claim 10 further including at
least one barrier layer adjacent the first graded interlayer and at
least one barrier layer adjacent the second graded interlayer,
wherein the barrier layers prevent propagation of threading
dislocations into adjacent ones of the solar subcells, wherein each
barrier layer has a composition that differs from a composition of
an adjacent graded interlayer or an adjacent solar subcell
layer.
15. The multijunction solar cell of claim 10 further including: a
first barrier layer adjacent one side of the first graded
interlayer; a second barrier layer adjacent a second side of the
first graded interlayer, a third barrier layer adjacent one side of
the second the graded interlayer, a fourth barrier layer adjacent a
second side of the second graded interlayer; wherein the barrier
layers prevent propagation of threading dislocations into adjacent
ones of the solar subcells, and wherein each barrier layer has a
composition that differs from a composition of an adjacent graded
interlayer or an adjacent solar subcell layer.
16. The multijunction solar cell of claim 10 wherein: the middle
portion of the multijunction solar cell includes a solar subcell
that has a GaInAs emitter and a GaInAs base, and the lower portion
of the multijunction solar cell includes a solar subcell that has a
GaInAs emitter and a GaInAs base.
17. The multijunction solar cell of claim 1 wherein the upper
portion of the multijunction solar cell includes a solar subcell
having an emitter in which a first region has a doping that is
graded and in which a second region has a doping that is constant,
the second region being directly disposed over the first
region.
18. The multijunction solar cell of claim 1 wherein the lower
portion of the multijunction solar cell further includes a third
graded interlayer that provides a transition in lattice constant
between two solar subcells in the lower portion that are lattice
mismatched to one another.
19. The multijunction solar cell of claim 1 wherein the upper
portion of the multijunction solar cell includes: an upper solar
subcell; a middle solar subcell disposed below the upper solar
subcell; and a lower solar subcell disposed below the middle solar
subcell, wherein the lower solar subcell is closer to the first
graded interlayer than the upper solar subcell; and wherein the
lower portion of the multijunction solar cell further includes a
third graded interlayer that provides a transition in lattice
constant between two solar subcells in the lower portion that are
lattice mismatched to one another.
20. The multijunction solar cell of claim 19 wherein the third
graded interlayer is composed of
(In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs with step graded changing
lattice constant, wherein 0<x<1 and 0<y<1.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 14/485,121, filed Sep. 12, 2014, which is a
continuation-in-part of U.S. patent application Ser. No.
13/401,181, filed Feb. 21, 2012, which is a continuation-in-part of
U.S. patent application Ser. No. 12/271,192 filed Nov. 14, 2008,
and of U.S. patent application Ser. No. 12/023,772, filed Jan. 31,
2008, which is in turn a continuation-in-part of U.S. patent
application Ser. No. 11/860,142 filed Sep. 24, 2007, and of U.S.
patent application Ser. No. 11/860,183, filed Sep. 24, 2007. The
contents of the earlier applications are incorporated herein by
reference in their entirety.
[0002] This application is related to U.S. patent application Ser.
No. 12/844,673 filed Jul. 27, 2010.
[0003] This application is related to U.S. patent application Ser.
No. 12/813,408 filed Jun. 10, 2010.
[0004] This application is related to U.S. patent application Ser.
No. 12/775,946 filed May 7, 2010.
[0005] This application is related to U.S. patent application Ser.
No. 12/756,926, filed Apr. 8, 2010.
[0006] This application is related to U.S. patent application Ser.
No. 12/730,018, filed Mar. 23, 2010.
[0007] This application is related to U.S. patent application Ser.
No. 12/716,814, filed Mar. 3, 2010.
[0008] This application is related to U.S. patent application Ser.
No. 12/708,361, filed Feb. 18, 2010.
[0009] This application is related to U.S. patent application Ser.
No. 12/637,241, filed Dec. 14, 2009.
[0010] This application is related to U.S. patent application Ser.
No. 12/623,134, filed Nov. 20, 2009.
[0011] This application is related to U.S. patent application Ser.
No. 12/544,001, filed Aug. 19, 2009.
[0012] This application is related to U.S. patent application Ser.
Nos. 12/401,137, 12/401,157, and 12/401,189, filed Mar. 10,
2009.
[0013] This application is related to U.S. patent application Ser.
No. 12/389,053, filed Feb. 19, 2009.
[0014] This application is related to U.S. patent application Ser.
No. 12/367,991, filed Feb. 9, 2009.
[0015] This application is related to U.S. patent application Ser.
No. 12/362,201, now U.S. Pat. No. 7,960,201; Ser. No. 12/362,213;
and Ser. No. 12/362,225, filed Jan. 29, 2009.
[0016] This application is related to U.S. patent application Ser.
No. 12/337,014 filed Dec. 17, 2008, now U.S. Pat. No. 7,785,989,
and Ser. No. 12/337,043 filed Dec. 17, 2008.
[0017] This application is related to U.S. patent application Ser.
No. 12/271,127 and Ser. No. 12/271,192 filed Nov. 14, 2008.
[0018] This application is related to U.S. patent application Ser.
No. 12/267,812 filed Nov. 10, 2008.
[0019] This application is related to U.S. patent application Ser.
No. 12/258,190 filed Oct. 24, 2008.
[0020] This application is related to U.S. patent application Ser.
No. 12/253,051 filed Oct. 16, 2008.
[0021] This application is related to U.S. patent application Ser.
No. 12/190,449, filed Aug. 12, 2008, now U.S. Pat. No. 7,741,146,
and its divisional patent application Ser. No. 12/816,205, filed
Jun. 15, 2010, now U.S. Pat. No. 8,039,291.
[0022] This application is related to U.S. patent application Ser.
No. 12/187,477, filed Aug. 7, 2008.
[0023] This application is related to U.S. patent application Ser.
No. 12/218,558 and U.S. patent application Ser. No. 12/218,582
filed Jul. 16, 2008.
[0024] This application is related to U.S. patent application Ser.
No. 12/123,864 filed May 20, 2008.
[0025] This application is related to U.S. patent application Ser.
No. 12/102,550 filed Apr. 14, 2008.
[0026] This application is related to U.S. Ser. No. 12/047,944,
filed Mar. 13, 2008.
[0027] This application is related to U.S. patent application Ser.
No. 12/023,772, filed Jan. 31, 2008.
[0028] This application is related to U.S. patent application Ser.
No. 11/956,069, filed Dec. 13, 2007, and its divisional application
Ser. No. 12/187,454 filed Aug. 7, 2008, now U.S. Pat. No.
7,727,795.
[0029] This application is also related to U.S. patent application
Ser. Nos. 11/860,142 and 11/860,183 filed Sep. 24, 2007.
[0030] This application is also related to U.S. patent application
Ser. No. 11/445,793 filed Jun. 2, 2006.
[0031] This application is also related to U.S. patent application
Ser. No. 11/500,053 filed Aug. 7, 2006, and its divisional
application Ser. No. 12/417,367 filed Apr. 2, 2009, and Ser. No.
12/549,340 filed Aug. 27, 2009.
BACKGROUND
[0033] 1. Field of the Disclosure
[0034] The present disclosure relates to multijunction solar cells
based on III-V semiconductor compounds, and to fabrication
processes and devices for multijunction (e.g., five and six
junction) solar cell structures including a metamorphic layer. Some
embodiments of such devices are also known as inverted metamorphic
multijunction solar cells.
[0035] 2. Description of the Related Art
[0036] Solar power from photovoltaic cells, also called solar
cells, has been predominantly provided by silicon semiconductor
technology. In the past several years, however, high-volume
manufacturing of III-V compound semiconductor multijunction solar
cells for space applications has accelerated the development of
such technology not only for use in space but also for terrestrial
solar power applications. Compared to silicon, III-V compound
semiconductor multijunction devices have greater energy conversion
efficiencies and generally more radiation resistance, although they
tend to be more complex to manufacture. Typical commercial II-V
compound semiconductor multijunction solar cells have energy
efficiencies that exceed 27% under one sun, air mass 0 (AM0),
illumination, whereas even the most efficient silicon technologies
generally reach only about 18% efficiency under comparable
conditions. Under high solar concentration (e.g., 500.times),
commercially available III-V compound semiconductor multijunction
solar cells in terrestrial applications (at AM1.5D) have energy
efficiencies that exceed 37%. The higher conversion efficiency of
III-V compound semiconductor solar cells compared to silicon solar
cells is in part based on the ability to achieve spectral splitting
of the incident radiation through the use of a plurality of
photovoltaic regions with different band gap energies, and
accumulating the current from each of the regions.
[0037] Typical III-V compound semiconductor solar cells are
fabricated on a semiconductor wafer in vertical, multijunction
structures. The individual solar cells or wafers are then disposed
in horizontal arrays, with the individual solar cells connected
together in an electrical series 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.
[0038] Inverted metamorphic solar cell structures based on III-V
compound semiconductor layers, such as described in M. W. Wanlass
et al., Lattice Mismatched Approaches for High Performance, III-V
Photovoltaic Energy Converters (Conference Proceedings of the
31.sup.st IEEE Photovoltaic Specialists Conference, Jan. 3-7, 2005,
IEEE Press, 2005), present an important conceptual starting point
for the development of future commercial high efficiency solar
cells. However, the materials and structures for a number of
different layers of the cell proposed and described in such
reference present a number of practical difficulties relating to
the appropriate choice of materials and fabrication steps.
[0039] Prior to the disclosures described in various ones or
combinations of this and the related applications noted above, the
materials and fabrication steps disclosed in the prior art have
various drawbacks and disadvantages in producing a commercially
viable inverted metamorphic multijunction solar cell using
commercially established fabrication processes.
SUMMARY
[0040] Briefly, and in general terms, the present disclosure
provides multi-junction solar cells utilizing two or more
metamorphic layers. Thus, for example, vertical stacks of three or
more subcells can include two or more metamorphic grading
interlayers, each of which provides a transition in lattice
constant between lattice mismatched subcells. Further, in some
cases, the use of more than two metamorphic grading interlayer may
be utilized. In general, it can be advantageous for the band gap of
each particular graded interlayer to remain constant throughout its
thickness.
[0041] In some instances, the metamorphic grading interlayers are
composed of AlGaInAs with compositionally step-graded series of
AlGaInAs, for example, having a monotonically or step graded
changing lattice constant, so as to achieve a transition in lattice
constant in the semiconductor structure between lattice mismatched
subcells. In some cases, the structure helps reduce or minimize the
occurrence of threading dislocations.
[0042] Significant improvement in conversion efficiency can be
obtained for devices in which one graded interlayer has a band gap
of about 1.5 eV-1.6 eV and a second graded interlayer has a band
gap of 1.1 eV. In some cases, the first graded interlayer has a
band gap of 1.5 eV (.+-.3%) or 1.6 eV (.+-.3%), and the second
graded interlayer has a band gap of 1.1 eV (.+-.3%). Thus, in some
cases, the first graded interlayer has a band gap in the range of
1.5 eV-1.65 eV.
[0043] For example, in one aspect, the present disclosure describes
a multijunction solar cell that includes an upper portion including
at least one solar subcell; a first graded interlayer, a middle
portion including at least one solar subcell; a second graded
interlayer; and a lower portion including at least one solar
subcell. The first graded interlayer can be composed, for example,
of (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs with a step graded
changing lattice constant, wherein 0<x<1 and 0<y<1, and
wherein the first graded interlayer has a band gap in a range of
1.5 eV-1.65 eV that is constant throughout its thickness such that
the first graded interlayer provides a transition in lattice
constant between a particular solar subcell in the upper portion
and a particular solar subcell in the middle portion that is
lattice mismatched to the particular solar subcell in the upper
portion. The second graded interlayer can be composed, for example,
of (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs with a step graded
changing lattice constant, wherein 0<x<1 and 0<y<1, and
wherein the second graded interlayer has a band gap in a range of
1.1 eV+3% that is constant throughout its thickness such that the
second graded interlayer provides a transition in lattice constant
between a specific solar subcell in the middle portion and a
specific solar subcell in the lower portion that is lattice
mismatched to the specific solar subcell in the middle portion.
[0044] Additional solar subcells can be provided in one or more of
the upper, middle or lower portions of the solar cell. Further, in
some implementations, additional graded interlayers may be
present.
[0045] Thus, in another aspect, the present disclosure provides a
multijunction solar cell including an upper first solar subcell
having a first band gap, and the base-emitter junction of the upper
first solar subcell being a homojunction; a second solar subcell
adjacent to said first solar subcell and having a second band gap
smaller than said first band gap; and a third solar subcell
adjacent to said second solar subcell and having a third band gap
smaller than said second band gap. A first graded interlayer is
provided adjacent to said third solar subcell; said first graded
interlayer having a fourth band gap greater than said third band
gap. A fourth solar subcell is provided adjacent to said first
graded interlayer, said fourth subcell having a fifth band gap
smaller than said third band gap such that said fourth subcell is
lattice mismatched with respect to said third subcell. A second
graded interlayer is provided adjacent to said fourth solar
subcell; said second graded interlayer having a sixth band gap
greater than said fifth band gap; and a lower fifth solar subcell
is provided adjacent to said second graded interlayer, said lower
fifth subcell having a seventh band gap smaller than said fifth
band gap such that said fifth subcell is lattice mismatched with
respect to said fourth subcell.
[0046] In another aspect, the present disclosure provides a five
junction solar cell utilizing three metamorphic layers. More
particularly the present disclosure provides a multijunction solar
cell including an upper first solar subcell having a first band
gap, and the base-emitter junction of the upper first solar subcell
being a homojunction; a second solar subcell adjacent to said first
solar subcell and having a second band gap smaller than said first
band gap. A first graded interlayer is provided adjacent to said
second solar subcell; said first graded interlayer having a third
band gap greater than said second band gap. A third solar subcell
is provided adjacent to said first graded interlayer and having a
fourth band gap smaller than said second band gap such that said
third subcell is lattice mismatched with respect to said second
subcell. A second graded interlayer is provided adjacent to said
third solar subcell; said second graded interlayer having a fifth
band gap greater than said fourth band gap. A fourth solar subcell
is provided adjacent to said second graded interlayer, said fourth
subcell having a sixth band gap smaller than said fourth band gap
such that said fourth subcell is lattice mismatched with respect to
said third subcell. A third graded interlayer is provided adjacent
to said fourth solar subcell; said third graded interlayer having a
seventh band gap greater than said sixth band gap. A lower fifth
solar subcell is provided adjacent to said third graded interlayer,
said lower fifth subcell having a eighth band gap smaller than said
seventh band gap such that said fifth subcell is lattice mismatched
with respect to said fourth subcell.
[0047] In another aspect the present disclosure provides a six
junction solar cell utilizing three metamorphic layers. More
particularly the present disclosure provides a multijunction solar
cell including an upper first solar subcell having a first band
gap, and the base-emitter junction of the upper first solar subcell
being a homojunction; a second solar subcell adjacent to said first
solar subcell and having a second band gap smaller than said first
band gap; a third solar subcell adjacent to said second solar
subcell and having a third band gap smaller than said second band
gap; a first graded interlayer adjacent to said third solar
subcell; said first graded interlayer having a fourth band gap
greater than said third band gap; and a fourth solar subcell
adjacent to said first graded interlayer, said fourth subcell
having a fifth band gap smaller than said third band gap such that
said fourth subcell is lattice mismatched with respect to said
third subcell; a second graded interlayer adjacent to said fourth
solar subcell; said second graded interlayer having a sixth band
gap greater than said fifth band gap; a fifth solar subcell
adjacent to said second graded interlayer, said fifth subcell
having a seventh band gap smaller than said fifth band gap such
that said fifth subcell is lattice mismatched with respect to said
fourth subcell; a third graded interlayer adjacent to said fifth
solar subcell; said third graded interlayer having a eighth band
gap greater than said seventh band gap; and a lower sixth solar
subcell adjacent to said third graded interlayer, said lower sixth
subcell having a ninth band gap smaller than said eighth band gap
such that said sixth subcell is lattice mismatched with respect to
said fifth subcell.
[0048] In another aspect the present disclosure provides a five
junction solar cell utilizing one metamorphic layer. More
particularly the present disclosure provides a multijunction solar
cell including an upper first solar subcell having a first band
gap, and the base-emitter junction of the upper first solar subcell
being a homojunction; a second solar subcell adjacent to said first
solar subcell and having a second band gap smaller than said first
band gap; and a third solar subcell adjacent to said second solar
subcell and having a third band gap smaller than said second band
gap. A graded interlayer is provided adjacent to said third solar
subcell; said graded interlayer having a fourth band gap greater
than said third band gap. A fourth solar subcell is provided
adjacent to said first graded interlayer, said fourth subcell
having a fifth band gap smaller than said third band gap such that
said fourth subcell is lattice mismatched with respect to said
third subcell. A lower fifth solar subcell is provided adjacent to
said fourth subcell, said lower fifth subcell having a sixth band
gap smaller than said fifth band gap such that said fourth subcell
is lattice matched with respect to said fourth subcell.
[0049] In another aspect the present disclosure provides a six
junction solar cell utilizing two metamorphic layers. More
particularly the present disclosure provides a multijunction solar
cell including an upper first solar subcell having a first band
gap, and the base-emitter junction of the upper first solar subcell
being a homojunction; a second solar subcell adjacent to said first
solar subcell and having a second band gap smaller than said first
band gap; and a third solar subcell adjacent to said second solar
subcell and having a third band gap smaller than said second band
gap. A first graded interlayer is provided adjacent to said third
solar subcell; said first graded interlayer having a fourth band
gap greater than said third band gap. A fourth solar subcell is
provided adjacent to said first graded interlayer, said fourth
subcell having a fifth band gap smaller than said third band gap
such that said fourth subcell is lattice mismatched with respect to
said third subcell. A fifth solar subcell is provided adjacent to
said fourth subcell, said fifth subcell having a sixth band gap
smaller than said fifth band gap such that said fifth subcell is
lattice matched with respect to said fourth subcell. A second
graded interlayer is provided adjacent to said fifth solar subcell;
said second graded interlayer having a seventh band gap greater
than said sixth band gap. A lower sixth solar subcell is provided
adjacent to said second graded interlayer, said lower sixth subcell
having a eighth band gap smaller than said sixth band gap such that
said sixth subcell is lattice mismatched with respect to said fifth
subcell.
[0050] In another aspect the present disclosure provides a method
of forming a five junction solar cell utilizing two metamorphic
layers. More particularly the present disclosure provides a method
of manufacturing a solar cell including providing a first
substrate; forming an upper first solar subcell having a first band
gap on the first substrate, the base-emitter junction of the upper
first solar subcell being a homojunction; forming a second solar
subcell adjacent to said first solar subcell and having a second
band gap smaller than said first band gap; and forming a third
solar subcell adjacent to said second solar subcell and having a
third band gap smaller than said second band gap. A first graded
interlayer is formed adjacent to said third solar subcell; said
first graded interlayer having a fourth band gap greater than said
third band gap. A fourth solar subcell is formed adjacent to said
first graded interlayer, said fourth subcell having a fifth band
gap smaller than said third band gap such that said fourth subcell
is lattice mismatched with respect to said third subcell. A second
graded interlayer is formed adjacent to said fourth solar subcell;
said second graded interlayer having a sixth band gap greater than
said fifth band gap; and a lower fifth solar subcell is formed
adjacent to said second graded interlayer, said lower fifth subcell
having a seventh band gap smaller than said fifth band gap such
that said fifth subcell is lattice mismatched with respect to said
fourth subcell. A surrogate substrate is mounted on top of lower
fifth solar subcell; and the first substrate is removed.
[0051] In another aspect, the present disclosure provides a method
of forming a five junction solar cell utilizing three metamorphic
layers. More particularly the present disclosure provides a method
of manufacturing a multijunction solar cell including providing a
first substrate; forming an upper first solar subcell having a
first band gap, the base-emitter junction of the upper first solar
subcell being a homojunction; forming a second solar subcell
adjacent to said first solar subcell and having a second band gap
smaller than said first band gap. A first graded interlayer is
formed adjacent to said second solar subcell; said first graded
interlayer having a third band gap greater than said second band
gap. A third solar subcell is formed adjacent to said first graded
interlayer having a fourth band gap smaller than said second band
gap such that said third subcell is lattice mismatched with respect
to said second subcell. A second graded interlayer is formed
adjacent to said third solar subcell; said second graded interlayer
having a fifth band gap greater than said fourth band gap. A fourth
solar subcell is formed adjacent to said second graded interlayer,
said fourth subcell having a sixth band gap smaller than said
fourth band gap such that said fourth subcell is lattice mismatched
with respect to said third subcell. A third graded interlayer is
formed adjacent to said fourth solar subcell; said third graded
interlayer having a seventh band gap greater than said sixth band
gap. A lower fifth solar subcell is formed adjacent to said third
graded interlayer, said lower fifth subcell having a eighth band
gap smaller than said seventh band gap such that said fifth subcell
is lattice mismatched with respect to said fourth subcell.
[0052] In another aspect the present disclosure provides a method
of forming a six junction solar cell utilizing three metamorphic
layers. More particularly the present disclosure provides a method
of manufacturing a multijunction solar cell including providing a
first substrate; forming an upper first solar subcell having a
first band gap on the first substrate, the base-emitter junction of
the upper first solar subcell being a homojunction; forming a
second solar subcell adjacent to said first solar subcell and
having a second band gap smaller than said first band gap; forming
a third solar subcell adjacent to said second solar subcell and
having a third band gap smaller than said second band gap; forming
a first graded interlayer adjacent to said third solar subcell;
said first graded interlayer having a fourth band gap greater than
said third band gap; forming a fourth solar subcell adjacent to
said first graded interlayer, said fourth subcell having a fifth
band gap smaller than said third band gap such that said fourth
subcell is lattice mismatched with respect to said third subcell;
forming a second graded interlayer adjacent to said fourth solar
subcell; said second graded interlayer having a sixth band gap
greater than said fifth band gap; forming a fifth solar subcell
adjacent to said second graded interlayer, said fifth subcell
having a seventh band gap smaller than said fifth band gap such
that said fifth subcell is lattice mismatched with respect to said
fourth subcell; forming a third graded interlayer adjacent to said
fifth solar subcell; said third graded interlayer having a eighth
band gap greater than said seventh band gap; and forming a lower
sixth solar subcell adjacent to said third graded interlayer, said
lower sixth subcell having a ninth band gap smaller than said
eighth band gap such that said sixth subcell is lattice mismatched
with respect to said fifth subcell. A surrogate substrate is
mounted on top of lower sixth solar subcell; and the first
substrate is removed.
[0053] In another aspect the present disclosure provides a method
of forming a five junction solar cell utilizing one metamorphic
layer. More particularly the present disclosure provides a method
of manufacturing a multijunction solar cell including providing a
first substrate; forming an upper first solar subcell having a
first band gap, and the base-emitter junction of the upper first
solar subcell being a homojunction; forming a second solar subcell
adjacent to said first solar subcell and having a second band gap
smaller than said first band gap; and forming a third solar subcell
adjacent to said second solar subcell and having a third band gap
smaller than said second band gap. A graded interlayer is formed
adjacent to said third solar subcell; said graded interlayer having
a fourth band gap greater than said third band gap. A fourth solar
subcell is formed adjacent to said first graded interlayer, said
fourth subcell having a fifth band gap smaller than said third band
gap such that said fourth subcell is lattice mismatched with
respect to said third subcell. A lower fifth solar subcell is
formed adjacent to said fourth subcell, said lower fifth subcell
having a sixth band gap smaller than said fifth band gap such that
said fourth subcell is lattice matched with respect to said fourth
subcell.
[0054] In another aspect the present disclosure provides a method
of forming a six junction solar cell utilizing two metamorphic
layers. More particularly the present disclosure provides a method
of manufacturing a multijunction solar cell including providing a
first substrate; forming an upper first solar subcell being a
homojunction; forming a second solar subcell adjacent to said first
solar subcell and having a second band gap smaller than said first
band gap; and forming a third solar subcell adjacent to said second
solar subcell and having a third band gap smaller than said second
band gap. A first graded interlayer is formed adjacent to said
third solar subcell; said first graded interlayer having a fourth
band gap greater than said third band gap. A fourth solar subcell
is formed adjacent to said first graded interlayer, said fourth
subcell having a fifth band gap smaller than said third band gap
such that said fourth subcell is lattice mismatched with respect to
said third subcell. A fifth solar subcell is formed adjacent to
said fourth subcell, said fifth subcell having a sixth band gap
smaller than said fifth band gap such that said fifth subcell is
lattice matched with respect to said fourth subcell. A second
graded interlayer is formed adjacent to said fifth solar subcell;
said second graded interlayer having a seventh band gap greater
than said sixth band gap. A lower sixth solar subcell is formed
adjacent to said second graded interlayer, said lower sixth subcell
having a eighth band gap smaller than said sixth band gap such that
said sixth subcell is lattice mismatched with respect to said fifth
subcell.
[0055] In some embodiments, the base and emitter of the upper first
solar subcell is composed of AlGaInP.
[0056] In some embodiments, the band gap of the base of the upper
first solar subcell is equal to or greater than 2.05 eV.
[0057] In some embodiments, the emitter of the upper first solar
subcell is composed of a first region in which the doping is graded
from 3.times.10.sup.18 to 1.times.10.sub.18 free carriers per cubic
centimeter, and a second region directly disposed over the first
region in which the doping is constant at 1.times.10.sup.17 free
carriers per cubic centimeter.
[0058] In some embodiments, the first region of the emitter of the
upper first solar subcell is directly adjacent to a window
layer.
[0059] In some embodiments, the emitter of the upper first solar
subcell has a thickness of 80 nm.
[0060] In some embodiments, there is a spacer layer between the
emitter and the base of the upper first solar subcell. In some
embodiments, the spacer layer between the emitter and the base of
the upper first solar subcell is composed of unintentionally doped
AlGaInP.
[0061] In some embodiments, the base of the upper first solar
subcell has a thickness of less than 400 nm.
[0062] In some embodiments, the base of the upper first solar
subcell has a thickness of 260 nm.
[0063] In some embodiments, the emitter section of the upper first
solar subcell has a free carrier density of 3.times.10.sup.18 to
9.times.10.sup.18 per cubic centimeter.
[0064] In some embodiments, in particular in connection with a five
junction solar cell utilizing two metamorphic layers, the band gap
of the first graded interlayer remains constant at 1.5 eV
throughout the thickness of the first graded interlayer, and the
band gap of the second graded interlayer remains constant at 1.5 eV
throughout the thickness of the second graded interlayer. More
specifically, in some embodiments, the present disclosure provides
a method of manufacturing a solar cell using an MOCVD process,
wherein the first and second graded interlayers are composed of
(In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs, and are formed in the MOCVD
reactor so that they are compositionally graded to lattice match
the third subcell on one side and the fourth subcell on the other
side, and the fourth subcell on one side and the bottom fifth
subcell on the other side, respectively, with the values for x and
y computed and the composition of the first and second graded
interlayers determined so that as the layers are grown in the MOCVD
reactor, the band gap of the first graded interlayer remains
constant at 1.5 eV throughout the thickness of the first graded
interlayer, and the band gap of the second graded interlayer
remains constant at 1.5 eV throughout the thickness of the second
graded interlayer.
[0065] In some embodiments, in particular in connection with a five
junction solar cell utilizing two metamorphic layers, the upper
subcell is composed of an AlGaInP emitter layer and an AlGaInP base
layer, the second subcell is composed of AlGaAs emitter layer and a
AlGaAs base layer, the third subcell is composed of a GaInP emitter
layer and a GaAs base layer, the fourth subcell is composed of a
GaInAs emitter layer and a GaInAs base layer, and the bottom fifth
subcell is composed of a GaInAs emitter layer and a GaInAs base
layer.
[0066] In some embodiments, in particular in connection with a five
junction solar cell utilizing two metamorphic layers, the lower
fifth subcell has a band gap in the range of approximately 0.85 to
0.95 eV, the fourth subcell has a band gap in the range of
approximately 1.0 to 1.2 eV; the third subcell has a band gap in
the range of approximately 1.3 to 1.5 eV, the second subcell has a
band gap in the range of approximately 1.65 to 1.80 eV and the
upper subcell has a band gap in the range of 1.9 to 2.2 eV.
[0067] In some implementations, it is advantageous for the band gap
of the first graded interlayer to be substantially constant at
approximately 1.6 eV and the band gap of the second graded
interlayer to be substantially constant at approximately 1.1 eV. In
some embodiments, in particular in connection with a six junction
solar cell utilizing three metamorphic layers, the band gap of the
first graded interlayer remains constant at 1.5 eV or 1.6 eV
throughout the thickness of the first graded interlayer, the band
gap of the second graded interlayer remains constant at 1.5 eV or
1.6 eV throughout the thickness of the second graded interlayer,
and the band gap of the third graded interlayer remains constant at
1.1 eV throughout the thickness of the third graded interlayer.
[0068] In some embodiments, in particular in connection with a six
junction solar cell utilizing three metamorphic layers, the upper
subcell is composed of an AlGaInP emitter layer and an AlGaInP base
layer, the second subcell is composed of AlGaAs emitter layer and a
AlGaAs base layer, the third subcell is composed of a GaInP emitter
layer and a GaAs base layer, the fourth subcell is composed of a
GaInAs emitter layer and a GaInAs base layer, the fifth subcell is
composed of a GaInAs emitter layer and a GaInAs base layer, and the
bottom sixth subcell is composed of a GaInAs emitter layer and a
GaInAs base layer.
[0069] In some embodiments, in particular in connection with a six
junction solar cell utilizing three metamorphic layers, the lower
sixth subcell has a band gap in the range of approximately 0.60 to
0.70 eV, the fifth subcell has a band gap in the range of
approximately 0.85 to 0.95 eV the fourth subcell has a band gap in
the range of approximately 1.0 to 1.2 eV; the third subcell has a
band gap in the range of approximately 1.3 to 1.5 eV, the second
subcell has a band gap in the range of approximately 1.65 to 1.80
eV and the upper subcell has a band gap in the range of 1.9 to 2.2
eV.
[0070] Various methods are described for fabricating solar cells
including formation of the graded interlayers.
[0071] Some implementations of the present disclosure may
incorporate or implement fewer of the aspects and features noted in
the foregoing summaries.
[0072] 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
[0073] 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:
[0074] FIG. 1 is a graph representing the band gap of certain
binary materials and their lattice constants;
[0075] FIG. 2A is a cross-sectional view of the solar cell of one
embodiment of a multijunction solar cell after an initial stage of
fabrication including the deposition of certain semiconductor
layers on the growth substrate;
[0076] FIG. 2B is a cross-sectional view of the solar cell of FIG.
2A after the next sequence of process steps;
[0077] FIG. 2C is a cross-sectional view of the solar cell of FIG.
2B after the next sequence of process steps;
[0078] FIG. 2D is a cross-sectional view of the solar cell of FIG.
2C after the next sequence of process steps;
[0079] FIG. 2E is a cross-sectional view of the solar cell of FIG.
2D after the next process step;
[0080] FIG. 2F is a cross-sectional view of the solar cell of FIG.
2E after the next process step in which a surrogate substrate is
attached;
[0081] FIG. 2G is a cross-sectional view of the solar cell of FIG.
2F after the next process step in which the original substrate is
removed;
[0082] FIG. 2H is another cross-sectional view of the solar cell of
FIG. 2G with the surrogate substrate on the bottom of the
Figure;
[0083] FIG. 3A is a cross-sectional view of the solar cell of an
embodiment of the present disclosure representing a five junction
solar cell utilizing two metamorphic layers after an initial stage
of fabrication including the deposition of certain semiconductor
layers on the growth substrate;
[0084] FIG. 3B is a cross-sectional view of the solar cell of FIG.
3A after the next sequence of process steps in which the lower two
subcells are grown;
[0085] FIG. 3C is a cross-sectional view of the a solar cell of an
embodiment of the present disclosure representing a five junction
solar cell utilizing three metamorphic layers after an initial
stage of fabrication including the deposition of certain
semiconductor layers on the growth substrate;
[0086] FIG. 4 is a cross-sectional view of a solar cell of an
embodiment of the present disclosure representing a six junction
solar cell utilizing three metamorphic layers after an initial
stage of fabrication including the deposition of certain
semiconductor layers on the growth substrate;
[0087] FIG. 5 is a cross-sectional view of a solar cell of another
embodiment of the present disclosure representing a five junction
solar cell utilizing one metamorphic layer after an initial stage
of fabrication including the deposition of certain semiconductor
layers on the growth substrate;
[0088] FIG. 6 is a cross-sectional view of a solar cell of another
embodiment of the present disclosure representing a six junction
solar cell utilizing two metamorphic layers after an initial stage
of fabrication including the deposition of certain semiconductor
layers on the growth substrate;
[0089] FIG. 7 is a simplified cross-sectional view of the solar
cell of either FIG. 2H, 3B, 3C, 4, 5, or 6 after the next sequence
of process steps in which a metallization layer is deposited over
the contact layer, and a surrogate substrate attached;
[0090] FIG. 8 is a cross-sectional view of the solar cell of FIG. 7
after the next sequence of process steps in which the growth
substrate is removed;
[0091] FIG. 9 is a another cross-sectional view of the solar cell
of FIG. 7, similar to that of FIG. 8, but here oriented and
depicted with the surrogate substrate at the bottom of the
figure;
[0092] FIG. 10 is a cross-sectional view of the solar cell of FIG.
9 after the next sequence of process steps;
[0093] FIG. 11 is a cross-sectional view of the solar cell of FIG.
10 after the next sequence of process steps;
[0094] FIG. 12 is a cross-sectional view of the solar cell of FIG.
11 after the next sequence of process steps;
[0095] FIG. 13A is a top plan view of a wafer in one embodiment of
the present disclosure in which the solar cells are fabricated;
[0096] FIG. 13B is a bottom plan view of a wafer in the embodiment
of FIG. 13A;
[0097] FIG. 14 is a cross-sectional view of the solar cell of FIG.
12 after the next sequence of process steps;
[0098] FIG. 15 is a cross-sectional view of the solar cell of FIG.
14 after the next sequence of process steps;
[0099] FIG. 16 is a top plan view of the wafer of FIG. 15 depicting
the surface view of the trench etched around the cell in one
embodiment of the present disclosure;
[0100] FIG. 17 is a cross-sectional view of the solar cell of FIG.
15 after the next sequence of process steps in one embodiment of
the present disclosure;
[0101] FIG. 18 is a cross-sectional view of the solar cell of FIG.
17 after the next sequence of process steps in one embodiment of
the present disclosure;
[0102] FIG. 19 is a cross-sectional view of the solar cell of FIG.
17 after the next sequence of process steps in another embodiment
of the present disclosure;
[0103] FIG. 20A is a graph of the doping profile of the emitter and
base layers of the top subcell in the solar cell according to the
present disclosure;
[0104] FIG. 20B is a graph of the doping profile of the emitter and
base layers of one or more of the middle subcells in the solar cell
according to the present disclosure;
[0105] FIG. 21 is a graph representing the Al, Ga and In mole
fractions versus the Al to In mole fraction in a AlGaInAs material
system that is necessary to achieve a constant 1.5 eV band gap;
[0106] FIG. 22 is a diagram representing the relative concentration
of Al, In, and Ga in an AlGaInAs material system needed to have a
constant band gap with various designated values (ranging from 0.4
eV to 2.1 eV) as represented by curves on the diagram;
[0107] FIG. 23 is a graph representing the Ga mole fraction to the
Al to In mole fraction in a AlGaInAs material system that is
necessary to achieve a constant 1.51 eV band gap;
[0108] FIG. 24 is a graph that depicts the current and voltage
characteristics of a test solar cell with the doped emitter
structure of FIG. 20A according to the present disclosure, compared
to a solar cell with a normally doped emitter structure; and
[0109] FIG. 25 is a graph that depicts the external quantum
efficiency (EQE) as a function of wavelength of a multijunction
solar cell with the doped emitter structure of FIG. 20A according
to the present disclosure, compared with that of a solar cell with
a normally doped emitter structure.
[0110] FIG. 26 is a graph that depicts band gap combinations versus
cell conversion efficiency for multi-junction solar cell devices
including two graded interlayers each of which has a substantially
constant band gap.
GLOSSARY OF TERMS
[0111] "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 UI 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).
[0112] "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.
[0113] "Compound semiconductor" refers to a semiconductor formed
using two or more chemical elements.
[0114] "Graded interlayer" (or "grading interlayer")--see
"metamorphic layer."
[0115] "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 deployed
configuration, are deposited or grown on a growth substrate prior
to depositing or growing the lower band gap subcells.
[0116] "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.
[0117] "Lattice mismatched" refers to two adjacently disposed
materials having different lattice constants from one another.
[0118] "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.
[0119] "Multijunction solar cell" refers to a solar cell containing
two or more discrete and distinct p-n junctions, each of which may
be tuned to a different wavelength of light.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0120] 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.
[0121] The basic concept of fabricating an inverted metamorphic
multijunction (IMM) solar cell is to grow the subcells of the solar
cell on a substrate in a "reverse" sequence. That is, the high band
gap subcells (i.e. subcells with band gaps in the range of 1.8 to
2.2 eV), which would normally be the "top" subcells facing the
solar radiation, are grown epitaxially on a semiconductor growth
substrate, such as for example GaAs or Ge, and such subcells are
therefore lattice matched to such substrate. One or more lower band
gap middle subcells (i.e. with band gaps in the range of 1.2 to 1.8
eV) can then be grown on the high band gap subcells.
[0122] At least one lower subcell is formed over the middle subcell
such that the at least one lower subcell is substantially lattice
mismatched with respect to the growth substrate and such that the
at least one lower subcell has a third lower band gap (i.e. a band
gap in the range of 0.7 to 1.2 eV). A surrogate substrate or
support structure is then attached or provided over the "bottom" or
substantially lattice mismatched lower subcell, and the growth
semiconductor substrate is subsequently removed. (The growth
substrate may then subsequently be re-used for the growth of a
second and subsequent solar cells).
[0123] A variety of different features of 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 solar cells of the
present disclosure. However, more particularly, the present
disclosure is directed to the fabrication of a five or six junction
inverted metamorphic solar cell using either one, two or three
different metamorphic layers, all grown on a single growth
substrate. In the present disclosure, the resulting construction
may include four, five, or six subcells, with band gaps in the
range of 1.8 to 2.2 eV (or higher) for the top subcell, and 1.3 to
1.8 eV, 0.9 to 1.2 eV for the middle subcells, and 0.6 to 0.8 eV,
for the bottom subcell, respectively.
[0124] It should be apparent to one skilled in the art that in
addition to the one or two different metamorphic layers discussed
in the present disclosure, additional types of semiconductor layers
within the cell are also within the scope of the present
invention.
[0125] Reference throughout this specification to "one embodiment"
or "an embodiment" means that a particular feature, structure, or
characteristic described in connection with the embodiment is
included in at least one embodiment of the present invention. Thus,
the appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily all referring to the same embodiment. Furthermore, the
particular features, structures, or characteristics may be combined
in any suitable manner in one or more embodiments.
[0126] FIG. 1 is a graph representing the band gap of certain
binary materials and their lattice constants. The band gap and
lattice constants of ternary materials are located on the lines
drawn between typical associated binary materials (such as the
ternary material AlGaAs being located between the GaAs and AlAs
points on the graph, with the band gap of the ternary material
lying between 1.42 eV for GaAs and 2.16 eV for AlAs depending upon
the relative amount of the individual constituents). Thus,
depending upon the desired band gap, the material constituents of
ternary materials can be appropriately selected for growth.
[0127] 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 vapor deposition method, such as Organo Metallic Vapor Phase
Epitaxy (OMVPE), Metal Organic Chemical Vapor Deposition (MOCVD),
Molecular Beam Epitaxy (MBE), or other vapor deposition methods for
the reverse 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.
[0128] The present disclosure is 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. More particularly, the present disclosure is directed
to the materials and fabrication steps that are particularly
suitable for producing commercially viable 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.
[0129] In order to provide appropriate background FIG. 2A through
2H depicts the sequence of steps in forming a four junction solar
cell solar cell generally as set forth in parent U.S. patent
application Ser. No. 12/271,192 filed Nov. 14, 2008, herein
incorporated by reference.
[0130] FIG. 2A depicts the sequential formation of the three
subcells A, B and C on a GaAs growth substrate. More particularly,
there is shown a substrate 101, which is preferably gallium
arsenide (GaAs), but may also be germanium (Ge) or other suitable
material. For GaAs, the substrate is preferably a 15.degree.
off-cut substrate, that is to say, its surface is orientated
15.degree. off the (100) plane towards the (111)A plane, as more
fully described in U.S. patent application Ser. No. 12/047,944,
filed Mar. 13, 2008.
[0131] In the case of a Ge substrate, a nucleation layer (not
shown) is deposited directly on the substrate 101. On the
substrate, or over the nucleation layer (in the case of a Ge
substrate), a buffer layer 102 and an etch stop layer 103 are
further deposited. In the case of GaAs substrate, the buffer layer
102 is preferably GaAs. In the case of Ge substrate, the buffer
layer 102 is preferably GaInAs. A contact layer 104 of GaAs is then
deposited on layer 103, and a window layer 105 of AlInP is
deposited on the contact layer. The subcell A, consisting of an n+
emitter layer 106 and a p-type base layer 107, is then epitaxially
deposited on the window layer 105. The subcell A is generally
lattice matched to the growth substrate 101.
[0132] It should be noted that the multijunction solar cell
structure could be formed by any suitable combination of group III
to V elements listed in the periodic table subject to lattice
constant and band gap requirements, wherein the group II includes
boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium
(T). The group IV includes carbon (C), silicon (Si), germanium
(Ge), and tin (Sn). The group V includes nitrogen (N), phosphorus
(P), arsenic (As), antimony (Sb), and bismuth (Bi).
[0133] In one embodiment, the emitter layer 106 is composed of
GaInP and the base layer 107 is composed of AlGaInP. In some
embodiments, more generally, the base-emitter junction may be a
heterojunction. In other embodiments, the base layer may be
composed of (Al)GaInP, where the aluminum or Al term in parenthesis
in the preceding formula means that Al is an optional constituent,
and in this instance may be used in an amount ranging from 0% to
30%. The doping profile of the emitter and base layers 106 and 107
according to the present invention will be discussed in conjunction
with FIG. 20.
[0134] In some embodiments, the band gap of the base layer 107 is
1.91 eV or greater.
[0135] Subcell A will ultimately become the "top" subcell of the
inverted metamorphic structure after completion of the process
steps according to the present invention to be described
hereinafter.
[0136] On top of the base layer 107 a back surface field ("BSF")
layer 108 preferably p+ AlGaInP is deposited and used to reduce
recombination loss.
[0137] The BSF layer 108 drives minority carriers from the region
near the base/BSF interface surface to minimize the effect of
recombination loss. In other words, the BSF layer 18 reduces
recombination loss at the backside of the solar subcell A and
thereby reduces the recombination in the base.
[0138] On top of the BSF layer 108 a sequence of heavily doped
p-type and n-type layers 109a and 109b is deposited that forms a
tunnel diode, i.e. an ohmic circuit element that forms an
electrical connection between subcell A to subcell B. Layer 109a is
preferably composed of p++ AlGaAs, and layer 109b is preferably
composed of n++ GaInP.
[0139] On top of the tunnel diode layers 109 a window layer 110 is
deposited, preferably n+ GaInP. The advantage of utilizing GaInP as
the material constituent of the window layer 110 is that it has an
index of refraction that closely matches the adjacent emitter layer
111, as more fully described in U.S. patent application Ser. No.
12/258,190, filed Oct. 24, 2008. The window layer 110 used in the
subcell B also operates to reduce the interface recombination loss.
It should be apparent to one skilled in the art, that additional
layer(s) may be added or deleted in the cell structure without
departing from the scope of the present disclosure.
[0140] On top of the window layer 110 the layers of subcell B are
deposited: the n-type emitter layer 111 and the p-type base layer
112. These layers are preferably composed of GaInP and
GaIn.sub.0.0015As respectively (for a Ge substrate or growth
template), or GaInP and GaAs respectively (for a GaAs substrate),
although any other suitable materials consistent with lattice
constant and band gap requirements may be used as well. Thus,
subcell B may be composed of a GaAs, GaInP, GaInAs, GaAsSb, or
GaInAsN emitter region and a GaAs, GaInAs, GaAsSb, or GaInAsN base
region. The doping profile of layers 111 and 112 according to the
present disclosure will be discussed in conjunction with FIG.
20B.
[0141] In some previously disclosed implementations of an inverted
metamorphic solar cell, the middle cell was a homostructure. In
some embodiments of the present disclosure, similarly to the
structure disclosed in U.S. patent application Ser. No. 12/023,772,
the middle subcell becomes a heterostructure with an GaInP emitter
and its window is converted from AlInP to GaInP. This modification
eliminated the refractive index discontinuity at the window/emitter
interface of the middle subcell, as more fully described in U.S.
patent application Ser. No. 12/258,190, filed Oct. 24, 2008.
Moreover, the window layer 110 is preferably doped three times that
of the emitter 111 to move the Fermi level up closer to the
conduction band and therefore create band bending at the
window/emitter interface which results in constraining the minority
carriers to the emitter layer.
[0142] In one embodiment of the present disclosure, the middle
subcell emitter has a band gap equal to the top subcell emitter,
and the third subcell emitter has a band gap greater than the band
gap of the base of the middle subcell. Therefore, after fabrication
of the solar cell, and implementation and operation, neither the
emitters of middle subcell B nor the third subcell C will be
exposed to absorbable radiation. Substantially all of the photons
representing absorbable radiation will be absorbed in the bases of
cells B and C, which have narrower band gaps than the emitters.
Therefore, the advantages of using heterojunction subcells are: (i)
the short wavelength response for both subcells will improve, and
(ii) the bulk of the radiation is more effectively absorbed and
collected in the narrower band gap base. The effect will be to
increase the short circuit current J.sub.sc.
[0143] On top of the cell B is deposited a BSF layer 113 which
performs the same function as the BSF layer 109. The p++/n++ tunnel
diode layers 114a and 114b respectively are deposited over the BSF
layer 113, similar to the layers 109a and 109b, forming an ohmic
circuit element to connect subcell B to subcell C. The layer 114a
may be composed of p++ AlGaAs, and layer 114b may be composed of
n++ GaAs or GaInP.
[0144] In some embodiments a barrier layer 115, composed of n-type
(Al)GaInP, is deposited over the tunnel diode 114a/I 14b, to a
thickness of about 0.5 micron. Such barrier layer is intended to
prevent threading dislocations from propagating, either opposite to
the direction of growth into the middle and top subcells B and C,
or in the direction of growth into the bottom subcell A, and is
more particularly described in copending U.S. patent application
Ser. No. 11/860,183, filed Sep. 24, 2007.
[0145] A metamorphic layer (or graded interlayer) 116 is deposited
over the barrier layer 115. Layer 116 is preferably a
compositionally step-graded series of AlGaInAs layers, preferably
with monotonically changing lattice constant, so as to achieve a
gradual transition in lattice constant in the semiconductor
structure from subcell B to subcell C while minimizing threading
dislocations from occurring. In some embodiments, the band gap of
layer 116 is constant throughout its thickness, preferably
approximately equal to 1.5 eV, or otherwise consistent with a value
slightly greater than the band gap of the middle subcell B. One
embodiment of the graded interlayer may also be expressed as being
composed of (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs, with x and y
selected such that the band gap of the interlayer remains constant
at approximately 1.50 eV or other appropriate band gap. In other
embodiments, the graded interlayer is composed of
(In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs, with x and y selected such
that the band gap of the interlayer remains substantially constant
at 1.6 eV or approximately 1.6 eV (e.g., in the range of 1.55 eV to
1.65 eV).
[0146] In an alternative embodiment where the solar cell has only
two subcells, and the "middle" cell B is the uppermost or top
subcell in the final solar cell, wherein the "top" subcell B would
typically have a band gap of 1.8 to 1.9 eV, then the band gap of
the interlayer would remain constant at 1.9 eV.
[0147] In the inverted metamorphic structure described in the
Wanlass et al. paper cited above, the metamorphic layer consists of
nine compositionally graded GaInP steps, with each step layer
having a thickness of 0.25 micron. As a result, each layer of
Wanlass et al. has a different band gap. In one embodiment of the
present invention, the layer 116 is composed of a plurality of
layers of AlGaInAs, with monotonically changing lattice constant,
each layer having the same band gap, approximately 1.5 eV. In some
embodiments, the layer 116 is composed of a plurality of layers of
AlGaInAs, with monotonically changing lattice constant, each layer
having the same band gap, approximately 1.6 eV.
[0148] The advantage of utilizing a constant band gap material such
as AlGaInAs is that arsenide-based semiconductor material is much
easier to process from a manufacturing standpoint in standard
commercial MOCVD reactors than materials incorporating phosphorus,
while the small amount of aluminum in the band gap material assures
radiation transparency of the metamorphic layers.
[0149] Although one embodiment of the present disclosure utilizes a
plurality of layers of AlGaInAs for the metamorphic layer 116 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 B to subcell C. Other embodiments of the present disclosure
may utilize continuously graded, as opposed to step graded,
materials. More generally, the graded interlayer may be composed of
any of the As, P, N, Sb based III-V compound semiconductors subject
to the constraints of having the in-plane lattice parameter greater
or equal to that of the second solar cell and less than or equal to
that of the third solar cell, and having a band gap energy greater
than that of the second solar cell.
[0150] In another embodiment of the present disclosure, an optional
second barrier layer 117 may be deposited over the AlGaInAs
metamorphic layer 116. The second barrier layer 117 will typically
have a different composition than that of barrier layer 115, and
performs essentially the same function of preventing threading
dislocations from propagating. In one embodiment, barrier layer 117
is n+ type GaInP. Further, each barrier layer in this and other
implementations described here, has a composition that differs from
the composition of an adjacent graded interlayer and/or adjacent
solar subcell layer.
[0151] A window layer 118 preferably composed of n+ type GaInP is
then deposited over the barrier layer 117 (or directly over layer
116, in the absence of a second barrier layer). This window layer
operates to reduce the recombination loss in subcell "C". It should
be apparent to one skilled in the art that additional layers may be
added or deleted in the cell structure without departing from the
scope of the present disclosure.
[0152] On top of the window layer 118, the layers of cell C are
deposited: the n+ emitter layer 119, and the p-type base layer 120.
These layers are preferably composed of n+ type GaInAs and p+ type
GaInAs respectively, or n+ type GaInP and p type GaInAs for a
heterojunction subcell, although other suitable materials
consistent with lattice constant and band gap requirements may be
used as well. The doping profile of layers 119 and 120 will be
discussed in connection with FIG. 20.
[0153] A BSF layer 121, preferably composed of AlGaInAs, is then
deposited on top of the cell C, the BSF layer performing the same
function as the BSF layers 108 and 113.
[0154] The p++/n++ tunnel diode layers 122a and 122b respectively
are deposited over the BSF layer 121, similar to the layers 114a
and 114b, forming an ohmic circuit element to connect subcell C to
subcell D. The layer 122a is composed of p++ AlGaInAs, and layer
122b is composed of n++ GaInP.
[0155] FIG. 2B depicts a cross-sectional view of the solar cell of
FIG. 2A after the next sequence of process steps. In some
embodiments a barrier layer 123, preferably composed of n-type
GaInP, is deposited over the tunnel diode 122a/122b, to a thickness
of about 0.5 micron. Such barrier layer is intended to prevent
threading dislocations from propagating, either opposite to the
direction of growth into the top and middle subcells A, B and C, or
in the direction of growth into the subcell D, and is more
particularly described in copending U.S. patent application Ser.
No. 11/860,183, filed Sep. 24, 2007.
[0156] A metamorphic layer (or graded interlayer) 124 is deposited
over the barrier layer 123. Layer 124 is preferably a
compositionally step-graded series of AlGaInAs layers, preferably
with monotonically changing lattice constant, so as to achieve a
gradual transition in lattice constant in the semiconductor
structure from subcell C to subcell D while minimizing threading
dislocations from occurring. In some embodiments the band gap of
layer 124 is constant throughout its thickness, preferably
approximately equal to 1.1 eV, or otherwise consistent with a value
slightly greater than the band gap of the middle subcell C. One
embodiment of the graded interlayer may also be expressed as being
composed of (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs, with x and y
selected such that the band gap of the interlayer remains constant
at approximately 1.1 eV or other appropriate band gap. In some
implementations, it is advantageous for the band gap of the first
graded interlayer 116 to be substantially constant at approximately
1.6 eV and the band gap of the second graded interlayer 124 to be
substantially constant at approximately 1.1 eV.
[0157] A window layer 125 preferably composed of n+ type AlGaInAs
is then deposited over layer 124 (or over a second barrier layer,
if there is one, disposed over layer 124,). This window layer
operates to reduce the recombination loss in the fourth subcell
"D". It should be apparent to one skilled in the art that
additional layers may be added or deleted in the cell structure
without departing from the scope of the present invention.
[0158] FIG. 2C depicts a cross-sectional view of the solar cell of
FIG. 2B after the next sequence of process steps. On top of the
window layer 125, the layers of cell D are deposited: the n+
emitter layer 126, and the p-type base layer 127. These layers are
preferably composed of n+ type GaInAs and p type GaInAs
respectively, although other suitable materials consistent with
lattice constant and band gap requirements may be used as well. The
doping profile of layers 126 and 127 will be discussed in
connection with FIG. 20.
[0159] Turning next to FIG. 2D, a BSF layer 128, preferably
composed of p+ type AlGaInAs, is then deposited on top of the cell
D, the BSF layer performing the same function as the BSF layers
108, 113 and 121.
[0160] Finally a high band gap contact layer 129, preferably
composed of p++ type AlGaInAs, is deposited on the BSF layer
128.
[0161] The composition of this contact layer 129 located at the
bottom (non-illuminated) side of the lowest band gap photovoltaic
cell (i.e., subcell "D" in the depicted embodiment) in a
multijunction photovoltaic cell, can be formulated to reduce
absorption of the light that passes through the cell, so that (i)
the backside ohmic metal contact layer below it (on the
non-illuminated side) will also act as a mirror layer, and (ii) the
contact layer doesn't have to be selectively etched off, to prevent
absorption.
[0162] It should be apparent to one skilled in the art, that
additional layer(s) may be added or deleted in the cell structure
without departing from the scope of the present invention.
[0163] FIG. 2E is a cross-sectional view of the solar cell of FIG.
2D after the next process step in which a metal contact layer 130
is deposited over the p+ semiconductor contact layer 129. The metal
is the sequence of metal layers Ti/Au/Ag/Au.
[0164] Also, the metal contact scheme chosen is one that has a
planar interface with the semiconductor, after heat treatment to
activate the ohmic contact. This is done so that (1) a dielectric
layer separating the metal from the semiconductor doesn't have to
be deposited and selectively etched in the metal contact areas; and
(2) the contact layer is specularly reflective over the wavelength
range of interest.
[0165] FIG. 2F is a cross-sectional view of the solar cell of FIG.
2E after the next process step in which an adhesive layer 131 is
deposited over the metal layer 130. The adhesive may be CR 200
(manufactured by Brewer Science, Inc. of Rolla, Mo.).
[0166] In the next process step, a surrogate substrate 132,
preferably sapphire, is attached. Alternatively, the surrogate
substrate may be GaAs, Ge or Si, or other suitable material. The
surrogate substrate is about 40 mils in thickness, and is
perforated with holes about 1 mm in diameter, spaced 4 mm apart, to
aid in subsequent removal of the adhesive and the substrate. Of
course, surrogate substrates with other thicknesses and perforation
configurations may be used as well. As an alternative to using an
adhesive layer 131, a suitable substrate (e.g., GaAs) may be
eutectically or permanently bonded to the metal layer 130.
[0167] FIG. 2G is a cross-sectional view of the solar cell of FIG.
2F after the next process step in which the original substrate is
removed, in one embodiment, by a sequence of lapping and/or etching
steps in which the substrate 101, and the buffer layer 103 are
removed. The choice of a particular etchant is growth substrate
dependent.
[0168] As described above, in some implementations, it is
particularly advantageous for the band gap of the first graded
interlayer 116 to be substantially constant at approximately 1.6 eV
(i.e., 1.6 eV.+-.3% or in the range of about 1.55 eV to 1.65 eV)
and the band gap of the second graded interlayer 124 to be
substantially constant at approximately 1.1 eV (i.e., in the range
of 1.05 eV to 1.15 eV).
[0169] Four-junction IMM solar cell devices, each of which included
two different graded interlayers, were fabricated by an IMM growth
process. The devices included the following semiconductor layers
deposited over one another: an upper solar subcell (A) of
GaInP.sub.2 having a band gap of 1.9 eV; a second solar subcell (B)
of GaAs having a band gap of 1.41 eV; a first graded interlayer of
InGaAlAs below the second subcell; a third solar subcell (C) of
In.sub.0.285Ga.sub.0.715As having a band gap of 1.02 eV; a second
graded interlayer of InGaAlAs below the third subcell; and a lower
solar subcell (D) of In.sub.0.57Ga.sub.0.43As having a band gap of
0.67 eV. The devices included a GaAs substrate having a band gap of
1.41 eV. In each sample, the band gap of the first graded
interlayer was substantially constant at about 1.4 eV, 1.5 eV or
1.6 eV, whereas the band gap of the second graded interlayer was
substantially constant at about 1.0 eV, 1.1 eV or 1.2 eV. The
higher band gap graded interlayer was disposed between the 1.41 eV
and 1.02 eV junctions, and the lower band gap graded interlayer was
disposed between the 1.02 eV and the 0.67 eV junctions. Solar cells
with the following band gap combinations were processed into
working devices:
TABLE-US-00001 Combination ID: 1 2 3 4 5 6 7 8 9 Band gap (eV) of
1.4 1.5 1.6 1.4 1.5 1.6 1.4 1.5 1.6 first grading interlayer: Band
gap (eV) of 1.0 1.0 1.0 1.1 1.1 1.1 1.2 1.2 1.2 second grading
interlayer:
[0170] The performance of the cells was measured, and the cell
conversion efficiency was calculated, as indicated by FIG. 26.
[0171] The comparative test data of FIG. 26 indicate that the
optimal combination of band gaps for the first and second graded
interlayers occurs when the band gap of the first graded interlayer
is in the range of about 1.5 eV-1.6 eV, and the band gap of the
second graded interlayer is approximately 1.1 eV (i.e., combination
IDs 5 and 6). These particular combination of band gaps provides a
relatively large increase in conversion efficiency. This is
advantageous, particularly in the field of solar cell devices in
which even smaller improvements in conversion efficiency are
typically considered significant. Of particular surprise is the
significant improvement in conversion efficiency obtained for
devices in which the first graded interlayer has a band gap of 1.6
eV and the second graded interlayer has a band gap of 1.1 eV. The
combination of 1.6 eV and 1.1 eV band gaps for the graded
interlayers can be advantageous for multi-junctions devices,
including five-junction and six-junction devices as well as for
three-junction and four-junction devices. In general, it can be
advantageous for the first graded interlayer to have a band gap of
1.6 eV (.+-.3%) and the second graded interlayer to have a band gap
of 1.1 eV (.+-.3%).
[0172] In some implementations, forming a particular one of the
graded interlayers includes selecting an interlayer composed of
InGaAlAs, and identifying a set of compositions of the formula
(In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs defined by specific values
of x and y, wherein 0<x<1 and 0<y<1, each composition
having the same particular band gap (e.g., a particular value in
the range 1.6 eV.+-.3% eV for the first graded interlayer or a
particular value in the range 1.1 eV.+-.3% eV for the second graded
interlayer). Forming a particular graded interlayer also can
include identifying appropriate lattice constants for either side
of the graded interlayer so that they match, respectively, the
adjacent solar subcells. Forming a particular graded interlayer
further can include identifying a subset of compositions of the
formula (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs having the
particular band gap that are defined by specific values of x and y,
wherein 0<x<1 and 0<y<1, wherein the subset of
compositions have lattice constants ranging from the identified
lattice constant that matches the solar subcell on one side of the
graded interlayer to the identified lattice constant that matches
the solar subcell on the opposing side of the graded
interlayer.
[0173] In some instances, one or more of the steps can be performed
by a computer program. For example, identifying a set of
compositions of the formula (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs
defined by specific values of x and y, each composition having the
same particular band gap (e.g., a particular value in the range 1.6
eV.+-.3% eV), can include using a computer program. Likewise,
identifying a subset of compositions of the formula
(In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs having the particular band
gap that are defined by specific values of x and y, wherein the
subset of compositions have lattice constants ranging from the
identified lattice constant that matches the solar subcell on one
side of the graded interlayer to the identified lattice constant
that matches the solar subcell on the opposing side of the graded
interlayer, can include using a computer program.
[0174] The fabrication method can include precisely controlling and
incrementally adjusting a mole fraction of each of indium (In),
gallium (Ga) and aluminum (Al) to form a continuously graded
interlayer as the first, second or other graded interlayer. Forming
a particular one of the graded interlayers also can include
providing a metal organic chemical vapor deposition (MOCVD) system
configured to independently control the flow of source gases for
gallium, indium, aluminum, and arsenic, and selecting a reaction
time, a temperature and a flow rate for each source gas to form a
continuously graded interlayer as the particular graded
interlayer.
[0175] FIG. 2H is a cross-sectional view of the solar cell of FIG.
2G with the orientation with the surrogate substrate 132 being at
the bottom of the Figure. Subsequent Figures in this application
will assume such orientation.
[0176] Five Junction Solar Cell with Two Metamorphic Layers
[0177] FIG. 3A through 3B depicts the sequence of steps in forming
a multijunction solar cell in an embodiment according to the
present disclosure in which a five junction solar cell with two
metamorphic buffer layers is fabricated.
[0178] FIG. 3A depicts the initial sequence of steps in forming a
multijunction solar cell in an embodiment according to the present
disclosure in which the first three cells of one embodiment of a
five junction solar cell is fabricated.
[0179] FIG. 3B is a cross-sectional view of the solar cell of FIG.
3A in the embodiment after the next sequence of process steps in
which the lower two subcells are grown.
[0180] FIG. 3C is a cross-sectional view of a solar cell in an
embodiment of a five junction solar cell with three metamorphic
layers.
[0181] Turning first to FIG. 3A, the sequential formation of the
three subcells A, B and C on a GaAs growth substrate is depicted.
More particularly, there is shown a substrate 201, which is
preferably gallium arsenide (GaAs), but may also be germanium (Ge)
or other suitable material. For GaAs, the substrate is preferably a
15.degree. off-cut substrate, that is to say, its surface is
orientated 15.degree. off the (100) plane towards the (111)A plane,
as more fully described in U.S. patent application Ser. No.
12/047,944, filed Mar. 13, 2008.
[0182] In the case of a Ge substrate, a nucleation layer (not
shown) is deposited directly on the substrate 201. On the
substrate, or over the nucleation layer (in the case of a Ge
substrate), a buffer layer 202 and an etch stop layer 203 are
further deposited. In the case of GaAs substrate, the buffer layer
202 is preferably GaAs. In the case of Ge substrate, the buffer
layer 202 is preferably GaInAs. A contact layer 204 of GaAs is then
deposited on layer 203, and a window layer 205 of AlInP is
deposited on the contact layer. The subcell A, which will be the
upper first solar subcell of the structure, consisting of an n+
emitter layer 206 and a p-type base layer 207, is then epitaxially
deposited on the window layer 205. The subcell A is generally
lattice matched to the growth substrate 201.
[0183] It should be noted that the multijunction solar cell
structure could be formed by any suitable combination of group III
to V elements listed in the periodic table subject to lattice
constant and band gap requirements, wherein the group III includes
boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium
(T). The group IV includes carbon (C), silicon (Si), germanium
(Ge), and tin (Sn). The group V includes nitrogen (N), phosphorus
(P), arsenic (As), antimony (Sb), and bismuth (Bi).
[0184] In one embodiment, the emitter layer is composed of AlGaInP
and the base layer 207 is composed of AlGaInP, and thus the p/n
junction of this subcell is a homojunction. More particularly, the
emitter layer 206 is composed of two regions: an n+ type emitter
region 206a directly grown on the window layer 205, and an n type
emitter region 206b directly grown on the emitter region 206a. The
doping profile of the different emitter regions 206a and 206b, and
base layer 207 according to the present disclosure will be
discussed in conjunction with FIG. 20A.
[0185] In some embodiments, a spacer layer 206c composed of
unintentionally doped AlGaInP is then grown directly on top of the
n type emitter region 206b.
[0186] The base layer 207 is composed of AlGaInP is grown over the
spacer layer 206c.
[0187] In some embodiments, the band gap of the base layer 207 is
1.92 eV or greater.
[0188] In some embodiments, the band gap of the base of the upper
first solar subcell is equal to or greater than 2.05 eV.
[0189] In some embodiments, the emitter of the upper first solar
subcell is composed of a first region in which the doping is graded
from 3.times.10.sup.18 to 1.times.10.sup.18 free carriers per cubic
centimeter, and a second region directly disposed over the first
region in which the doping is constant at 1.times.10.sup.17 free
carriers per cubic centimeter.
[0190] In some embodiments, the first region of the emitter of the
upper first solar subcell is directly adjacent to a window
layer.
[0191] In some embodiments, the emitter of the upper first solar
subcell has a thickness of 80 nm.
[0192] In some embodiments, the base of the upper first solar
subcell has a thickness of less than 400 nm.
[0193] In some embodiments, the base of the upper first solar
subcell has a thickness of 260 nm.
[0194] In some embodiments, the emitter section of the upper first
solar subcell has a first region in which the doping is graded, and
a second region directly disposed over the first region in which
the doping is constant.
[0195] Subcell A will ultimately become the "top" subcell of the
inverted metamorphic structure after completion of the process
steps according to the present invention to be described
hereinafter.
[0196] On top of the base layer 207 a back surface field ("BSF")
layer 208 preferably p+ AlInP is deposited and used to reduce
recombination loss.
[0197] The BSF layer 208 drives minority carriers from the region
near the base/BSF interface surface to minimize the effect of
recombination loss. In other words, the BSF layer 208 reduces
recombination loss at the backside of the solar subcell A and
thereby reduces the recombination in the base.
[0198] On top of the BSF layer 208 a sequence of heavily doped
p-type and n-type layers 209a and 209b is deposited that forms a
tunnel diode, i.e. an ohmic circuit element that forms an
electrical connection between subcell A to subcell B. Layer 209a
may be composed of p++ AlGaAs, and layer 209b may be composed of
n++ GaInP.
[0199] On top of the tunnel diode layers 209 a window layer 210 is
deposited, which may be n+ AlInP. The advantage of utilizing AlInP
as the material constituent of the window layer 210 is that it has
an index of refraction that closely matches the adjacent emitter
layer 211, as more fully described in U.S. patent application Ser.
No. 12/258,190, filed Oct. 24, 2008. The window layer 210 used in
the subcell B also operates to reduce the interface recombination
loss. It should be apparent to one skilled in the art, that
additional layer(s) may be added or deleted in the cell structure
without departing from the scope of the present disclosure.
[0200] On top of the window layer 210 the layers of subcell B are
deposited: the n+ type emitter layer 211 and the p-type base layer
212. These layers are composed of AlGaAs and AlGaAs respectively
(for a GaAs substrate), although any other suitable materials
consistent with lattice constant and band gap requirements may be
used as well. Thus, subcell B may be composed of a GaAs, GaInP,
GaInAs, GaAsSb, or GaInAsN emitter region and a GaAs, GaInAs,
GaAsSb, or GaInAsN base region. The doping profile of layers 211
and 212 according to the present disclosure will be discussed in
conjunction with FIG. 20B.
[0201] In embodiments of the present disclosure, similarly to the
structure disclosed in U.S. patent application Ser. No. 12/023,772,
the subcell B may be a heterostructure with an GaInP emitter and
its window is converted from AlInP to GaInP. This modification
eliminated the refractive index discontinuity at the window/emitter
interface of the middle subcell, as more fully described in U.S.
patent application Ser. No. 12/258,190, filed Oct. 24, 2008.
Moreover, the window layer 210 is preferably doped three times that
of the emitter 211 to move the Fermi level up closer to the
conduction band and therefore create band bending at the
window/emitter interface which results in constraining the minority
carriers to the emitter layer.
[0202] On top of the cell B is deposited a BSF layer 213 which
performs the same function as the BSF layer 209. The p++/n++ tunnel
diode layers 214a and 214b respectively are deposited over the BSF
layer 213, similar to the layers 209a and 209b, forming an ohmic
circuit element to connect subcell B to subcell C. The layer 214a
may be composed of p++ AlGaAs, and layer 214b may be composed of
n++ GaInP.
[0203] A window layer 215 composed of n+ type GaInP is then
deposited over the tunnel diode layers 214a, 214b. This window
layer operates to reduce the recombination loss in subcell "C". It
should be apparent to one skilled in the art that additional layers
may be added or deleted in the cell structure without departing
from the scope of the present disclosure.
[0204] On top of the window layer 215, the layers of subcell C are
deposited: the n+ emitter layer 216, and the p-type base layer 217.
These layers are composed of n+ type GaInP and p+ type GaAs
respectively, although other suitable materials consistent with
lattice constant and band gap requirements may be used as well. The
doping profile of layers 216 and 217 will be discussed in
connection with FIG. 27B.
[0205] A BSF layer 218, preferably composed of AlGaAs, is then
deposited on top of the cell C, the BSF layer performing the same
function as the BSF layers 208 and 213.
[0206] The p++/n++ tunnel diode layers 219a and 219b respectively
are deposited over the BSF layer 218, similar to the layers 214a
and 214b, forming an ohmic circuit element to connect subcell C to
subcell D. The layer 219a is preferably composed of p AlGaAs, and
layer 219b is preferably composed of n++ GaInP.
[0207] FIG. 3B is a cross-sectional view of the solar cell of FIG.
3A in a first embodiment of a five junction solar cell after the
next sequence of process steps in which the lower two subcells D
and E are grown on the initial structure of FIG. 3A.
[0208] Turning to FIG. 3B, a sequence of layers 220 through 235 are
grown on top of the tunnel diode layers 219a and 219b.
[0209] In some embodiments a barrier layer 220, composed of n-type
(Al)GaInP, is deposited over the tunnel diode 219a/219b, to a
thickness of about 0.5 micron. Such barrier layer is intended to
prevent threading dislocations from propagating, either opposite to
the direction of growth into the middle subcells B and C, or in the
direction of growth into the lower subcell D and E, and is more
particularly described in copending U.S. patent application Ser.
No. 11/860,183, filed Sep. 24, 2007.
[0210] A first metamorphic layer (or graded interlayer) 221 is
deposited over the barrier layer 220. Layer 221 is preferably a
compositionally step-graded series of AlGaInAs layers, preferably
with monotonically changing lattice constant, so as to achieve a
gradual transition in lattice constant in the semiconductor
structure from subcell C to subcell D while minimizing threading
dislocations from occurring. Stated another way, the layer 221 has
a lattice constant on one surface adjacent to subcell C which
matches that of subcell C, and a lattice constant on the opposing
surface adjacent to subcell D which matches that of subcell D, and
a gradation in lattice constant throughout its thickness. In some
embodiments, the band gap of layer 221 is constant throughout its
thickness, preferably approximately equal to 1.5 eV, or otherwise
consistent with a value slightly greater than the band gap of the
middle subcell C. One embodiment of the graded interlayer may also
be expressed as being composed of
Al.sub.y(Ga.sub.xIn.sub.1-x).sub.1-yAs, or alternatively written as
(In.sub.xGa.sub.1-x).sub.1-yAl.sub.yAs, with the positive values
for x and y selected such that the band gap of the interlayer
remains constant at approximately 1.50 eV or other appropriate band
gap.
[0211] In the inverted metamorphic structure described in the
Wanlass et al. paper cited above, the metamorphic layer consists of
nine compositionally graded GaInP steps, with each step layer
having a thickness of 0.25 micron. As a result, each layer of
Wanlass et al. has a different band gap. In one embodiment of the
present disclosure, the layer 221 is composed of a plurality of
layers of AlGaInAs, with monotonically changing lattice constant,
each layer having the same band gap, approximately 1.5 eV. More
particularly, the first graded interlayer 221 is composed of
Al.sub.y(Ga.sub.xIn.sub.1-x).sub.1-yAs with the values for x and y
between 0 and 1 selected such that the band gap of each sublayer in
the interlayer remains constant throughout its thickness.
[0212] In some embodiments, the band gap of the first graded
interlayer remains constant at 1.5 eV or 1.6 eV throughout the
thickness of the first graded interlayer.
[0213] Since the present disclosure (and the related applications
noted above) are directed to high volume manufacturing processes
using metalorganic vapor phase epitaxy (MOVPE) reactors to form the
solar cell epitaxial layers, a short discussion of some of the
considerations associated with such processes and methods
associated with the formation of the graded interlayer(s) are in
order here.
[0214] First, it should be noted that the advantage of utilizing an
interlayer material such as AlGaInAs is that arsenide-based
semiconductor material is much easier to process from a
manufacturing standpoint using present state-of-the-art high volume
manufacturing metalorganic vapor phase epitaxy (MOVPE) reactors
than either the AlGaInAsP, or GaInP compounds, or in general any
material including phosphorus. Simply stated, the use of a III-V
arsenide compound is much more desirable than a III-V phosphide
compound from the perspectives of cost, ease of growth, reactor
maintenance, waste handling and personal safety.
[0215] The cost advantage of the use of the AlGaInAs quaternary
grading material relative to a GaInP ternary grading material, as
an example, is a consequence of several factors. First, the use of
a GaInP grading approach requires indium mole fractions of the
order of 60% (i.e., the required material is
Ga.sub.0.04In.sub.0.6P) whereas the use of the AlGaInAs quaternary
requires only 15% indium (i.e., the required material is
Al.sub.y(Ga.sub.0.85In.sub.0.15).sub.1-yAs). In addition to the
difference in the material itself, there is a further difference in
the amount of precursor gases (trimethylgallium, trimethylindium,
and arsine) that must be input to the reactor in order to achieve
the desired composition. In particular, the ratio of the amount of
precursor gases into the reactor to provide Group V elements, to
the amount of precursor gases to provide Group III elements (such
ratio being referred to as the "input V/III ratio") is typically
five to ten times greater to produce a phosphide compound compared
to producing an arsenide compound. As a illustrative quantification
of the cost of producing a phosphide compound in a commercial
operational MOPVE reactor process compared to the cost of producing
an arsenide compound, Table 1 below presents the typical pro-form a
costs of each element of the AlGaInAs and GaInP compounds for
producing a graded interlayer of the type described in the present
disclosure expressed on a per mole basis. Of course, like many
commodities, the price of chemical compounds fluctuate from time to
time and vary in different geographic locations and countries and
from supplier to supplier. The prices used in Table I are
representative for purchases in commercial quantities in the United
States at the time of the present disclosure. The cost calculations
make the assumption (typical for epitaxial processes using current
commercial MOVPE reactors) that the input V/III ratios are 20 and
120 for the arsenide and phosphide compounds respectively. Such a
choice of value of the ratio is merely illustrative for a typical
process, and some processes may use even higher ratios for
producing a graded interlayer of the type described in the present
disclosure. The practical consequence of the inlet V/III ratio is
that one will use 20 moles of As to one (1) mole of AlGaIn in the
formation of the Applicant's quaternary material AlGaInAs, or 120
moles of P to 1 mole of Gain in the formation of the interlayer
using the ternary material GaInP. These assumptions along with the
molar cost of each of the constituent elements indicate that the
cost of fabrication of the AlGaInAs based grading interlayer will
be approximately 25% of the cost of fabrication of a similar
phosphide based grading interlayer. Thus, there is an important
economic incentive from a commercial and manufacturing perspective
to utilize an arsenide compound as opposed to a phosphide compound
for the grading interlayer.
TABLE-US-00002 TABLE 1 Cost estimate of one mole of each of the
AlGaInAs and GaInP grading layers Cost Molecular Mole Cost
Molecular Mole Element MW (gms) S/gm Cost/mole ($) MF AlGaIn of
Al.17Ga.68In.15 MF GaInP of Ga.4In.6 Al 27 10.2 275.4 0.17 46.818 0
0 Ga 70 2.68 187.6 0.68 127.568 0.4 75.04 In 115 28.05 3225.75 0.15
483.8625 0.6 1935.45 Approx OM 658.2485 2010.49 Cost/mole =
Cost/mole V/III Cost/mole of ($) ratio Cost/mole of Arsenic
phosphorus AsH3 $7.56 20 $151.20 $151.20 PH3 $9.49 120 $1,138.80
$1,138.54 Approx $809.45 $3,149.03 cost/molecular mole =
[0216] The "ease of growth" of an arsenide compound as opposed to a
phosphide compound for the grading interlayer in a high volume
manufacturing environment is another important consideration and is
closely related to issues of reactor maintenance, waste handling
and personal safety. More particularly, in a high volume
manufacturing environment the abatement differences between
arsenide and phosphide based processes affect both cost and safety.
The abatement of phosphorus is more time consuming, and hazardous
than that required for arsenic. Each of these compounds builds up
over time in the downstream gas flow portions of the MOVPE growth
reactor. As such, periodic reactor maintenance for removal of these
deposited materials is necessary to prevent adverse affects on the
reactor flow dynamics, and thus the repeatability and uniformity of
the epitaxial structures grown in the reactor. The difference in
handling of these waste materials is significant. Arsenic as a
compound is stable in air, non-flammable, and only represents a
mild irritant upon skin contact. Phosphorus however, must be
handled with considerably more care. Phosphorus is very flammable
and produces toxic fumes upon burning and it is only moderately
stable in air. Essentially the differences are manifest by the need
for special handling and containment materials and procedures when
handling phosphorus to prevent either combustion or toxic exposure
to this material whereas using common personal protection equipment
such as gloves, and a particle respirator easily accommodates the
handling of arsenic.
[0217] Another consideration related to "ease of growth" that
should be noted in connection with the advantages of a AlGaInAs
based grading interlayer process compared to a AlGaInAsP compound
derives from a frequently encountered issue when using an AlGaInAsP
compound: the miscibility gap. A miscibility gap will occur if the
enthalpy of mixing exceeds the entropy of mixing of two binary
compounds AC and BC, where A, B and C are different elements. It is
an established fact that the enthalpies of mixing of all ternary
crystalline alloys of the form A.sub.xB.sub.1-xC, based upon the
binary semiconductor forms AC and BC are positive leading to
miscibility gaps in these compounds. See, for example, the
discussion in reference [1] noted below. In this example, the
letters A and B designate group III elements and letter C
designates a group V element. As such, mixing of the binary
compounds is said to occur on the group III sublattice. However,
because OMVPE growth takes place under non-equilibrium conditions,
the miscibility gap is not really a practical problem for accessing
the entire ternary semiconductor phase space. For the case of
quaternary compounds of the form A.sub.xB.sub.1-xC.sub.yD.sub.1-y
where mixing of the binary alloys, AC, AD, BC, and BD occurs on
both the group III and group V sublattices, the immiscibility
problem is accentuated. Specifically for the GaP, InP, GaAs, InAs
system, the region of immiscibility is quite large at growth
temperatures appropriate for the OMVPE technique. See, for example,
the discussion in reference [2] noted below. The resulting
miscibility gap will prevent one from producing the requisite
AlGaInAsP compounds needed for optical transparent grading of the
IMM solar cell.
REFERENCES
[0218] [1] V. A. Elyukhin, E. L. Portnoi, E. A. Avrutin, and J. H.
Marsh, J. Crystal Growth 173 (1997) pp 69-72. [0219] [2] G. B.
Stringfellow, Organometallic Vapor-Phase Epitaxy (Academic Press,
New York 1989).
[0220] The fabrication of a step graded (or continuous graded)
interlayer in an MOCVD process can be more explicitly described in
a sequence of conceptual and operational steps which we describe
here for pedagogical clarity. First, the appropriate band gap for
the interlayer must be selected. In one of the disclosed
embodiments, the desired constant band gap is 1.5 eV; in other
embodiments, the desired constant band gap may be, for example, 1.6
eV or 1.1 eV. Second, the most appropriate material system (i.e.,
the specific semiconductor elements to form a compound
semiconductor alloy) must be identified. In the disclosed
embodiment, these elements are Al, Ga, In, and As. Third, a
computation must be made, for example using a computer program, to
identify the class of compounds of
Al.sub.y(Ga.sub.xIn.sub.1-x).sub.1-yAs for specific x and y that
have a band gap of 1.5 eV. An example of such a computer program
output that provides a very rough indication of these compounds is
illustrated in FIG. 22. Fourth, based upon the lattice constants of
the epitaxial layers adjoining the graded interlayer, a
specification of the required lattice constants for the top surface
of the interlayer (to match the adjacent semiconductor layer), and
the bottom surface of the interlayer (to match the adjacent
semiconductor layer) must be made. Fifth, based on the required
lattice constants, the compounds of
Al.sub.y(Ga.sub.xIn.sub.1-x).sub.1-yAs for specific x and y that
have a band gap of 1.5 eV may be identified. Again, a computation
must be made, and as an example, the data may be displayed in a
graph such as FIG. 21 representing the Al, Ga and In mole fractions
versus the Al to In mole fraction in a AlGaInAs material system
that is necessary to achieve a constant 1.5 eV band gap. Assuming
there is a small number (e.g. typically in the range of seven,
eight, nine, ten, etc.) of steps or grades between the top surface
and the bottom surface, and that the lattice constant difference
between each step is made equal, the bold markings in FIG. 21
represent selected lattice constants for each successive sublayer
in the interlayer, and the corresponding mole fraction of Al, Ga
and In needed to achieve that lattice constant in that respective
sublayer may be readily obtained by reference to the axes of the
graph. Thus, based on an analysis of the data in FIG. 21, the
reactor may be programmed to introduce the appropriate quantities
of precursor gases (as determined by flow rates at certain timed
intervals) into the reactor so as to achieve a desired specific
Al.sub.y(Ga.sub.xIn.sub.1-x).sub.1-yAs composition in that sublayer
so that each successive sublayer maintains the constant band gap of
1.5 eV and a monotonically increasing lattice constant. The
execution of this sequence of steps, with calculated and
determinate precursor gas composition, flow rate, temperature, and
reactor time to achieve the growth of a
Al.sub.y(Ga.sub.xIn.sub.1-x).sub.1-yAs composition of the
interlayer with the desired properties (lattice constant change
over thickness, constant band gap over the entire thickness), in a
repeatable, manufacturable process, is not to be minimalized or
trivialized.
[0221] Although one embodiment of the present disclosure utilizes a
plurality of layers of AlGaInAs for the metamorphic layer 221 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, N, Sb based III-V compound semiconductors subject to
the constraints of having the in-plane lattice parameter greater or
equal to that of the third solar cell and less than or equal to
that of the fourth solar cell, and having a band gap energy greater
than that of the third solar cell.
[0222] In one embodiment of the present disclosure, an optional
second barrier layer 222 may be deposited over the AlGaInAs
metamorphic layer 221. The second barrier layer 222 will typically
have a different composition than that of barrier layer 220, and
performs essentially the same function of preventing threading
dislocations from propagating. In one embodiment, barrier layer 222
is n+ type GaInP.
[0223] A window layer 223 preferably composed of n+ type GaInP is
then deposited over the second barrier layer, if there is one,
disposed over layer 221. This window layer operates to reduce the
recombination loss in the fourth subcell "D". It should be apparent
to one skilled in the art that additional layers may be added or
deleted in the cell structure without departing from the scope of
the present invention.
[0224] On top of the window layer 223, the layers of cell D are
deposited: the n+ emitter layer 224, and the p-type base layer 225.
These layers are preferably composed of n+ type GaInAs and p type
GaInAs respectively, although other suitable materials consistent
with lattice constant and band gap requirements may be used as
well. The doping profile of layers 224 and 225 will be discussed in
connection with FIG. 20B.
[0225] A BSF layer 226, preferably composed of p+ type AlGaInAs, is
then deposited on top of the cell D, the BSF layer performing the
same function as the BSF layers 208, 213 and 218.
[0226] The p++/n++ tunnel diode layers 227a and 227b respectively
are deposited over the BSF layer 226, similar to the layers
214a/214b and 219a/219b, forming an ohmic circuit element to
connect subcell D to subcell E. The layer 227a is preferably
composed of p++ AlGaInAs, and layer 227b is preferably composed of
n++ GaInP.
[0227] In some embodiments a barrier layer 228, preferably composed
of n-type GaInP, is deposited over the tunnel diode 227a/227b, to a
thickness of about 0.5 micron. Such bather layer is intended to
prevent threading dislocations from propagating, either opposite to
the direction of growth into the middle subcells B, C and D, or in
the direction of growth into the subcell E, and is more
particularly described in copending U.S. patent application Ser.
No. 11/860,183, filed Sep. 24, 2007.
[0228] A second metamorphic layer (or graded interlayer) 229 is
deposited over the bather layer 228. Layer 229 is preferably a
compositionally step-graded series of AlGaInAs 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 E while minimizing threading
dislocations from occurring. In some embodiments the band gap of
layer 229 is constant throughout its thickness, preferably
approximately equal to 1.1 eV, or otherwise consistent with a value
slightly greater than the band gap of the middle subcell D. One
embodiment of the graded interlayer may also be expressed as being
composed of (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs, with x and y
selected such that the band gap of the interlayer remains constant
at approximately 1.1 eV or other appropriate band gap.
[0229] In one embodiment of the present disclosure, an optional
second barrier layer 230 may be deposited over the AlGaInAs
metamorphic layer 229. The second barrier layer 230 performs
essentially the same function as the first barrier layer 228 of
preventing threading dislocations from propagating. In one
embodiment, barrier layer 230 has not the same composition than
that of barrier layer 228, i.e. n+ type GaInP.
[0230] A window layer 231 preferably composed of n+ type GaInP is
then deposited over the barrier layer 230. This window layer
operates to reduce the recombination loss in the fifth subcell "E".
It should be apparent to one skilled in the art that additional
layers may be added or deleted in the cell structure without
departing from the scope of the present invention.
[0231] On top of the window layer 231, the layers of cell E are
deposited: the n+ emitter layer 232, and the p-type base layer 233.
These layers are preferably composed of n+ type GaInAs and p type
GaInAs respectively, although other suitable materials consistent
with lattice constant and band gap requirements may be used as
well.
[0232] The doping profile of layers 232 and 233 will be discussed
in connection with FIG. 20B.
[0233] A BSF layer 234, preferably composed of p+ type AlGaInAs, is
then deposited on top of the cell E, the BSF layer performing the
same function as the BSF layers 208, 213, 218, and 226.
[0234] Finally a high band gap contact layer 235, preferably
composed of p++ type AlGaInAs, is deposited on the BSF layer
234.
[0235] The composition of this contact layer 235 located at the
bottom (non-illuminated) side of the lowest band gap photovoltaic
cell (i.e., subcell "E" in the depicted embodiment) in a
multijunction photovoltaic cell, can be formulated to reduce
absorption of the light that passes through the cell, so that (i)
the backside ohmic metal contact layer below it (on the
non-illuminated side) will also act as a mirror layer, and (ii) the
contact layer doesn't have to be selectively etched off, to prevent
absorption.
[0236] It should be apparent to one skilled in the art, that
additional layer(s) may be added or deleted in the cell structure
without departing from the scope of the present invention.
[0237] The subsequent remaining steps in the fabrication of the
multijunction solar cell according to the illustrated embodiment,
including the deposition of metallization over the contact layer,
and the attachment of a surrogate substrate, will be discussed and
depicted at a later point in connection with FIG. 7 and subsequent
Figures.
[0238] Five Junction Solar Cell with Three Metamorphic Layers
[0239] FIG. 3C is a cross-sectional view of the solar cell of FIG.
3A in an embodiment of a five junction solar cell with three
metamorphic layers. The layers 201 through 214b of this embodiment
are substantially identical to those discussed in connection with
the embodiment of FIG. 3A, and therefore in the interest of brevity
of this disclosure, the description of such layers will not be
repeated here.
[0240] As depicted in FIG. 3C, the embodiment of a five junction
solar cell with three metamorphic layers, a sequence of layers 250
through 273 are grown on top of the tunnel diode layers 214a and
214b of the structure of FIG. 3A.
[0241] In some embodiments a barrier layer 250, composed of n-type
(Al)GaInP, is deposited over the tunnel diode 214a/214b, to a
thickness of about 1.0 micron. Such barrier layer is intended to
prevent threading dislocations from propagating, either opposite to
the direction of growth into the subcells A and B, or in the
direction of growth into the middle subcells C and D, and is more
particularly described in copending U.S. patent application Ser.
No. 11/860,183, filed Sep. 24, 2007.
[0242] A first metamorphic layer (or graded interlayer) 251 is
deposited over the barrier layer 250. Layer 251 is preferably a
compositionally step-graded series of AlGaInAs layers, preferably
with monotonically changing lattice constant, so as to achieve a
gradual transition in lattice constant in the semiconductor
structure from subcell B to subcell C while minimizing threading
dislocations from occurring. Stated another way, the layer 251 has
a lattice constant on one surface adjacent to subcell B which
matches that of subcell B, and a lattice constant on the opposing
surface adjacent to subcell C which matches that of subcell C, and
a gradation in lattice constant throughout its thickness. In some
embodiments, the band gap of layer 251 is constant throughout its
thickness, preferably approximately equal to 1.5 eV, or otherwise
consistent with a value slightly greater than the band gap of the
middle subcell B. One embodiment of the graded interlayer may also
be expressed as being composed of
(In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs, with x and y selected such
that the band gap of the interlayer remains constant at
approximately 1.50 eV or other appropriate band gap.
[0243] In the inverted metamorphic structure described in the
Wanlass et al. paper cited above, the metamorphic layer consists of
nine compositionally graded GaInP steps, with each step layer
having a thickness of 0.25 micron. As a result, each layer of
Wanlass et al. has a different band gap. In one embodiment of the
present disclosure, the layer 251 is composed of a plurality of
layers of AlGaInAs, with monotonically changing lattice constant,
each layer having the same band gap, approximately 1.5 eV.
[0244] The advantage of utilizing a constant band gap material such
as AlGaInAs is that arsenide-based semiconductor material is much
easier to process from a manufacturing standpoint in standard
commercial MOCVD reactors than materials incorporating phosphorus,
while the small amount of aluminum in the band gap material assures
radiation transparency of the metamorphic layers.
[0245] Although one embodiment of the present disclosure utilizes a
plurality of layers of AlGaInAs for the metamorphic layer 251 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 B to subcell C. 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, N, Sb based III-V compound semiconductors subject to
the constraints of having the in-plane lattice parameter greater or
equal to that of the second solar subcell and less than or equal to
that of the third solar subcellcell, and having a band gap energy
greater than that of the third solar cell.
[0246] In one embodiment of the present disclosure, an optional
second barrier layer 252 may be deposited over the AlGaInAs
metamorphic layer 251. The second barrier layer 252 will typically
have a different composition than that of barrier layer 250, and
performs essentially the same function of preventing threading
dislocations from propagating. In one embodiment, barrier layer 252
is n+ type GaInP.
[0247] A window layer 253 composed of n+ type GaInP is then
deposited over the barrier layer 253. This window layer operates to
reduce the recombination loss in the third subcell "C". It should
be apparent to one skilled in the art that additional layers may be
added or deleted in the cell structure without departing from the
scope of the present invention.
[0248] On top of the window layer 253, the layers of cell C are
deposited: the n+ emitter layer 254, and the p-type base layer 255.
These layers are preferably composed of n+ type GaInAs and p type
GaInAs respectively, although other suitable materials consistent
with lattice constant and band gap requirements may be used as
well.
[0249] The doping profile of layers 254 and 255 will be discussed
in connection with FIG. 20B.
[0250] A BSF layer 256, preferably composed of p+ type AlGaInAs, is
then deposited on top of the cell C, the BSF layer performing the
same function as the BSF layers 208, and 213.
[0251] The p++/n++ tunnel diode layers 257a and 257b respectively
are deposited over the BSF layer 256, similar to the layers
209a/209b and 214a/214b, forming an ohmic circuit element to
connect subcell C to subcell D. The layer 257a is preferably
composed of p++ AlGaInAs, and layer 257b is preferably composed of
n++ AlGaInAs.
[0252] In some embodiments a barrier layer 258, may be composed of
n-type GaInP, is deposited over the tunnel diode 257a/257b, to a
thickness of about 0.5 micron. Such barrier layer is intended to
prevent threading dislocations from propagating, either opposite to
the direction of growth into the middle subcells B and C, or in the
direction of growth into the subcell D, and is more particularly
described in copending U.S. patent application Ser. No. 11/860,183,
filed Sep. 24, 2007.
[0253] A second metamorphic layer (or graded interlayer) 259 is
deposited over the barrier layer 258. Layer 259 may be a
compositionally step-graded series of AlGaInAs layers, preferably
with monotonically changing lattice constant, so as to achieve a
gradual transition in lattice constant in the semiconductor
structure from subcell C to subcell D while minimizing threading
dislocations from occurring. In some embodiments the band gap of
layer 259 is constant throughout its thickness, preferably
approximately equal to 1.1 eV, or otherwise consistent with a value
slightly greater than the band gap of the middle subcell C. One
embodiment of the graded interlayer may also be expressed as being
composed of (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs, with x and y
selected such that the band gap of the interlayer remains constant
at approximately 1.1 eV or other appropriate band gap.
[0254] In one embodiment of the present disclosure, an optional
second bather layer 260 may be deposited over the AlGaInAs
metamorphic layer 259. The second barrier layer will typically have
a different composition than that of bather layer 258, and performs
essentially the same function of preventing threading dislocations
from propagating.
[0255] A window layer 261 composed of n+ type GaInP is then
deposited over the bather layer 260. This window layer operates to
reduce the recombination loss in the subcell "D". It should be
apparent to one skilled in the art that additional layers may be
added or deleted in the cell structure without departing from the
scope of the present invention.
[0256] On top of the window layer 260, the layers of cell D are
deposited: the n+ emitter layer 262, and the p-type base layer 263.
These layers are illustrated as being composed of n+ type GaInAs
and p type GaInAs respectively, although other suitable materials
consistent with lattice constant and band gap requirements may be
used as well. The doping profile of layers 260 and 261 will be
discussed in connection with FIG. 20B.
[0257] A BSF layer 264, may be composed of p+ type AlGaInAs, is
then deposited on top of the cell D, the BSF layer performing the
same function as the BSF layers 208, 213, and 256.
[0258] The p++/n++ tunnel diode layers 265a and 265b respectively
are deposited over the BSF layer 264, similar to the layers
209a/209b, 214a/214b, and 257a/257b, forming an ohmic circuit
element to connect subcell D to subcell E. The layer 265a may be
composed of p++ AlGaInAs, and layer 265b may be composed of n++
AlGaInAs.
[0259] In some embodiments a barrier layer 266, may be composed of
n-type GaInP, is deposited over the tunnel diode 265a/265b, to a
thickness of about 0.5 micron. Such barrier layer is intended to
prevent threading dislocations from propagating, either opposite to
the direction of growth into the middle subcells B, C and D, or in
the direction of growth into the subcell E, and is more
particularly described in copending U.S. patent application Ser.
No. 11/860,183, filed Sep. 24, 2007.
[0260] A third metamorphic layer (or graded interlayer) 267 is
deposited over the barrier layer 266. Layer 267 may be a
compositionally step-graded series of AlGaInAs 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 E while minimizing threading
dislocations from occurring. In some embodiments the band gap of
layer 267 is constant throughout its thickness, preferably
approximately equal to 1.1 eV, or otherwise consistent with a value
slightly greater than the band gap of the middle subcell D. One
embodiment of the graded interlayer may also be expressed as being
composed of (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs, with x and y
selected such that the band gap of the interlayer remains constant
at approximately 1.1 eV or other appropriate band gap.
[0261] In one embodiment of the present disclosure, an optional
second barrier layer 268 may be deposited over the AlGaInAs
metamorphic layer 267. The second barrier layer is a different
alloy than that of bather layer 266, and performs essentially the
same function of preventing threading dislocations from
propagating.
[0262] A window layer 269 preferably composed of n+ type AlGaInAs
is then deposited over the second bather layer 268, if there is one
disposed over layer 267, or directly over third metamorphic layer
267. This window layer operates to reduce the recombination loss in
the fifth subcell "E". It should be apparent to one skilled in the
art that additional layers may be added or deleted in the cell
structure without departing from the scope of the present
invention.
[0263] On top of the window layer 269, the layers of cell E are
deposited: the n+ emitter layer 270, and the p-type base layer 271.
These layers are preferably composed of n+ type GaInAs and p type
GaInAs respectively, although other suitable materials consistent
with lattice constant and band gap requirements may be used as
well. The doping profile of layers 270 and 271 will be discussed in
connection with FIG. 20B.
[0264] A BSF layer 272, preferably composed of p+ type AlGaInAs, is
then deposited on top of the cell E, the BSF layer performing the
same function as the BSF layers 208, 213, 256, and 264.
[0265] Finally a high band gap contact layer 273, preferably
composed of p++ type AlGaInAs, is deposited on the BSF layer
272.
[0266] The composition of this contact layer 273 located at the
bottom (non-illuminated) side of the lowest band gap photovoltaic
cell (i.e., subcell "E" in the depicted embodiment) in a
multijunction photovoltaic cell, can be formulated to reduce
absorption of the light that passes through the cell, so that (i)
the backside ohmic metal contact layer below it (on the
non-illuminated side) will also act as a mirror layer, and (ii) the
contact layer doesn't have to be selectively etched off, to prevent
absorption.
[0267] It should be apparent to one skilled in the art, that
additional layer(s) may be added or deleted in the cell structure
without departing from the scope of the present invention.
[0268] The subsequent remaining steps in the fabrication of the
multijunction solar cell according to the illustrated embodiment,
including the deposition of metallization over the contact layer,
and the attachment of a surrogate substrate, will be discussed and
depicted at a later point in connection with FIG. 7 and subsequent
Figures.
[0269] Six Junction Solar Cell with Three Metamorphic Layers
[0270] FIG. 4 depicts a multijunction solar cell in an embodiment
according to the present disclosure in which a six junction solar
cell with three metamorphic buffer layers is fabricated.
[0271] In particular, FIG. 4 depicts the sequential formation of
the six subcells A, B, C, D, E and F on a GaAs growth substrate.
The sequence of layers 302 through 314b that are grown on the
growth substrate are similar to layers 102 to 122b discussed in
connection with FIG. 3A, but the description of such layers with
new reference numbers will be repeated here for clarity of the
presentation.
[0272] More particularly, there is shown a substrate 301, which is
preferably gallium arsenide (GaAs), but may also be germanium (Ge)
or other suitable material. For GaAs, the substrate is preferably a
15.degree. off-cut substrate, that is to say, its surface is
orientated 15.degree. off the (100) plane towards the (111)A plane,
as more fully described in U.S. patent application Ser. No.
12/047,944, filed Mar. 13, 2008.
[0273] In the case of a Ge substrate, a nucleation layer (not
shown) is deposited directly on the substrate 301. On the
substrate, or over the nucleation layer (in the case of a Ge
substrate), a buffer layer 302 and an etch stop layer 303 are
further deposited. In the case of GaAs substrate, the buffer layer
302 is preferably GaAs. In the case of Ge substrate, the buffer
layer 302 is preferably GaInAs. A contact layer 304 of GaAs is then
deposited on layer 303, and a window layer 305 of AlInP is
deposited on the contact layer. The subcell A, which will be the
upper first solar subcell of the structure, consisting of an n+
emitter layer 306a and 306b and a p-type base layer 307, is then
epitaxially deposited on the window layer 305. The subcell A is
generally lattice matched to the growth substrate 301.
[0274] It should be noted that the multijunction solar cell
structure could be formed by any suitable combination of group III
to V elements listed in the periodic table subject to lattice
constant and band gap requirements, wherein the group III includes
boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium
(T). The group IV includes carbon (C), silicon (Si), germanium
(Ge), and tin (Sn). The group V includes nitrogen (N), phosphorus
(P), arsenic (As), antimony (Sb), and bismuth (Bi).
[0275] In one embodiment, the emitter layer is composed of AlGaInP
and the base layer 307 is composed of AlGaInP, and thus the p/n
junction of this subcell is a homojunction. More particularly, the
emitter layer is composed of two regions: an n+ type emitter region
306a directly grown on the window layer 305, and an n type emitter
region 306b directly grown on the emitter region 306a. The doping
profile of the different emitter regions 306a and 306b, and base
layer 307 according to the present disclosure will be discussed in
conjunction with FIG. 20A.
[0276] In some embodiments, a spacer layer 306c composed of
unintentionally doped AlGaInP is then grown directly on top of the
n type emitter region 306b.
[0277] The base layer 307 is composed of AlGaInP is grown over the
spacer layer 306c.
[0278] In some embodiments, the band gap of the base layer 307 is
1.92 eV or greater.
[0279] In some embodiments, the band gap of the base of the upper
first solar subcell is equal to or greater than 2.05 eV.
[0280] In some embodiments, the emitter of the upper first solar
subcell is composed of a first region in which the doping is graded
from 3.times.10.sup.18 to 1.times.10.sup.18 free carriers per cubic
centimeter, and a second region directly disposed over the first
region in which the doping is constant at 1.times.10.sup.17 free
carriers per cubic centimeter.
[0281] In some embodiments, the first region of the emitter of the
upper first solar subcell is directly adjacent to a window
layer.
[0282] In some embodiments, the emitter of the upper first solar
subcell has a thickness of 80 nm.
[0283] In some embodiments, the base of the upper first solar
subcell has a thickness of less than 400 nm.
[0284] In some embodiments, the base of the upper first solar
subcell has a thickness of 260 nm.
[0285] In some embodiments, the emitter section of the upper first
solar subcell has a first region in which the doping is graded, and
a second region directly disposed over the first region in which
the doping is constant.
[0286] Subcell A will ultimately become the "top" subcell of the
inverted metamorphic structure after completion of the process
steps according to the present invention to be described
hereinafter.
[0287] On top of the base layer 307 a back surface field ("BSF")
layer 308 preferably p+ AlInP is deposited and used to reduce
recombination loss.
[0288] The BSF layer 308 drives minority carriers from the region
near the base/BSF interface surface to minimize the effect of
recombination loss. In other words, the BSF layer 308 reduces
recombination loss at the backside of the solar subcell A and
thereby reduces the recombination in the base.
[0289] On top of the BSF layer 308 a sequence of heavily doped
p-type and n-type layers 309a and 309b is deposited that forms a
tunnel diode, i.e. an ohmic circuit element that forms an
electrical connection between subcell A to subcell B. Layer 309a
may be composed of p++ AlGaAs, and layer 309b may be composed of
n++ GaInP.
[0290] On top of the tunnel diode layers 309 a window layer 310 is
deposited, which may be n+ AlInP. The window layer 310 used in the
subcell B also operates to reduce the interface recombination loss.
It should be apparent to one skilled in the art, that additional
layer(s) may be added or deleted in the cell structure without
departing from the scope of the present disclosure.
[0291] On top of the window layer 310 the layers of subcell B are
deposited: the n+ type emitter layer 311 and the p-type base layer
312. These layers are preferably composed of AlGaAs and AlGaAs
respectively, although any other suitable materials consistent with
lattice constant and band gap requirements may be used as well.
Thus, subcell B may be composed of a GaAs, GaInP, GaInAs, GaAsSb,
or GaInAsN emitter region and a GaAs, GaInAs, GaAsSb, or GaInAsN
base region. The doping profile of layers 311 and 312 according to
the present disclosure will be discussed in conjunction with FIG.
20B.
[0292] On top of the cell B is deposited a BSF layer 313 which
performs the same function as the BSF layer 308. The p++/n++ tunnel
diode layers 314a and 314b respectively are deposited over the BSF
layer 308, similar to the layers 309a and 309b, forming an ohmic
circuit element to connect subcell B to subcell C. The layer 314a
may be composed of p++ AlGaAs, and layer 314b may be composed of
n++ GaInP.
[0293] A window layer 315 preferably composed of n+ type GaInP is
then deposited over the tunnel diode layers 314a/314b. This window
layer operates to reduce the recombination loss in subcell "C". It
should be apparent to one skilled in the art that additional layers
may be added or deleted in the cell structure without departing
from the scope of the present disclosure.
[0294] On top of the window layer 315, the layers of subcell C are
deposited: the n+ emitter layer 316, and the p-type base layer 317.
These layers are preferably composed of n+ type GaInP and p type
GaAs respectively, although other suitable materials consistent
with lattice constant and band gap requirements may be used as
well. The doping profile of layers 316 and 317 will be discussed in
connection with FIG. 20B.
[0295] A BSF layer 318, preferably composed of AlGaAs, is then
deposited on top of the cell C, the BSF layer performing the same
function as the BSF layers 308 and 313.
[0296] The p++/n++ tunnel diode layers 319a and 319b respectively
are deposited over the BSF layer 318, similar to the layers 314a
and 314b, forming an ohmic circuit element to connect subcell C to
subcell D. The layer 319a is preferably composed of p++ AlGaAs, and
layer 319b may be composed of n++ GaAs.
[0297] In some embodiments a barrier layer 320, preferably composed
of n-type GaInP, is deposited over the tunnel diode 319a/319b, to a
thickness of about 0.5 micron. Such barrier layer is intended to
prevent threading dislocations from propagating, either opposite to
the direction of growth into the top and middle subcells A, B and
C, or in the direction of growth into the subcell D, and is more
particularly described in copending U.S. patent application Ser.
No. 11/860,183, filed Sep. 24, 2007.
[0298] A first metamorphic layer (or graded interlayer) 321 is
deposited over the barrier layer 320. Layer 321 is preferably a
compositionally step-graded series of AlGaInAs layers, preferably
with monotonically changing lattice constant, so as to achieve a
gradual transition in lattice constant in the semiconductor
structure from subcell C to subcell D while minimizing threading
dislocations from occurring. In some embodiments the band gap of
layer 321 is constant throughout its thickness, preferably
approximately equal to 1.5 eV, approximately 1.6 eV, or otherwise
consistent with a value slightly greater than the band gap of the
middle subcell C. One embodiment of the graded interlayer may also
be expressed as being composed of
(In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs, with x and y selected such
that the band gap of the interlayer remains constant at
approximately 1.5 eV, approximately 1.6 eV, or other appropriate
band gap.
[0299] In one embodiment of the present disclosure, an optional
second barrier layer 322 may be deposited over the AlGaInAs
metamorphic layer 321. The second barrier layer 322 will typically
have a different composition than that of barrier layer 320, and
performs essentially the same function of preventing threading
dislocations from propagating. In one embodiment, barrier layer 322
is n+ type GaInP.
[0300] A window layer 323 preferably composed of n+ type GaInP is
then deposited over the second bather layer, if there is one,
disposed over layer 322. This window layer operates to reduce the
recombination loss in the fourth subcell "D". It should be apparent
to one skilled in the art that additional layers may be added or
deleted in the cell structure without departing from the scope of
the present invention.
[0301] On top of the window layer 323, the layers of cell D are
deposited: the n+ emitter layer 324, and the p-type base layer 325.
These layers are preferably composed of n+ type GaInAs and p type
GaInAs respectively, although other suitable materials consistent
with lattice constant and band gap requirements may be used as
well. The doping profile of layers 324 and 325 will be discussed in
connection with FIG. 20B.
[0302] A BSF layer 326, preferably composed of p+ type AlGaInAs, is
then deposited on top of the cell D, the BSF layer performing the
same function as the BSF layers 308, 313, and 318.
[0303] The p++/n++ tunnel diode layers 327a and 327b respectively
are deposited over the BSF layer 326, similar to the layers
309a/309b, and 319a/319b, forming an ohmic circuit element to
connect subcell D to subcell E. The layer 327a is preferably
composed of p++ AlGaInAs, and layer 327b is preferably composed of
n++ GaInP.
[0304] In some embodiments a barrier layer 328, preferably composed
of n-type GaInP, is deposited over the tunnel diode 327a/327b, to a
thickness of about 0.5 micron. Such barrier layer is intended to
prevent threading dislocations from propagating, either opposite to
the direction of growth into the middle subcells B, C and D, or in
the direction of growth into the subcell E, and is more
particularly described in copending U.S. patent application Ser.
No. 11/860,183, filed Sep. 24, 2007.
[0305] A second metamorphic layer (or graded interlayer) 329 is
deposited over the bather layer 328. Layer 329 is preferably a
compositionally step-graded series of AlGaInAs 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 E while minimizing threading
dislocations from occurring. In some embodiments the band gap of
layer 329 is constant throughout its thickness, preferably
approximately equal to 1.5 eV, approximately 1.6 eV, or otherwise
consistent with a value slightly greater than the band gap of the
middle subcell D. One embodiment of the graded interlayer may also
be expressed as being composed of
(In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs, with x and y selected such
that the band gap of the interlayer remains constant at
approximately 1.5 eV approximately 1.6 eV, or other appropriate
band gap.
[0306] In one embodiment of the present disclosure, an optional
second barrier layer 330 may be deposited over the AlGaInAs
metamorphic layer 329. The second barrier layer will typically have
a different composition than that of barrier layer 328, and
performs essentially the same function of preventing threading
dislocations from propagating.
[0307] A window layer 331 preferably composed of n+ type GaInP is
then deposited over the second barrier layer 330, if there is one
disposed over layer 329, or directly over second metamorphic layer
329. This window layer operates to reduce the recombination loss in
the fifth subcell "E". It should be apparent to one skilled in the
art that additional layers may be added or deleted in the cell
structure without departing from the scope of the present
invention.
[0308] On top of the window layer 331, the layers of cell E are
deposited: the n+ emitter layer 332, and the p-type base layer 333.
These layers are preferably composed of n+ type GaInAs and p type
GaInAs respectively, although other suitable materials consistent
with lattice constant and band gap requirements may be used as
well. The doping profile of layers 332 and 333 will be discussed in
connection with FIG. 20B.
[0309] A BSF layer 334, preferably composed of p+ type AlGaInAs, is
then deposited on top of the cell D, the BSF layer performing the
same function as the BSF layers 308, 313, 318, and 326.
[0310] The p++/n++ tunnel diode layers 335a and 335b respectively
are deposited over the BSF layer 334, similar to the layers
309a/309b, and 319a/319b, forming an ohmic circuit element to
connect subcell E to subcell F. The layer 335a is preferably
composed of p++ AlGaInAs, and layer 335b is preferably composed of
n++ GaInP.
[0311] In some embodiments a barrier layer 336, preferably composed
of n-type GaInP, is deposited over the tunnel diode 335a/335b, to a
thickness of about 0.5 micron. Such bather layer is intended to
prevent threading dislocations from propagating, either opposite to
the direction of growth into the middle subcells D and E, or in the
direction of growth into the subcell F, and is more particularly
described in copending U.S. patent application Ser. No. 11/860,183,
filed Sep. 24, 2007.
[0312] A third metamorphic layer (or graded interlayer) 337 is
deposited over the bather layer 336. Layer 337 is preferably a
compositionally step-graded series of AlGaInAs 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 E while minimizing threading
dislocations from occurring. In some embodiments the band gap of
layer 258 is constant throughout its thickness, preferably
approximately equal to 1.1 eV, or otherwise consistent with a value
slightly greater than the band gap of the middle subcell D. One
embodiment of the graded interlayer may also be expressed as being
composed of (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs, with x and y
selected such that the band gap of the interlayer remains constant
at approximately 1.1 eV or other appropriate band gap.
[0313] In one embodiment of the present disclosure, an optional
second barrier layer (not shown) may be deposited over the AlGaInAs
metamorphic layer 337. The second bather layer will typically be a
different alloy than that of barrier layer 336, and performs
essentially the same function of preventing threading dislocations
from propagating.
[0314] A window layer 338 preferably composed of n+ type AlGaInAs
is then deposited over the second bather layer, if there is one
disposed over layer 337, or directly over second metamorphic layer
337. This window layer operates to reduce the recombination loss in
the sixth subcell "F". It should be apparent to one skilled in the
art that additional layers may be added or deleted in the cell
structure without departing from the scope of the present
invention.
[0315] On top of the window layer 338, the layers of cell F are
deposited: the n+ emitter layer 339, and the p-type base layer 340.
These layers are preferably composed of n+ type GaInAs and p type
GaInAs respectively, although other suitable materials consistent
with lattice constant and band gap requirements may be used as
well.
[0316] The doping profile of layers 339 and 340 will be discussed
in connection with FIG. 20B.
[0317] A BSF layer 341, preferably composed of p+ type AlGaInAs, is
then deposited on top of the cell F, the BSF layer performing the
same function as the BSF layers 308, 313, 326, and 334.
[0318] Finally a high band gap contact layer 342, preferably
composed of p++ type AlGaInAs, is deposited on the BSF layer
341.
[0319] The composition of this contact layer 342 located at the
bottom (non-illuminated) side of the lowest band gap photovoltaic
cell (i.e., subcell "F" in the depicted embodiment) in a
multijunction photovoltaic cell, can be formulated to reduce
absorption of the light that passes through the cell, so that (i)
the backside ohmic metal contact layer below it (on the
non-illuminated side) will also act as a mirror layer, and (ii) the
contact layer doesn't have to be selectively etched off, to prevent
absorption.
[0320] It should be apparent to one skilled in the art, that
additional layer(s) may be added or deleted in the cell structure
without departing from the scope of the present invention.
[0321] The subsequent remaining steps in the fabrication of the
multijunction solar cell according to the illustrated embodiment,
including the deposition of metallization over the contact layer,
and the attachment of a surrogate substrate, will be discussed and
depicted at a later point in connection with FIG. 7 and subsequent
Figures.
[0322] Five Junction Solar Cell with One Metamorphic Layer
[0323] FIG. 5 depicts a multijunction solar cell in an embodiment
according to the present disclosure in which a five junction solar
cell with one metamorphic buffer layer is fabricated.
[0324] FIG. 5 depicts the sequential formation of the five subcells
A, B, C, D, and E on a GaAs growth substrate. The layers 401
through 423 of this embodiment are substantially identical to
layers 201 through 223 discussed in connection with the embodiment
of FIG. 3B, and therefore in the interest of brevity of this
disclosure, the description of such layers will not be repeated
here.
[0325] On top of the window layer 423, the layers of cell D are
deposited: the n+ emitter layer 424, and the p-type base layer 425.
These layers are preferably composed of n+ type GaInP and p type
AlGaInAs respectively, although other suitable materials consistent
with lattice constant and band gap requirements may be used as
well. The doping profile of layers 424 and 425 will be discussed in
connection with FIG. 20B.
[0326] A BSF layer 426, preferably composed of p+ type AlGaInAs, is
then deposited on top of the cell D, the BSF layer performing the
same function as the BSF layers 408,413 and 418.
[0327] The p++/n++ tunnel diode layers 427a and 427b respectively
are deposited over the BSF layer 426, similar to the layers
414a/414b and 419a/419b, forming an ohmic circuit element to
connect subcell D to subcell E. The layer 427a is composed of p++
AlGaInAs, and layer 427b is composed of n++ AlGaInAs.
[0328] A window layer 428 preferably composed of n+ type GaInP is
then deposited over the tunnel diode layers 427a and 427b. This
window layer operates to reduce the recombination loss in the fifth
subcell "E". It should be apparent to one skilled in the art that
additional layers may be added or deleted in the cell structure
without departing from the scope of the present invention.
[0329] On top of the window layer 428, the layers of cell E are
deposited: the n+ emitter layer 429, and the p-type base layer 430.
These layers are preferably composed of n+ type GaInP and p type
GaInAs respectively, although other suitable materials consistent
with lattice constant and band gap requirements may be used as
well. The doping profile of layers 429 and 430 will be discussed in
connection with FIG. 20B.
[0330] A BSF layer 431, preferably composed of p+ type AlGaInAs, is
then deposited on top of the cell E, the BSF layer performing the
same function as the BSF layers 408, 413, 418, and 426.
[0331] Finally a high band gap contact layer 432, preferably
composed of p++ type AlGaInAs, is deposited on the BSF layer
431.
[0332] The composition of this contact layer 432 located at the
bottom (non-illuminated) side of the lowest band gap photovoltaic
cell (i.e., subcell "E" in the depicted embodiment) in a
multijunction photovoltaic cell, can be formulated to reduce
absorption of the light that passes through the cell, so that (i)
the backside ohmic metal contact layer below it (on the
non-illuminated side) will also act as a mirror layer, and (ii) the
contact layer doesn't have to be selectively etched off, to prevent
absorption.
[0333] It should be apparent to one skilled in the art, that
additional layer(s) may be added or deleted in the cell structure
without departing from the scope of the present invention.
[0334] The subsequent remaining steps in the fabrication of the
multijunction solar cell according to the illustrated embodiment,
including the deposition of metallization over the contact layer,
and the attachment of a surrogate substrate, will be discussed and
depicted at a later point in connection with FIG. 7 and subsequent
Figures.
[0335] Six Junction Solar Cell with Two Metamorphic Layers
[0336] FIG. 6 depicts the sequence of steps in forming a
multijunction solar cell in an embodiment according to the present
disclosure in which a six junction solar cell with two metamorphic
buffer layers is fabricated.
[0337] FIG. 6 depicts the sequential formation of the six subcells
A, B, C, D, E and F on a GaAs growth substrate. The layers 501
through 527b of this embodiment are substantially identical to
layers 301 through 327b discussed in connection with the embodiment
of FIG. 4, and layers 528 through 539 of this embodiment are
substantially identical to layers 331 through 342 discussed in
connection with the embodiment of FIG. 4, and therefore in the
interest of brevity of this disclosure, the description of such
layers will not be repeated here.
[0338] It should be apparent to one skilled in the art, that
additional layer(s) may be added or deleted in the cell structure
without departing from the scope of the present invention.
[0339] The subsequent remaining steps in the fabrication of the
multijunction solar cell according to the illustrated embodiment,
including the deposition of metallization over the contact layer,
and the attachment of a surrogate substrate, will be discussed and
depicted at a later point in connection with FIG. 7 and subsequent
Figures.
[0340] FIG. 7 is a simplified cross-sectional view of the solar
cell of either FIG. 2H, 3B, 3C, 4, 5, or 6 depicting just a few of
the top layers and lower layers after the next sequence of process
steps in which a metallization layer 130 is deposited over the p
type contact layer and a surrogate substrate 132 attached using an
adhesive or other type of bonding material or layer 131.
[0341] FIG. 8 is a cross-sectional view of the solar cell of FIG. 7
after the next sequence of process steps in which the growth
substrate 101 is removed.
[0342] FIG. 9 is a another cross-sectional view of the solar cell
of FIG. 8, but here oriented and depicted with the surrogate
substrate 132 at the bottom of the figure.
[0343] FIG. 10 is a cross-sectional view of the solar cell of FIG.
9 after the next process step in which the buffer layer 102, and
the etch stop layer 103 is removed by a HCl/H.sub.2O solution.
[0344] FIG. 11 is a cross-sectional view of the solar cell of FIG.
10 after the next sequence of process steps in which a photoresist
mask (not shown) is placed over the contact layer 104 to form the
grid lines 601. As will be described in greater detail below, a
photoresist layer is deposited over the contact layer 104, and
lithographically patterned with the desired grid pattern. A metal
layer is then deposited over the patterned photoresist by
evaporation. The photoresist mask is then subsequently lifted off,
leaving the finished metal grid lines 601 as depicted in the
Figures.
[0345] As more fully described in U.S. patent application Ser. No.
12/218,582 filed Jul. 18, 2008, hereby incorporated by reference,
the grid lines 601 are preferably composed of a sequence of layers
Pd/Ge/Ti/Pd/Au, although other suitable materials and layered
sequences may be used as well.
[0346] FIG. 12 is a cross-sectional view of the solar cell of FIG.
11 after the next process step in which the grid lines are used as
a mask to etch down the surface to the window layer 105 using a
citric acid/peroxide etching mixture.
[0347] FIG. 13A is a top plan view of a wafer 600 according to the
present disclosure in which four solar cells are implemented. The
depiction of four cells is for illustration purposes only, and the
present disclosure is not limited to any specific number of cells
per wafer.
[0348] In each cell there are grid lines 601 (more particularly
shown in cross-section in FIG. 12), an interconnecting bus line
602, and a contact pad 603. The geometry and number of grid and bus
lines and the contact pad are illustrative and the present
disclosure is not limited to the illustrated embodiment.
[0349] FIG. 13B is a bottom plan view of the wafer according to the
present disclosure with four solar cells shown in FIG. 13A.
[0350] FIG. 14 is a cross-sectional view of the solar cell of FIG.
12 after the next process step in which an antireflective (ARC)
dielectric coating layer 140 is applied over the entire surface of
the "top" side of the wafer over the grid lines 601.
[0351] FIG. 15 is a cross-sectional view of the solar cell of FIG.
14 after the next process step in one embodiment according to the
present disclosure in which first and second annular channels 610
and 611, or portion of the semiconductor structure are etched down
to the metal layer 130 using phosphide and arsenide etchants. These
channels define a peripheral boundary between the cell and the rest
of the wafer, and leave a mesa structure which constitutes the
solar cell. The cross-section depicted in FIG. 15 is that as seen
from the A-A plane shown in FIG. 16. In one embodiment, channel 610
is substantially wider than that of channel 611.
[0352] FIG. 17 is a cross-sectional view of the solar cell of FIG.
15 after the next process step in another embodiment of the present
disclosure in which a cover glass 614 is secured to the top of the
cell by an adhesive 613. The cover glass 614 preferably covers the
entire channel 610, but does not extend to the periphery of the
cell near the channel 611. Although the use of a cover glass is one
embodiment, it is not necessary for all implementations, and
additional layers or structures may also be utilized for providing
additional support or environmental protection to the solar
cell.
[0353] FIG. 18 is a cross-sectional view of the solar cell of FIG.
17 after the next process step of the present disclosure in an
embodiment in which the bond layer 131, the surrogate substrate 132
and the peripheral portion 612 of the wafer is entirely removed,
breaking off in the region of the channel 611, leaving only the
solar cell with the cover glass 614 (or other supporting layers or
structures) on the top, and the metal contact layer 130 on the
bottom. The metal contact layer 130 forms the backside contact of
the solar cell. The surrogate substrate is removed by the use of
the Wafer Bond solvent, or other techniques. As noted above, the
surrogate substrate includes perforations over its surface that
allow the flow of solvent through the surrogate substrate 132 to
permit its lift off. The surrogate substrate may be reused in
subsequent wafer processing operations.
[0354] FIG. 19 is a cross-sectional view of the solar cell of FIG.
18 after the next sequence of process steps in an embodiment in
which the solar cell is attached to a support. In some embodiments,
the support may be a thin metallic flexible film 140. More
particularly, in such embodiments, the metal contact layer 130 may
be attached to the flexible film 140 by an adhesive (either
metallic or non-metallic), or by metal sputtering evaporation, or
soldering. In one embodiment, the thin film 140 may be Kapton.TM.
or another suitable polyimide material which has a metallic layer
on the surface adjoining the metal contact layer 130. Reference may
be made to U.S. patent application Ser. No. 11/860,142 filed Sep.
24, 2007, depicting utilization of a portion of the metal contact
layer 130 as a contact pad for making electrical contact to an
adjacent solar cell.
[0355] One aspect of some implementations of the present
disclosure, such as described in U.S. patent application Ser. No.
12/637,241, filed Dec. 14, 2009, is that the metallic flexible film
140 has a predetermined coefficient of thermal expansion, and the
coefficient of thermal expansion of the semiconductor body closely
matches the predetermined coefficient of thermal expansion of the
metallic film 140. More particularly, in some embodiments the
coefficient of thermal expansion of the metallic film that has a
value within 50% of the coefficient of thermal expansion of the
adjacent semiconductor material.
[0356] In some implementations, the metallic film 141 is a solid
metallic foil. In other implementations, the metallic film 141
comprises a metallic layer deposited on a surface of a Kapton or
polyimide material. In some implementations, the metallic layer is
composed of copper.
[0357] In some implementations, the semiconductor solar cell has a
thickness of less than 50 microns, and the metallic flexible film
141 has a thickness of approximately 75 microns.
[0358] In some implementations, the metal electrode layer may have
a coefficient of thermal expansion within a range of 0 to 10 ppm
per degree Kelvin different from that of the adjacent semiconductor
material of the semiconductor solar cell. The coefficient of
thermal expansion of the metal electrode layer may be in the range
of 5 to 7 ppm per degree Kelvin.
[0359] In some implementations, the metallic flexible film
comprises molybdenum, and in some implementations, the metal
electrode layer includes molybdenum.
[0360] In some implementations, the metal electrode layer includes
a Mo/Ti/Ag/Au, Ti/Mo/Ti/Ag, or Ti/Au/Mo sequence of layers.
[0361] FIG. 20A is a graph of a doping profile in the emitter and
base layers in the top subcell "A" of the inverted metamorphic
multijunction solar cell of the present disclosure. As noted in the
description of FIG. 3A, the emitter of the upper first solar
subcell is composed of a first region 206a in which the doping is
graded from 3.times.10.sup.18 to 1.times.10.sup.18 free carriers
per cubic centimeter, and a second region 206b directly disposed
over the first region in which the doping is constant at
1.times.10.sup.17 free carriers per cubic centimeter. Adjacent to
the second region 206b is a the first surface of a spacer region
206c, and adjacent to the second surface of the spacer region is
the base layer 108a.
[0362] The specific doping profiles depicted herein (e.g., a linear
profile) are merely illustrative, and other more complex profiles
may be utilized as would be apparent to those skilled in the art
without departing from the scope of the present disclosure.
[0363] FIG. 20B is a graph of a doping profile in the emitter and
base layers in one or more of the other subcells (i.e., other than
the top subcell) of the inverted metamorphic multijunction solar
cell of the present disclosure. The various doping profiles within
the scope of the present disclosure, and the advantages of such
doping profiles are more particularly described in copending U.S.
patent application Ser. No. 11/956,069 filed Dec. 13, 2007, herein
incorporated by reference. The doping profiles depicted herein are
merely illustrative, and other more complex profiles may be
utilized as would be apparent to those skilled in the art without
departing from the scope of the present disclosure.
[0364] FIG. 21 is a graph representing the Al, Ga and In mole
fractions versus the Al to In mole fraction in a AlGaInAs material
system that is necessary to achieve a constant 1.5 eV band gap.
[0365] FIG. 22 is a diagram representing the relative concentration
of Al, In, and Ga in an AlGaInAs material system needed to have a
constant band gap with various designated values (ranging from 0.4
eV to 2.1 eV) as represented by curves on the diagram. The range of
band gaps of various GaInAlAs materials is represented as a
function of the relative concentration of Al, In, and Ga. This
diagram illustrates how the selection of a constant band gap
sequence of layers of GaInAlAs used in the metamorphic layer may be
designed through the appropriate selection of the relative
concentration of Al, In, and Ga to meet the different lattice
constant requirements for each successive layer. Thus, whether 1.5
eV or 1.1 eV or other band gap value is the desired constant band
gap, the diagram illustrates a continuous curve for each band gap,
representing the incremental changes in constituent proportions as
the lattice constant changes, in order for the layer to have the
required band gap and lattice constant.
[0366] FIG. 23 is a graph that further illustrates the selection of
a constant band gap sequence of layers of GaInAlAs used in the
metamorphic layer by representing the Ga mole fraction versus the
Al to In mole fraction in GaInAlAs materials that is necessary to
achieve a constant 1.51 eV band gap.
[0367] FIG. 24 is a graph that depicts the current and voltage
characteristics of two test multijunction solar cells fabricated
according to the present disclosure. The current and voltage
characteristics of the first test solar cell, is shown in solid
lines. In the first test cell, the emitter of the top solar subcell
is composed of a first region in which the doping is graded from
3.times.10.sup.18 to 1.times.10.sup.18 free carriers per cubic
centimeter, and a second region directly disposed over the first
region in which the doping is constant at 1.times.10.sup.17 free
carriers per cubic centimeter, as depicted in the solar cell of
FIG. 3A. The current and voltage characteristics of the second test
solar cell, is shown in dashed lines ( - - - ). In the second test
solar cell, the emitter of the top solar subcell is composed of a
first region in which the doping is graded from 3 to
1.times.10.sup.18/cm.sup.3 and a second region directly disposed
over the first region in which the doping is constant at
1.times.10.sup.18/cm.sup.3, as is representative of the solar cell
depicted in FIG. 2A and others described in the parent U.S. patent
application Ser. No. 12/271,192 filed Nov. 14, 2008.
[0368] FIG. 25 is a graph that depicts the measured external
quantum efficiency (EQE) as a function of wavelength of the two
test multijunction solar cells noted in FIG. 24 above. The external
quantum efficiency of the first test solar cell, is shown in a
dashed line. The external quantum efficiency of the second test
solar cell, is shown in a solid line ( - - - ) A comparison of the
external quantum efficiency (EQE) measurements shown in FIG. 25
indicate that the EQE was substantially higher in the wavelength
range from 350 nm to 600 nm for the first test solar cell compared
with that of the second test solar cell.
[0369] 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.
[0370] Although described embodiments of the present disclosure
utilizes a vertical stack of four, five, or six 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, seven junction cells, etc. Thus, stacks of
three or more subcells can include two or more metamorphic grading
interlayers, each of which provides a transition in lattice
constant between lattice mismatched subcells. Further, in some
cases (e.g., in the case of seven or more junction cells), the use
of more than two metamorphic grading interlayer may also be
utilized.
[0371] 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.
[0372] As noted above, the solar cell described in the present
disclosure may utilize an arrangement of one or more, or all,
homojunction cells or subcells, i.e., a cell or subcell in which
the p-n junction is formed between a p-type semiconductor and an
n-type semiconductor both of which have the same chemical
composition and the same band gap, differing only in the dopant
species and types, and one or more heterojunction cells or
subcells. Subcell A, with p-type and n-type GaInP is one example of
a homojunction subcell. Alternatively, as more particularly
described in U.S. patent application Ser. No. 12/023,772 filed Jan.
31, 2008, the solar cell of the present disclosure may utilize one
or more, or all, heterojunction 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 having different chemical
compositions of the semiconductor material in the n-type regions,
and/or different band gap energies in the p-type regions, in
addition to utilizing different dopant species and type in the
p-type and n-type regions that form the p-n junction.
[0373] 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.
[0374] 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, GaInAs, GaInPAs, AlGaAs,
AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb,
AlGaInSb, AlN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe,
CdSSe, and similar materials, and still fall within the spirit of
the present invention.
[0375] While the solar cell described in the present disclosure has
been illustrated and described as embodied in an inverted
metamorphic multijunction solar cell, it is not intended to be
limited to the details shown, since various modifications and
structural changes may be made without departing in any way from
the spirit of the present invention.
[0376] 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.
[0377] 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. Accordingly, other implementations are within the
scope of the claims.
* * * * *