U.S. patent application number 14/610177 was filed with the patent office on 2015-12-24 for high efficiency multijunction solar cells.
The applicant listed for this patent is SOLAR JUNCTION CORPORATION. Invention is credited to Daniel Derkacs, Rebecca Elizabeth Jones-Albertus, Ting Liu, Pranob Misra, Vijit Sabnis, Michael J. Sheldon, Ferran Suarez, Michael West Wiemer, Homan B. Yuen.
Application Number | 20150372178 14/610177 |
Document ID | / |
Family ID | 47324413 |
Filed Date | 2015-12-24 |
United States Patent
Application |
20150372178 |
Kind Code |
A1 |
Jones-Albertus; Rebecca Elizabeth ;
et al. |
December 24, 2015 |
HIGH EFFICIENCY MULTIJUNCTION SOLAR CELLS
Abstract
Multijunction solar cells having at least four subcells are
disclosed, in which at least one of the subcells comprises a base
layer formed of an alloy of one or more elements from group III on
the periodic table, nitrogen, arsenic, and at least one element
selected from the group consisting of Sb and Bi, and each of the
subcells is substantially lattice matched. Methods of manufacturing
solar cells and photovoltaic systems comprising at least one of the
multijunction solar cells are also disclosed.
Inventors: |
Jones-Albertus; Rebecca
Elizabeth; (Washington, DC) ; Misra; Pranob;
(Santa Clara, CA) ; Sheldon; Michael J.; (Cortaro,
AZ) ; Yuen; Homan B.; (Santa Clara, CA) ; Liu;
Ting; (San Jose, CA) ; Derkacs; Daniel;
(Albuquerque, NM) ; Sabnis; Vijit; (Cupertino,
CA) ; Wiemer; Michael West; (Campbell, CA) ;
Suarez; Ferran; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SOLAR JUNCTION CORPORATION |
San Jose |
CA |
US |
|
|
Family ID: |
47324413 |
Appl. No.: |
14/610177 |
Filed: |
January 30, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13708791 |
Dec 7, 2012 |
8962993 |
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14610177 |
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13678389 |
Nov 15, 2012 |
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13708791 |
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61559982 |
Nov 15, 2011 |
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Current U.S.
Class: |
136/255 |
Current CPC
Class: |
Y02E 10/544 20130101;
H01L 31/06 20130101; H01L 31/03048 20130101; H01L 31/1852 20130101;
H01L 31/1856 20130101; H01L 31/1876 20130101; H01L 31/0687
20130101; H01L 31/0693 20130101; H01L 31/028 20130101; H01L 31/078
20130101; H01L 31/1844 20130101; Y02P 70/50 20151101; Y02E 10/547
20130101 |
International
Class: |
H01L 31/0687 20060101
H01L031/0687; H01L 31/028 20060101 H01L031/028; H01L 31/0304
20060101 H01L031/0304; H01L 31/0693 20060101 H01L031/0693 |
Claims
1-20. (canceled)
21. A photovoltaic cell comprising at least four subcells, wherein,
at least one of the at least four subcells comprises a base layer
formed of Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which
values for x, y and z are 0.06.ltoreq.x.ltoreq.0.24,
0.001.ltoreq.y.ltoreq.0.03, and 0.001.ltoreq.z.ltoreq.0.02; and
each of the at least four subcells is lattice matched to each of
the other subcells.
22. The photovoltaic cell of claim 21, wherein each of the at least
four subcells is lattice matched to Ge.
23. The photovoltaic cell of claim 21, wherein the at least one
subcell is characterized by a bandgap selected from the group
consisting of 0.7 eV to 1.1 eV, 0.8 to 0.9 eV, 0.9 eV to 1.0 eV,
0.9 eV to 1.3 eV, 1.0 eV to 1.1 eV, 1.0 eV to 1.2 eV, 1.1 eV to 1.2
eV, 1.1 eV to 1.4 eV, and 1.2 eV to 1.4 eV.
24. The photovoltaic cell of claim 21, comprising: a first subcell
having a first base layer formed of a material selected from the
group consisting of Ge, SiGe(Sn), and
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.06.ltoreq.x.ltoreq.0.24,
0.001.ltoreq.y.ltoreq.0.03, and 0.001.ltoreq.z.ltoreq.0.02, and
characterized by a band gap of 0.7 eV to 1.1 eV; a second subcell
having a second base layer overlying the first subcell, wherein the
second base layer is formed of an alloy of one or more elements
from group III on the periodic table, nitrogen, arsenic, and at
least one element selected from the group consisting of Sb and Bi,
and characterized by a band gap of 0.9 eV to 1.3 eV; a third
subcell having a third base layer overlying the second subcell,
wherein the third base layer is formed of a material selected from
the group consisting of GaInPAs and (Al,In)GaAs, and characterized
by a band gap from 1.4 eV to 1.7 eV; and a fourth subcell having a
fourth base layer overlying the third subcell, wherein the fourth
base layer is formed of (Al)InGaP and characterized by a band gap
from 1.9 eV to 2.2 eV; wherein the first base layer, the second
base layer, or the first base layer and the second base layer are
formed of Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which
values for x, y and z are 0.06.ltoreq.x.ltoreq.0.24,
0.001.ltoreq.y.ltoreq.0.03, and 0.001.ltoreq.z.ltoreq.0.02.
25. The photovoltaic cell of claim 24, wherein the band gap of the
first base layer is 0.7 eV to 0.9 eV, the band gap of the second
base layer is 1.0 eV to 1.2 eV, the band gap of the third base
layer is 1.5 eV to 1.6 eV, and the band gap of the fourth base
layer is 1.9 eV to 2.1 eV.
26. The photovoltaic cell of claim 21, comprising: a first subcell
having a first base layer formed of a material selected from the
group consisting of Ge, SiGe(Sn) and
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.06.ltoreq.x.ltoreq.0.24,
0.001.ltoreq.y.ltoreq.0.03, and 0.001.ltoreq.z.ltoreq.0.02, and
characterized by a band gap of 0.7 eV to 1.1 eV; a second subcell
having a second base layer overlying the first subcell, wherein the
second base layer is formed of an alloy of one or more elements
from group III on the periodic table, nitrogen, arsenic, and at
least one element selected from the group consisting of Sb and Bi,
and characterized by a band gap of 0.9 eV to 1.3 eV; a third
subcell having a third base layer overlying the second subcell,
wherein the third base layer is formed of a material selected from
the group consisting of GaInPAs, (Al,In)GaAs, and an alloy of one
or more elements from group III on the periodic table, nitrogen,
arsenic, and at least one element selected from the group
consisting of Sb and Bi, and characterized by a band gap of 1.2 eV
to 1.6 eV; a fourth subcell having a fourth base layer overlying
the third subcell, wherein the fourth base layer is formed of a
material selected from the group consisting of GaInPAs and
(Al,In)GaAs, and characterized by a band gap from 1.6 eV to 1.9 eV;
and a fifth subcell having a fifth base layer overlying the fourth
subcell, wherein the fifth base layer is formed of (Al)InGaP and
characterized by a band gap from 1.9 eV to 2.2 eV; wherein the
first base layer, the second base layer, or the first base layer
and the second base layer are formed of
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.06.ltoreq.x.ltoreq.0.24,
0.001.ltoreq.y.ltoreq.0.03, and 0.001.ltoreq.z.ltoreq.0.02.
27. The photovoltaic cell of claim 21, comprising: a first subcell
having a first base layer formed of a material selected from the
group consisting of Ge, SiGe(Sn), and
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.06.ltoreq.x.ltoreq.0.24,
0.001.ltoreq.y.ltoreq.0.03, and 0.001.ltoreq.z.ltoreq.0.02, and
characterized by a band gap of 0.7 eV to 1.1 eV; a second subcell
having a second base layer overlying the first subcell, wherein the
second base layer is formed of an alloy of one or more elements
from group III on the periodic table, nitrogen, arsenic, and at
least one element selected from the group consisting of Sb and Bi,
and characterized by a band gap of 0.9 eV to 1.3 eV; a third
subcell having a third base layer overlying the second subcell,
wherein the third base layer is formed of a material selected from
the group consisting of GaInPAs, (Al,In)GaAs, and an alloy of one
or more elements from group III on the periodic table, nitrogen,
arsenic, and at least one element selected from the group
consisting of Sb and Bi, and characterized by a band gap of 1.1 eV
to 1.5 eV; a fourth subcell having a fourth base layer overlying
the third subcell, wherein the fourth base layer is formed of a
material selected from the group consisting of (Al,In)GaAs and
(Al)InGa(P)As, and characterized by a band gap from 1.4 eV to 1.7
eV; a fifth subcell having a fifth base layer overlying the fourth
subcell, wherein the fifth base layer is formed of a material
selected from the group consisting of (Al)InGaP and Al(In)Ga(P)As,
and characterized by a band gap from 1.6 eV to 2.0 eV; and a sixth
subcell having a sixth base layer overlying the fifth subcell,
wherein the sixth base layer is formed of (Al)InGaP, and
characterized by a band gap from 1.9 eV to 2.3 eV; wherein the
first base layer, the second base layer, or the first base layer
and the second base layer are formed of
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.06.ltoreq.x.ltoreq.0.24,
0.001.ltoreq.y.ltoreq.0.03, and 0.001.ltoreq.z.ltoreq.0.02.
28. A photovoltaic apparatus comprising at least one photovoltaic
cell of claim 21.
29. A photovoltaic cell comprising at least four subcells, wherein,
at least one of the at least four subcells comprises a base layer
formed of Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which
values for x, y and z are 0.ltoreq.x.ltoreq.0.15,
0.015.ltoreq.y.ltoreq.0.04, and 0.005.ltoreq.z.ltoreq.0.02; and
each of the at least four subcells is lattice matched to each of
the other subcells.
30. The photovoltaic cell of claim 29, wherein each of the at least
four subcells is lattice matched to GaAs.
31. The photovoltaic cell of claim 29, wherein the at least one
subcell is characterized by a bandgap selected from the group
consisting of 0.7 eV to 1.1 eV, 0.8 to 0.9 eV, 0.9 eV to 1.0 eV,
0.9 eV to 1.3 eV, 1.0 eV to 1.1 eV, 1.0 eV to 1.2 eV, 1.1 eV to 1.2
eV, 1.1 eV to 1.4 eV, and 1.2 eV to 1.4 eV.
32. The photovoltaic cell of claim 29, comprising: a first subcell
having a first base layer formed of a material selected from the
group consisting of Ge, SiGe(Sn), and
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.ltoreq.x.ltoreq.0.15, 0.015.ltoreq.y.ltoreq.0.04,
and 0.005.ltoreq.z.ltoreq.0.02, and characterized by a band gap of
0.7 eV to 1.1 eV; a second subcell having a second base layer
overlying the first subcell, wherein the second base layer is
formed of an alloy of one or more elements from group III on the
periodic table, nitrogen, arsenic, and at least one element
selected from the group consisting of Sb and Bi, and characterized
by a band gap of 0.9 eV to 1.3 eV; a third subcell having a third
base layer overlying the second subcell, wherein the third base
layer is formed of a material selected from the group consisting of
GaInPAs and (Al,In)GaAs, and characterized by a band gap from 1.4
eV to 1.7 eV; and a fourth subcell having a fourth base layer
overlying the third subcell, wherein the fourth base layer is
formed of (Al)InGaP and characterized by a band gap from 1.9 eV to
2.2 eV; wherein the first base layer, the second base layer, or the
first base layer and the second base layer are formed of
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.ltoreq.x.ltoreq.0.15, 0.015.ltoreq.y.ltoreq.0.04,
and 0.005.ltoreq.z.ltoreq.0.02.
33. The photovoltaic cell of claim 32, wherein the band gap of the
first base layer is 0.7 eV to 0.9 eV, the band gap of the second
base layer is 1.0 eV to 1.2 eV, the band gap of the third base
layer is 1.5 eV to 1.6 eV, and the band gap of the fourth base
layer is 1.9 eV to 2.1 eV.
34. The photovoltaic cell of claim 29, comprising: a first subcell
having a first base layer formed of a material selected from the
group consisting of Ge, SiGe(Sn) and
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.ltoreq.x.ltoreq.0.15, 0.015.ltoreq.y.ltoreq.0.04,
and 0.005.ltoreq.z.ltoreq.0.02, and characterized by a band gap of
0.7 eV to 1.1 eV; a second subcell having a second base layer
overlying the first subcell, wherein the second base layer is
formed of an alloy of one or more elements from group III on the
periodic table, nitrogen, arsenic, and at least one element
selected from the group consisting of Sb and Bi, and characterized
by a band gap of 0.9 eV to 1.3 eV; a third subcell having a third
base layer overlying the second subcell, wherein the third base
layer is formed of a material selected from the group consisting of
GaInPAs, (Al,In)GaAs, and an alloy of one or more elements from
group III on the periodic table, nitrogen, arsenic, and at least
one element selected from the group consisting of Sb and Bi, and
characterized by a band gap of 1.2 eV to 1.6 eV; a fourth subcell
having a fourth base layer overlying the third subcell, wherein the
fourth base layer is formed of a material selected from the group
consisting of GaInPAs and (Al,In)GaAs, and characterized by a band
gap from 1.6 eV to 1.9 eV; and a fifth subcell having a fifth base
layer overlying the fourth subcell, wherein the fifth base layer is
formed of (Al)InGaP and characterized by a band gap from 1.9 eV to
2.2 eV; wherein the first base layer, the second base layer, or the
first base layer and the second base layer are formed of
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.ltoreq.x.ltoreq.0.15, 0.015.ltoreq.y.ltoreq.0.04,
and 0.005.ltoreq.z.ltoreq.0.02.
35. The photovoltaic cell of claim 29, comprising: a first subcell
having a first base layer formed of a material selected from the
group consisting of Ge, SiGe(Sn), and
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.ltoreq.x.ltoreq.0.15, 0.015.ltoreq.y.ltoreq.0.04,
and 0.005.ltoreq.z.ltoreq.0.02, and characterized by a band gap of
0.7 eV to 1.1 eV; a second subcell having a second base layer
overlying the first subcell, wherein the second base layer is
formed of an alloy of one or more elements from group III on the
periodic table, nitrogen, arsenic, and at least one element
selected from the group consisting of Sb and Bi, and characterized
by a band gap of 0.9 eV to 1.3 eV; a third subcell having a third
base layer overlying the second subcell, wherein the third base
layer is formed of a material selected from the group consisting of
GaInPAs, (Al,In)GaAs, and an alloy of one or more elements from
group III on the periodic table, nitrogen, arsenic, and at least
one element selected from the group consisting of Sb and Bi, and
characterized by a band gap of 1.1 eV to 1.5 eV; a fourth subcell
having a fourth base layer overlying the third subcell, wherein the
fourth base layer is formed of a material selected from the group
consisting of (Al,In)GaAs and (Al)InGa(P)As, and characterized by a
band gap from 1.4 eV to 1.7 eV; a fifth subcell having a fifth base
layer overlying the fourth subcell, wherein the fifth base layer is
formed of a material selected from the group consisting of
(Al)InGaP and Al(In)Ga(P)As, and characterized by a band gap from
1.6 eV to 2.0 eV; and a sixth subcell having a sixth base layer
overlying the fifth subcell, wherein the sixth base layer is formed
of (Al)InGaP, and characterized by a band gap from 1.9 eV to 2.3
eV; wherein the first base layer, the second base layer, or the
first base layer and the second base layer are formed of
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.ltoreq.x.ltoreq.0.15, 0.015.ltoreq.y.ltoreq.0.04,
and 0.005.ltoreq.z.ltoreq.0.02.
36. A photovoltaic apparatus comprising at least one photovoltaic
cell of claim 29.
Description
[0001] This application claims the benefit under 35 U.S.C.
.sctn.119(e) of U.S. Provisional Application No. 61/559,982 filed
on Nov. 15, 2011, which is incorporated by reference in its
entirety.
BACKGROUND
[0002] The present invention relates to solar cells, and in
particular to high efficiency, multijunction solar cells formed
primarily of III-V semiconductor alloys.
[0003] The highest known solar cell efficiencies have been produced
by multijunction solar cells comprised primarily of III-V
semiconductor alloys. Such alloys are combinations of elements
drawn from columns IIIA and VA of the standard Periodic Table,
identified hereinafter by their standard chemical symbols, names
and abbreviations, and wherein the total number of elements from
column IIIA is substantially equal to the total number of elements
from column VA. The high efficiencies of these solar cells make
them attractive for terrestrial concentrating photovoltaic systems
and systems designed to operate in space.
[0004] Historically, the highest efficiency solar cells have
consisted of a monolithic stack of three subcells, which are
equivalently referred to as junctions, grown on germanium (Ge) or
gallium arsenide (GaAs) substrates. The subcells contain the
regions of the solar cell where light energy in a range of
wavelengths is absorbed and converted into electrical energy that
may be collected externally. The subcells may be interconnected
with one another by tunnel junctions. Other layers, such as buffer
layers, may also exist between the subcells. In the highest
efficiency solar cells demonstrated to date, the top subcell has
one or more absorbing layers made of (Al)GaInP, the intermediate
subcell has one or more absorbing layers made of (In)GaAs, and the
bottom subcell includes a Ge substrate or has absorbing layers made
of a III-V material. The foregoing nomenclature for a III-V alloy,
wherein a constituent element is shown parenthetically, such as Al
in (Al)InGaP, denotes a condition of variability in which that
particular element can be zero.
[0005] Each subcell comprises several associated layers, typically
including a window, emitter, base and back surface field (BSF).
These terms are well known to those skilled in the art and do not
need further definition here. Each of the foregoing layers may
itself include one or more sublayers. The window and emitter will
be of one doping polarity (e.g., n-type) and the base and back
surface field will be of the opposite polarity (e.g., p-type), with
a p-n or n-p junction formed between the base and the emitter. If
the base contains an intrinsic region in addition to an
intentionally doped region, then it may be considered a p-i-n or
n-i-p junction, as is well known to those skilled in the art. By
convention, the specific alloy and the band gap of a given subcell
are considered to be the name and the band gap, respectively, of
the material forming the base. This material may or may not also be
used for the window, emitter and back surface field of the subcell.
For example, a subcell comprising an AlInP window, an InGaP
emitter, a GaAs base and an AlGaAs back surface field would be
denoted a GaAs subcell and its band gap would be the GaAs band gap
of 1.4 eV. A subcell comprising an AlInP window, an InGaP emitter,
an InGaP base and an InGaP back surface field would be denoted an
InGaP subcell, and its band gap would be that of the InGaP base.
The subcell may include layers in addition to those listed above.
Those skilled in the art will also recognize that subcells may also
be constructed without one or more of the foregoing layers. For
example, subcells may be constructed without a window or without a
back surface field.
[0006] When speaking about the stacking order of the subcells from
top to bottom, the top subcell is defined to be the subcell closest
to the light source during operation of the solar cell, and the
bottom subcell is furthest from the light source. Relative terms
like "above," "below," "upper," and "lower" also refer to position
in the stack with respect to the light source. The order in which
the subcells were grown is not relevant to this definition. The top
subcell is also denoted "J1," with "J2" being the second subcell
from the top, "J3" being third from the top, and the highest number
going to the bottom subcell.
[0007] Three junction solar cells have reached the highest
efficiencies of any solar cells to date. See M. A. Green et al.,
Progress in Photovoltaics: Research and Applications 19 (2011)
565-572. However, these three junction solar cells are approaching
their practical efficiency limits. To reach significantly higher
efficiencies, additional junctions or subcells are needed. With
additional subcells, photons can be absorbed more efficiently by
materials with band gaps closer to the photon energies, which are
able to convert more light energy into electrical energy rather
than heat. In addition, the total solar cell current with
additional subcells may be lower for a given amount of incident
light, which may reduce series resistance losses. Another mechanism
for increasing efficiency is to absorb a larger fraction of the
solar spectrum with additional subcell(s). For many years, there
has been widespread recognition of the need for higher numbers of
junctions, but to date, the attempt to build cells of four, five
and six junctions has failed to produce solar cells with
efficiencies that exceeded the efficiencies of the best
three-junction solar cells. The reasons for failure have been
unclear, although material and design flaws have been suspected,
including poor material quality, which can be a result of
dislocations produced by the use of lattice-mismatched layers.
There are additional challenges related to the increased number of
tunnel junctions required to interconnect the additional subcells,
including the loss of light by tunnel junction absorption.
[0008] There has long been interest in high efficiency,
lattice-matched multijunction solar cells with four or more
subcells, but suitable materials for creating high efficiencies
while maintaining lattice matching among the subcells and to a
substrate have previously been elusive. For example, U.S. Pat. No.
7,807,921 discusses the possibility of a four junction,
lattice-matched solar cell with GaInNAs as the material for a 1.0
eV subcell. However, the applicant concluded that this design is
impractical because GaInNAs that is lattice matched to the other
subcells exhibited poor quality when produced by then known
techniques. To overcome the problems with finding feasible,
lattice-matched structures, the patent teaches the use of
metamorphic materials including a graded metamorphic layer of
GaInNAs that is not lattice matched. In another attempt to make a 1
eV subcell that may be lattice-matched to the traditional
InGaP/(In)GaAs/Ge solar cell, a material consisting of gallium,
indium, nitrogen, arsenic and various concentrations of antimony
was studied, but these investigators concluded that antimony, even
in small concentrations should be avoided as it was considered
detrimental to device performance. See Ptak et al., Journal of
Vacuum Science Technology B 25(3) May/June 2007 pp. 955-959.
[0009] Prior work in this general field demonstrates that a high
level of skill in the art exists for making materials, so that it
is not necessary to disclose specific details of the processes of
making the materials for use in solar cells. Several representative
U.S. patents are exemplary. U.S. Pat. No. 6,281,426 discloses
certain structures and compositions without disclosing fabrication
techniques and refers to other documents for guidance on growth of
materials. U.S. Pat. No. 7,727,795 relates to inverted metamorphic
structures for solar cells in which exponential doping is
disclosed.
[0010] What is needed to continue the progress toward higher
efficiency solar cells are designs for multijunction solar cells
with four or more subcells that can reach higher efficiencies than
can be practically attained with three junction solar cells. It is
conventionally assumed that substantially lattice-matched designs
are desirable because they have proven reliability and because they
use less semiconductor material than metamorphic solar cells, which
require relatively thick buffer layers to accommodate differences
in the lattice constants of the various materials. It is to be
noted that the general understanding of "substantially lattice
matched" is that the in-plane lattice constants of the materials in
their fully relaxed states differ by less than 0.6% when the
materials are present in thicknesses greater than 100 nm. Further,
subcells that are substantially lattice matched to each other as
used herein means that all materials in the subcells that are
present in thicknesses greater than 100 nm have in-plane lattice
constants in their fully relaxed states that differ by less than
0.6%.
SUMMARY
[0011] The invention includes multijunction solar cells comprising
four, five or more subcells having efficiencies that can exceed
those of known best solar cells. The multijunction solar cells
incorporate at least one subcell that has a base comprising a III-V
material containing As, N and at least one additional element from
the group containing Sb and Bi, referred to as III-AsNV materials,
wherein the composition of the material is tailored for band gap
and lattice constant. The aforementioned subcells comprise the
bottom subcell and/or the subcell immediately adjacent to the
bottom subcell in each of the multijunction solar cells of the
invention. The subcells of the multijunction solar cells of the
invention are substantially lattice-matched to each other. In
certain embodiments, the subcells of the multijunction solar cells
are substantially lattice-matched to a substrate. The methodology
for determining the physical parameters of the subcells is based
upon an accurate simulation that specifies subcell thicknesses and
runs an optimization procedure to find band gaps, and therefore
material ratios in alloys, by imposing lattice-matching and
current-matching between subcells. Solar cells of the desired high
quality material composition may then be fabricated based on the
material compositions specified by the simulation.
[0012] In a specific embodiment two III-AsNV subcells of differing
band gaps are fabricated in a single multijunction solar cell,
where at least one of the subcells has a band gap higher than
previously achievable or suggested. In another specific embodiment,
three III-AsNV subcells of differing band gaps are fabricated in a
single multijunction solar cell, where at least one of the subcells
has a band gap higher than previously achievable or suggested.
[0013] In further specific embodiments, designs with four to six
junctions are disclosed wherein the bottom subcell has a higher
bottom band gap than previously disclosed or suggested.
[0014] In further embodiments, solar cells with a bottom III-AsNV
subcell with a band gap lower than has previously been achievable
for a III-AsNV alloy that is substantially lattice-matched to a
substrate are disclosed.
[0015] While there has been a body of work in multijunction cells,
the material parameters and specific structures developed in the
present invention and discussed herein have not been disclosed.
[0016] The invention will be better understood by reference to the
following detailed description in connection with the accompanying
tables and figures which constitute the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1A is a schematic cross-section of a multijunction
solar cell with five subcells illustrating an embodiment of the
invention.
[0018] FIG. 1B is a schematic cross-section of a multijunction
solar cell with six subcells illustrating an embodiment of the
invention.
[0019] FIG. 2A is a schematic cross-section of a multijunction
solar cell with five subcells illustrating still another embodiment
of the invention.
[0020] FIG. 2B depicts a schematic cross-section of a multijunction
solar cell with six subcells illustrating still another embodiment
of the invention.
[0021] FIG. 3A depicts a schematic cross-section of a multijunction
solar cell with four subcells illustrating still another embodiment
of the invention.
[0022] FIG. 3B depicts a schematic cross-section of a multijunction
solar cell with four subcells illustrating still another embodiment
of the invention.
[0023] FIG. 3C depicts a schematic cross-section of a multijunction
solar cell with four subcells illustrating still another embodiment
of the invention.
[0024] FIG. 3D depicts a schematic cross-section of a multijunction
solar cell with four subcells illustrating still another embodiment
of the invention.
[0025] FIG. 4 shows the efficiency as a function of bottom subcell
band gap for a specific embodiment of the invention with four
subcells under the AM1.5D spectrum at 25.degree. C.
[0026] FIG. 5 shows the efficiency as a function of bottom subcell
band gap for a specific embodiment of the invention with four
subcells under the AM0 spectrum at 25.degree. C.
[0027] FIG. 6 depicts a schematic cross-section of a multijunction
solar cell with four subcells illustrating still another embodiment
of the invention.
[0028] FIG. 7 depicts a schematic cross-section of a multijunction
solar cell with five subcells illustrating still another embodiment
of the invention.
[0029] FIG. 8 shows the efficiency as a function of bottom subcell
band gap for a specific embodiment of the invention with five
subcells.
[0030] FIG. 9 depicts a schematic cross-section of a multijunction
solar cell with five subcells illustrating still another embodiment
of the invention.
[0031] FIG. 10 depicts a schematic cross-section of a multijunction
solar cell with five subcells illustrating still another embodiment
of the invention.
[0032] FIG. 11 depicts a schematic cross-section of a multijunction
solar cell with six subcells illustrating still another embodiment
of the invention.
[0033] FIG. 12 depicts a schematic cross-section of a multijunction
solar cell with six subcells illustrating still another embodiment
of the invention.
[0034] FIG. 13A depicts a schematic cross-section of a
multijunction solar cell with six subcells illustrating still
another embodiment of the invention.
[0035] FIG. 13B depicts a schematic cross-section of a
multijunction solar cell with six subcells illustrating still
another embodiment of the invention.
[0036] FIG. 14 shows the efficiency as a function of bottom subcell
band gap for a specific embodiment of the invention with six
subcells.
[0037] FIG. 15 illustrates elements of a multijunction solar cell
device as found in certain embodiments of the invention.
[0038] FIG. 16 illustrates a schematic cross section of a more
specific example of a multijunction solar cell according to the
invention.
[0039] FIG. 17A shows current-voltage curves for the multijunction
solar cell with five subcells according to the invention compared
to a state-of-the-art multijunction solar cell with three
subcells.
[0040] FIG. 17B depicts a schematic cross section of a
multijunction solar cell with five subcells illustrating still
another example of the embodiment of the invention depicted in FIG.
7.
[0041] FIG. 18A shows current-voltage curves for the multijunction
solar cell with four subcells according to the invention compared
to a state-of-the-art multijunction solar cell with three subcells
measured under the AM0 spectrum.
[0042] FIG. 18B shows the solar cell with four subcells for which
the simulation depicted in FIG. 18A was produced.
[0043] FIG. 19A shows current-voltage curves for the multijunction
solar cell with six subcells according to the invention compared to
a state-of-the-art multijunction solar cell with three subcells
measured under the AM0 spectrum.
[0044] FIG. 19B shows the solar cell with six subcells for which
the simulation depicted in FIG. 19A was produced.
[0045] FIG. 20 shows the calculated band gap as a function of
composition for
Ga.sub.1.01-3wIn.sub.3w-0.01N.sub.wAs.sub.0.99-wSb.sub.0.01.
DETAILED DESCRIPTION
[0046] "III-AsNV" materials are herein defined to be alloys of
elements from group IIIA (i.e., B, Al, Ga, In, Tl) and group VA
(i.e., N, P, As, Sb, Bi) of the periodic table, which alloys
include As, N and at least one additional element from Sb and Bi.
In certain embodiments, the at least one additional element is Sb.
In certain embodiments, the at least one additional element is Bi.
The alloys may comprise approximately one-half group IIIA elements
and one-half group-VA elements. An element may only be considered
to be part of the alloy if it is present in an atomic composition
of at least 0.05%. Thus, dopants used to create n-type or p-type
conductivity (e.g., Mg, Be, Si or Te) are not considered to be part
of the alloy. Examples of III-AsNV materials include GaNAsSb,
GaInNAsSbBi, and AlInGaNAsSb. In certain embodiments, a III-AsNV
material is an alloy of one or more elements from group III on the
periodic table, nitrogen, arsenic, and at least one element
selected from Sb and Bi. The expression "elements from group III on
the periodic table" as used herein refers to one or more elements
from group III on the periodic table. For example, in certain
embodiments, an alloy comprises one element from group III on the
periodic table, and in certain embodiments, more than one element
from group III on the periodic table, such as two elements from
group III on the periodic table.
[0047] III-AsNV materials are advantageous as solar cell materials
because their lattice constants can be varied to be substantially
matched to a broad range of substrates and/or subcells formed from
other than III-AsNV materials. Their lattice constants can be
controlled by the relative fractions of the different group IIIA
and group VA elements. Thus, by tailoring the compositions (i.e.,
the elements and quantities) of III-AsNV materials, a wide range of
lattice constants and band gaps may be obtained. Further, high
quality material may be obtained by optimizing the composition
around a specific lattice constant and band gap, while limiting the
total Sb and/or Bi composition to no more than 20 percent of the
Group V lattice sites, in certain embodiments to no more than 3
percent of the Group V lattice sites, and in certain embodiments,
to no more than 1 percent of the Group V lattice sites. The Sb
and/or Bi are believed to act as surfactants to promote smooth
growth morphology of the III-AsNV alloys. In addition, they
facilitate uniform incorporation of nitrogen and minimize the
formation of nitrogen-related defects. The incorporation of Sb
and/or Bi enhances the overall nitrogen incorporation and reduces
the alloy band gap, aiding the realization of lower band gap
alloys. However, there are additional defects created by Sb and/or
Bi; for this reason, their total concentration should be limited to
no more than 20 percent of the Group V lattice sites. Further, the
limit to the Sb and/or Bi composition decreases with decreasing
nitrogen composition. Alloys that include In have even lower limits
to the total composition of Sb and/or Bi because the In reduces the
amount of Sb and/or Bi needed to tailor the lattice constant. For
alloys that include In, the total composition of Sb and/or Bi may
be limited to no more than 3 percent of the Group V lattice sites,
and in certain embodiments, to no more than 1 percent of the Group
V lattice sites. For example,
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, disclosed in U.S.
Application Publication No. 2010-0319764, which is incorporated by
reference, is known to have produced high quality material when
substantially lattice-matched to a GaAs or Ge substrate and in the
composition range of 0.08.ltoreq.x.ltoreq.0.18,
0.025.ltoreq.y.ltoreq.0.04 and 0.001.ltoreq.z.ltoreq.0.03, with a
band gap of at least 0.9 eV. This material is used as the bottom
subcell of the solar cell holding the world record for conversion
efficiency as of the filing date of the priority application.
[0048] In certain embodiments of the invention, the N composition
is not more than 7 percent of the Group V lattice sites. In certain
embodiments the N composition is not more than 4 percent and in
certain embodiments, not more than 3 percent.
[0049] The present invention includes multijunction solar cells
with four or more subcells incorporating at least one III-AsNV
subcell. The band gaps of the III-AsNV materials can be tailored by
varying the compositions while limiting the overall composition of
Sb and Bi. Thus, III-AsNV subcells with optimal band gaps for
integrating with the other subcells may be fabricated while
maintaining substantial lattice-matching to the other subcells. The
band gaps and compositions of the III-AsNV subcells are tailored so
that the short-circuit currents produced by the III-AsNV subcells
will be the same as or slightly greater than the other subcells in
the solar cell. Because the III-AsNV materials provide high
quality, lattice-matched and band gap tunable subcells, the
disclosed solar cells comprising III-AsNV subcells will reach
conversion efficiencies exceeding those of triple junction solar
cells. The boost in efficiency is largely due to less light energy
being lost as heat, as the extra subcells allow more of the
incident photons to be absorbed by materials with band gaps closer
to the energy level of the incident photons. In addition, there
will be lower series resistance losses in these multijunction solar
cells compared with triple junction solar cells due to the lower
operating currents. At higher concentrations of sunlight, the
reduced series resistance losses become more pronounced. Depending
on the band gap of the bottom subcell, the collection of a wider
range of photons in the solar spectrum may also contribute to the
efficiency boost.
[0050] Designs of multijunction solar cells with more than three
subcells in the prior art predominantly rely on metamorphic growth
structures, new materials, or dramatic improvements in the quality
of existing subcell materials in order to predict structures that
can achieve high efficiencies. Solar cells containing metamorphic
buffer layers may have reliability concerns due to the potential
for dislocations from the buffer layers to propagate over time into
the subcells, causing degradation in performance. In contrast,
III-AsNV materials can be used today in solar cells with more than
three subcells to attain high efficiencies while maintaining
substantial lattice-matching between subcells, which is
advantageous for reliability. For example, Reliability testing on
III-AsNV subcells provided by the present disclosure has shown that
such devices survived the equivalent of 390 years of on-sun
operation at 100.degree. C. with no failures. The maximum
degradation seen in these subcells was a decrease in open-circuit
voltage of 1.2%.
[0051] For application in space, radiation hardness, which refers
to minimal degradation in device performance when exposed to
ionizing radiation including electrons and protons, is of great
importance. Multijunction solar cells incorporating III-AsNV
subcells of the present invention have been subjected to proton
radiation testing to examine the effects of degradation in space
environments. Compared to Ge-based triple junction solar cells, the
results demonstrate that these III-AsNV containing devices have
similar power degradation rates and superior voltage retention
rates. Compared to non-lattice matched (metamorphic) triple
junction solar cells, all metrics are superior for III-AsNV
containing devices. In certain embodiments of the invention, the
solar cells contain (Al)GaInPAs subcells to improve radiation
hardness compared to (Al,In)GaAs subcells.
[0052] An enhanced simulation model was used to determine the
designs and efficiencies of multijunction solar cells with 4, 5, or
6 subcells. The simulation relied upon the use of standard solar
cell equations (see, for example, Nelson, The Physics of Solar
Cells. London: Imperial College Press, 2003, pp. 145-176; or Kurtz
et al., Journal of Applied Physics 68 (1990) 1890) to calculate
quantum efficiency, dark current, current, and voltage for an
individual subcell, independent of the surrounding subcells, and
standard circuit equations for calculating the overall
multijunction current-voltage curve from the current-voltage curves
of the component subcells, including a single lumped series
resistance element. When high illumination intensities (>10
W/cm.sup.2) were used in these simulations, the dark current was
assumed to be dominated by the diffusion current; the contribution
from Shockley-Read-Hall recombination in the depletion region was
neglected. The simulation varied the band gaps, and thus
compositions of the subcells, until the subcells were
current-matched. (By current-matched, it is understood to mean that
the current generating capacity is substantially the same for each
subcell, which is defined as differing by no more than 2 percent
and preferably not more than 1 percent. Note that in any
multijunction solar cell with subcells connected in series, the
current flowing through each subcell must necessarily be the same.
It can be convenient, however, to talk about the short-circuit
currents that would be produced by each individual subcell if the
subcells were not connected in series, and as though the light to
lower subcells was still filtered by the upper subcells. This is
what is meant by reference to the current generating capacity of a
subcell.) The subcell materials were specified in the simulation,
and the band gaps, or the compositions, were varied within
specified ranges for each given material alloy system. When the
band gap of a given subcell hit the upper end of its allowed range
but current-matching had not been achieved, the base thickness was
reduced to achieve current-matching. When the band gap of a given
subcell hit the lower end of its allowed range, the
current-matching requirement was limited to the given subcell and
those above it; lower subcells had higher current generating
capacities. This is a distinct departure from earlier simulations
of this type, which typically vary only the base thickness to match
the currents between subcells. The simulation used for this
invention is further distinct from other types of simulations in
the prior art that vary the band gap of individual subcells to
match currents but assume ideal or arbitrary material properties
for some or all of the subcells. Such simulations in the prior art
will give different results than the simulations employed here,
which use experimentally determined material parameters for all
subcells except for Ge. The simulation can be used to optimize band
gap relationships and current-match for any incident spectrum of
light energy, and at any reasonable operating temperature. The
simulations used for the invention were carried out using the
AM1.5D spectrum at temperatures between 25.degree. C. and
90.degree. C. as inputs. The simulations were run on Matlab
software on a Windows operating system.
[0053] The simulation model was also used with the AM0 spectrum at
25.degree. C. to predict the designs and efficiencies of
multijunction solar cells with 4, 5, and 6 subcells for application
in space. Because these simulations were run at 1 sun of
illumination, the contribution to the dark current from
Shockley-Read-Hall recombination in the depletion region was
included. Depending on the application of interest, solar cells for
space may be optimized for other operating temperatures and the
resulting structures may change slightly from those here.
[0054] For the predictive simulations, a perfect anti-reflection
coating (ARC) was assumed in order to reduce the computational
expense of optimizing the ARC. This can cause the predictive
efficiencies to be inflated by approximately two to four percent
(e.g., 40.8% instead of 40.0%). For the simulation of existing
single and triple junction solar cells, a realistic ARC was
included in the simulation of solar cells having an ARC, in order
to more accurately model the experimental results.
[0055] Unique to the simulation used for this invention is the use
of accurate material parameters for the alloy systems of interest
that are substantially lattice-matched to GaAs and Ge substrates,
including (Al)InGaP, (Al,In)GaAs, and GaInNAsSb, an example of
III-AsNV material. These material parameters can be used to
predict, among other values, the quantum efficiency and the dark
current. For these material systems, a range of compositions were
found in which the primary effect on quantum efficiency and dark
current was the change in band gap. As a result, the other material
parameters could be treated as constant with accurate results. For
example, for (Al)InGaP, the range of compositions in which the
material parameters other than band gap could be treated as
constant for subcells substantially lattice-matched to GaAs or Ge
was x.ltoreq.0.2 for Al.sub.xIn.sub.0.5Ga.sub.1-xP.
[0056] Material parameters for (Al)GaInPAs and SiGe(Sn) were not
included in the simulation, but high efficiency solar cell
structures using (Al)GaInPAs and SiGe(Sn) may be designed with the
same methodology.
[0057] The material parameters for each material system in the
simulation included the band gap, the n and k values (i.e., the
refractive index), the hole and electron effective masses, the
static dielectric constant, the minority carrier mobilities, the
minority carrier lifetimes and the surface recombination velocities
for interfaces with relevant materials. The n and k values were
determined from ellipsometry measurements for some materials and
taken from the literature for other materials, and were shifted as
a function of band gap energy as necessary within a given alloy
system. The effective masses and static dielectric constants were
taken from the literature for (Al)InGaP and (Al,In)GaAs. For
GaInNAsSb the values used were 0.6 m.sub.0, 0.15 m.sub.0 and 13.3
for the hole and electron effective masses and the static
dielectric constant respectively, where m.sub.0 is the electron
effective mass. Material parameters were assumed to be constant
with temperature in the range simulated, except for the band gap
and the n and k values, which shifted with the band gap energy. The
minority carrier mobilities were initially estimated from the
majority carrier mobilities measured on uniformly doped layers
using Hall effect measurements, and were refined by fitting
experimental quantum efficiency data. The minority carrier
lifetimes and surface recombination velocities were determined by
time resolved photoluminescence measurements. For Ge, all material
parameters were estimates based on available numbers in the
literature. Average doping values and material parameters were used
in the simulations to treat cases where doping values were graded
throughout a layer. The doping values were between
1.times.10.sup.17 cm.sup.-3 and 1.times.10.sup.19 cm.sup.-3 for
n-type layers and between 5.times.10.sup.15 cm.sup.-3 and
2.times.10.sup.18 cm.sup.-3 for the p-type layers, and were
optimized for performance in experimental devices. The relationship
between composition and band gap is well known for the (Al)InGaP
and (Al,In)GaAs material systems lattice matched to GaAs and Ge,
specifying the composition for a given band gap. The band gap of
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z is not a simple
function of composition due to interactions between the different
elements, as well as factors such as the strain in the layer. The
composition that yields a desired band gap with a specific lattice
constant will be found by varying the composition in an
optimization procedure. As an example, the relationship between
band gap and composition w for
Ga.sub.1.01-3wIn.sub.3w-0.01N.sub.wAs.sub.0.99-wSb.sub.0.01 is
shown in FIG. 20. Here, the Sb composition is fixed. Similar plots
may be constructed for different Sb compositions or with other
elements held constant.
[0058] The thermal dose, which is controlled by the intensity of
heat applied for a given duration of time (e.g., application of a
temperature of 600.degree. C. to 900.degree. C. for a duration of
between 10 seconds to 10 hours), that a III-AsNV material receives
during growth and after growth, also affects the relationship
between band gap and composition. In general, the band gap
increases as thermal dose increases.
[0059] As development continues on the above-described materials,
it is expected that material quality will continue to improve,
enabling even higher efficiencies from the same structures
described in this invention. The simulation was also run with
improved minority carrier properties to predict structures and
performance of future devices.
[0060] The use of the simulation over the temperature ranges from
25.degree. C. to 90.degree. C. is supported by data for triple
junction solar cells with a bottom GaInNAsSb subcell operating from
25.degree. C. to 125.degree. C.
[0061] As composition is varied within a given alloy system, the
growth conditions need to be modified, as is well known to those
skilled in the art. For example, for (Al,In)GaAs, the growth
temperature will increase as the fraction of Al increases and
decrease as the fraction of In increases, in order to maintain the
same material quality. Thus, as a composition is changed, the
growth temperature as well as other growth conditions can be
adjusted accordingly.
[0062] Tables 1A and 1B show the short-circuit current, open
circuit voltage and fill factor from both simulated I-V curves and
experimental data for Al.sub.0.1In.sub.0.5Ga.sub.0.4P and
In.sub.0.5Ga.sub.0.5P subcells, respectively, exposed to incident
solar radiation of the stated intensity using an AM1.5D spectrum at
25.degree. C. The close conformance between simulation and
experimental results is a verification of the accuracy of the
simulation. The number of suns listed for each table gives
information about the illumination intensity incident on the cell
during testing. It is the number of multiples of "one sun"
intensity (0.1 W/cm.sup.2) incident on the cell. For example, the
term "800 suns" indicates 80 W/cm.sup.2. The numbers of suns
differs for each case because the existing experimental data was
taken at different intensities. The incident spectrum approximated
the AM1.5D spectrum in the experimental measurements and was the
AM1.5D spectrum in the simulations.
TABLE-US-00001 TABLE 1A Simulated and experimental solar cell data
for an Al.sub.0.1In.sub.0.5Ga.sub.0.4P subcell illuminated at 525
suns under the AM1.5D spectrum. Al.sub.0.1In.sub.0.5Ga.sub.0.4P
Subcell at 525 Suns Simulated Data Experimental Data Short-circuit
current 6.3 A/cm.sup.2 6.2 A/cm.sup.2 Open-circuit voltage 1.68 V
1.70 V Fill Factor 84% 82%
TABLE-US-00002 TABLE 1B Simulated and experimental solar cell data
for an In.sub.0.5Ga.sub.0.5P subcell illuminated at 925 suns under
the AM1.5D spectrum. In.sub.0.5Ga.sub.0.5P Subcell at 925 Suns
Simulated Data Experimental Data Short-circuit current 13.0
A/cm.sup.2 12.7 A/cm.sup.2 Open-circuit voltage 1.61 V 1.63 V Fill
Factor 79% 80%
[0063] Tables 1C and 1D show the short-circuit current, open
circuit voltage and fill factor from both simulated I-V curves and
experimental data for Al.sub.0.1In.sub.0.5Ga.sub.0.4P and
In.sub.0.5Ga.sub.0.5P subcells under the air mass zero (AM0)
spectrum. The incident spectrum approximated the AM0 spectrum at an
intensity of 1,353 W/cm.sup.2 at 25.degree. C. in the experimental
measurements and was the AM0 spectrum at 25.degree. C. in the
simulations. The close conformance between simulation and
experimental results is a verification of the accuracy of the
simulation.
TABLE-US-00003 TABLE 1C Simulated and experimental solar cell data
for an Al.sub.0.1In.sub.0.5Ga.sub.0.4P subcell illuminated under
the AM0 spectrum. Al.sub.0.1In.sub.0.5Ga.sub.0.4P Subcell Simulated
Data Experimental Data Short-circuit current 16.0 mA/cm.sup.2 16.2
mA/cm.sup.2 Open-circuit voltage 1.50 V 1.48 V Fill Factor 87%
87%
TABLE-US-00004 TABLE 1D Simulated and experimental solar cell data
for an In.sub.0.5Ga.sub.0.5P subcell under the AM0 spectrum.
In.sub.0.5Ga.sub.0.5P Subcell Simulated Data Experimental Data
Short-circuit current 18.1 A/cm.sup.2 17.7 mA/cm.sup.2 Open-circuit
voltage 1.41 V 1.41 V Fill Factor 87% 89%
[0064] Tables 2A and 2B show analogous data for
Al.sub.0.2Ga.sub.0.8As and GaAs subcells, respectively, and Tables
3A and 3B for
Ga.sub.0.96In.sub.0.04N.sub.0.01As.sub.0.98Sb.sub.0.01 and
Ga.sub.0.96In.sub.0.1N.sub.0.03As.sub.0.96Sb.sub.0.01 subcells,
respectively, exposed to incident solar radiation of the stated
intensity using an AM1.5D spectrum at 25.degree. C.
TABLE-US-00005 TABLE 2A Simulated and experimental solar cell data
for an Al.sub.0.2Ga.sub.0.8As subcell illuminated at 1130 suns
under the AM1.5D spectrum. Al.sub.0.2Ga.sub.0.8As Subcell at 1130
Suns Simulated Data Experimental Data Short-circuit current 10.3
A/cm.sup.2 10.4 A/cm.sup.2 Open-circuit voltage 1.46 V 1.43 V Fill
Factor 85% 83%
TABLE-US-00006 TABLE 2B Simulated and experimental solar cell data
for a GaAs subcell illuminated at 980 suns under the AM 1.5D
spectrum. GaAs Subcell at 980 Suns Simulated Data Experimental Data
Short-circuit current 17.2 A/cm.sup.2 17.1 A/cm.sup.2 Open-circuit
voltage 1.22 V 1.22 V Fill Factor 78% 80%
TABLE-US-00007 TABLE 3A Simulated and experimental solar cell data
for a Ga.sub.0.96In.sub.0.04N.sub.0.01As.sub.0.98Sb.sub.0.01
subcell illuminated at 1230 suns under the AM1.5D spectrum.
Ga.sub.0.96In.sub.0.04N.sub.0.01As.sub.0.98Sb.sub.0.01 Subcell at
1230 Suns Simulated Data Experimental Data Short-circuit current
6.5 A/cm.sup.2 6.6 A/cm.sup.2 Open-circuit voltage 0.84 V 0.82 V
Fill Factor 81% 77%
TABLE-US-00008 TABLE 3B Simulated and experimental solar cell data
for a Ga.sub.0.9In.sub.0.1N.sub.0.03As.sub.0.96Sb.sub.0.01 subcell
illuminated at 610 suns under the AM1.5D spectrum.
Ga.sub.0.9In.sub.0.1N.sub.0.03As.sub.0.96Sb.sub.0.01 Subcell at 610
Suns Simulated Data Experimental Data Short-circuit current 9.5
A/cm.sup.2 9.3 A/cm.sup.2 Open-circuit voltage 0.59 V 0.60 V Fill
Factor 71% 70%
[0065] Tables 2C and 2D show analogous data for
Al.sub.0.2Ga.sub.0.8As and GaAs subcells, respectively; and Tables
3C and 3D show data for
Ga.sub.0.96In.sub.0.04N.sub.0.01As.sub.0.98Sb.sub.0.01 and
Ga.sub.0.96In.sub.0.1N.sub.0.03As.sub.0.96Sb.sub.0.01 subcells,
respectively, under the AM0 spectrum at an intensity of 1,353
W/cm.sup.2 at 25.degree. C.
TABLE-US-00009 TABLE 2C Simulated and experimental solar cell data
for an Al.sub.0.2Ga.sub.0.8As subcell under the AM0 spectrum.
Al.sub.0.2Ga.sub.0.8As Subcell Simulated Data Experimental Data
Short-circuit current 10.3 mA/cm.sup.2 10.4 mA/cm.sup.2
Open-circuit voltage 1.23 V 1.18 V Fill Factor 85% 83%
TABLE-US-00010 TABLE 2D Simulated and experimental solar cell data
for a GaAs subcell under the AM0 spectrum. GaAs Subcell Simulated
Data Experimental Data Short-circuit current 18.9 mA/cm.sup.2 19.0
mA/cm.sup.2 Open-circuit voltage 1.01 V 1.00 V Fill Factor 83%
85%
TABLE-US-00011 TABLE 3C Simulated and experimental solar cell data
for a Ga.sub.0.96In.sub.0.04N.sub.0.01As.sub.0.98Sb.sub.0.01
subcell under the AM0 spectrum.
Ga.sub.0.96In.sub.0.04N.sub.0.01As.sub.0.98Sb.sub.0.01 Subcell
Simulated Data Experimental Data Short-circuit current 7.5
mA/cm.sup.2 7.7 mA/cm.sup.2 Open-circuit voltage 0.61 V 0.57 V Fill
Factor 76% 77%
TABLE-US-00012 TABLE 3D Simulated and experimental solar cell data
for a Ga.sub.0.96In.sub.0.1N.sub.0.03As.sub.0.96Sb.sub.0.01 subcell
under the AM0 spectrum.
Ga.sub.0.96In.sub.0.1N.sub.0.03As.sub.0.96Sb.sub.0.01 Subcell
Simulated Data Experimental Data Short-circuit current 18.2
mA/cm.sup.2 17.9 mA/cm.sup.2 Open-circuit voltage 0.40 V 0.41 V
Fill Factor 70% 71%
[0066] Validation of the model was also performed by simulating the
performance of a state-of-the-art multijunction solar cell
comprising three subcells: In.sub.0.5Ga.sub.0.5P (1.9 eV), GaAs
(1.4 eV), and Ga.sub.0.9In.sub.0.1N.sub.0.03As.sub.0.96Sb.sub.0.01
(1.0 eV), with the total subcell thicknesses being 1-2 .mu.m, 4-4.5
.mu.m and 2-3 .mu.m, respectively. Table 4A shows the short-circuit
current, open circuit voltage and fill factor of the simulated
solar cell under the AM1.5D spectrum concentrated to 525 suns (or
52.5 W/cm.sup.2) at 25.degree. C. Also shown is the measured
performance of a solar cell with these characteristics and
operating conditions. It can be seen that the model accurately
predicts the measured performance.
TABLE-US-00013 TABLE 4A Simulated and experimental solar cell data
for a solar cell with three subcells illuminated at 525 suns under
the AM1.5D spectrum. Solar Cell with 3 Subcells at 525 Suns
Simulated Data Experimental Data Short-circuit current 7.3
A/cm.sup.2 7.3 A/cm.sup.2 Open-circuit voltage 3.37 V 3.40 V Fill
Factor 87% 86%
[0067] Validation of the model was also performed by simulating the
performance of a state-of-the-art multijunction solar cell
comprising three subcells: Al.sub.0.1In.sub.0.5Ga.sub.0.4P (2.0
eV), GaAs (1.4 eV), and
Ga.sub.0.9In.sub.0.1N.sub.0.03As.sub.0.96Sb.sub.0.01 (1.0 eV), with
the total subcell thicknesses being 0.75-1.25 .mu.m, 3-4 .mu.m and
2-3 .mu.m, respectively, under the AM0 spectrum. Table 4B shows the
short-circuit current, open circuit voltage, and fill factor of the
simulated solar cell under the AM0 spectrum at 25.degree. C. Also
shown is the measured performance of a solar cell with these
characteristics and operating conditions. The model accurately
predicts the measured performance under the AM0 spectrum at
25.degree. C.
TABLE-US-00014 TABLE 4B Simulated and experimental solar cell data
for a solar cell with three subcells under the AM0 spectrum. Solar
Cell with 3 Subcells Simulated Data Experimental Data Short-circuit
current 17.1 mA/cm.sup.2 16.7 mA/cm.sup.2 Open-circuit voltage 2.90
V 2.92 V Fill Factor 85% 86%
[0068] The model was then used to predict the structures of
multijunction solar cells with 4, 5 and 6 subcells that exceed the
efficiencies of known best three junction solar cells. The
subsequent paragraphs describe more general and more specific
embodiments of the invention. In many cases, the structures were
constrained to be substantially lattice-matched to GaAs and Ge
substrates. All band gap ranges are given to one significant digit
to the right of the decimal point. While the simulation predicts
optimized structures where the subcells are typically
current-matched, other design criteria (e.g., desired solar cell
thickness) may lead to the modification of said structures, within
the band gap ranges specified below.
[0069] Of note is that the embodiments of the invention disclosed
below include multijunction solar cells with bottom subcells having
band gaps greater than 0.8 eV, up to 1.1 eV. The prior art
predominantly teaches that multijunction solar cells with more than
3 subcells should have a bottom subcell with a band gap less than
or equal to 0.8 eV, in order to collect light over a broader
fraction of the solar spectrum. Most commonly, the material
composing the base of the bottom subcell in the prior art is Ge or
InGaAs. Surprisingly, however, high efficiencies can be achieved
from solar cells of the invention using a bottom III-AsNV subcell
that has a band gap as high as 1.1 eV, due at least in part to the
higher voltages and efficient current extraction of such
subcells.
[0070] Another novel aspect of many of the embodiments disclosed
below is the inclusion of two or three III-AsNV subcells of
differing band gaps in a single multijunction solar cell. In these
embodiments, at least one of the III-AsNV subcells has a band gap
higher than has been previously achievable or suggested such as,
for example, a band gap of 1.3 eV.
[0071] Also of note is that certain embodiments have a bottom
III-AsNV subcell with a band gap lower than has been previously
achievable for a III-AsNV alloy that is substantially
lattice-matched to a substrate, such as, for example, a band gap of
0.8 eV.
[0072] FIGS. 1A-1B, 2A-2B, 3A-3D, 6-7, 9-12, 13A-B, and 15-16, 17B,
18B, and 19B exemplify, in additional detail, certain embodiments
of a 4, 5, and 6 multijunction solar cells according to the
invention. For simplicity, FIGS. 1A-1B, 2A-2B, 3A-3D, 6-7, 9-12,
13A-B, 17B, 18B, and 19B show only the subcells and interconnecting
tunnel junctions of the multijunction solar cells. As is well known
to those skilled in the art, additional elements may be necessary
to create a complete solar cell, including an anti-reflection
coating, contact layers, electrical contacts and a substrate or
wafer handle. As will be discussed below, FIG. 15 shows one example
structure with these additional elements. Further, additional
elements may be present in a complete solar cell, such as buffer
layers and additional tunnel junctions. In some of the embodiments
disclosed herein, the bottom subcell includes the substrate (e.g.,
a Ge subcell) and thus the substrate is shown in the figures. In
other embodiments, the substrate is not part of a subcell, and is
therefore typically not shown in the figures.
[0073] FIG. 1A depicts a multijunction solar cell according to the
invention that has five subcells, with the bottom subcell being a
III-AsNV subcell. All five subcells are substantially
lattice-matched to each other and may be interconnected by four
tunnel junctions, which are shown as dotted regions. The III-AsNV
subcell at the bottom of the stack has the lowest band gap of the
five subcells and absorbs the lowest-energy light that is converted
into electricity by the solar cell. The band gap of the III-AsNV
material in the bottom subcell is between 0.7-1.1 eV. The upper
subcells may comprise any suitable III-V, II-VI, or group IV
materials, including III-AsNV materials.
[0074] FIG. 1B depicts a multijunction solar cell according to the
invention that has six subcells, with the bottom subcell being a
III-AsNV subcell. All six subcells are substantially
lattice-matched to each other and may be interconnected by five
tunnel junctions, which are shown as dotted regions. The III-AsNV
subcell has the lowest band gap of the six subcells. The band gap
of the III-AsNV material in the bottom subcell is between 0.7-1.1
eV. The upper subcells may comprise any suitable III-V, II-VI, or
group IV materials, including III-AsNV materials.
[0075] In certain embodiments, the band gap of the III-AsNV alloy
in a bottom subcell is between 0.8-0.9 eV, and in other
embodiments, between 0.9-1.0 eV. In certain embodiments, the
composition of the base layer of a bottom III-AsNV subcell
comprises Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which
values for x, y and z are 0.ltoreq.x.ltoreq.0.24,
0.01.ltoreq.y.ltoreq.0.07 and 0.001.ltoreq.z.ltoreq.0.20, in
certain embodiments, 0.02.ltoreq.x.ltoreq.0.24,
0.01.ltoreq.y.ltoreq.0.07 and 0.001.ltoreq.z.ltoreq.0.03, in
certain embodiments, 0.02.ltoreq.x.ltoreq.0.18,
0.01.ltoreq.y.ltoreq.0.04 and 0.001.ltoreq.z.ltoreq.0.03, and in
certain embodiments, 0.06.ltoreq.x.ltoreq.0.20,
0.02.ltoreq.y.ltoreq.0.05 and 0.005.ltoreq.z.ltoreq.0.02.
[0076] In the embodiment of the invention illustrated in FIG. 2A, a
multijunction solar cell has five subcells, with J4, the subcell
directly above the bottom subcell and the fourth subcell from the
top, being a III-AsNV subcell. The band gap of the III-AsNV
material in J4 is between 0.9-1.3 eV. All five subcells are
substantially lattice-matched to each other and may be
interconnected by four tunnel junctions, which are shown as dotted
regions. The other four subcells may comprise any suitable III-V,
II-VI, or group IV materials, including III-AsNV materials.
[0077] FIG. 2B shows a multijunction solar cell according to the
invention that has six subcells, with J5, the subcell directly
above the bottom subcell, being a III-AsNV subcell. The band gap of
the III-AsNV material in the base of J5 is between 0.9-1.3 eV. All
six subcells are substantially lattice-matched to each other and
may be interconnected by five tunnel junctions, which are shown as
dotted regions. The other five subcells may comprise any suitable
III-V, II-VI or group IV materials, including III-AsNV
materials.
[0078] In certain embodiments, the band gap of the III-AsNV alloy
in the subcell directly above the bottom subcell is between 0.9-1.0
eV, and in certain embodiments, between 1.0-1.1 eV. In certain
embodiments, the composition of the base layer of the subcell
directly above the bottom subcell comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.ltoreq.x.ltoreq.0.18, 0.001.ltoreq.y.ltoreq.0.05
and 0.001.ltoreq.z.ltoreq.0.15, in certain embodiments
0.ltoreq.x.ltoreq.0.18, 0.001.ltoreq.y.ltoreq.0.05, and
0.001.ltoreq.z.ltoreq.0.03, and in certain embodiments
0.02.ltoreq.x.ltoreq.0.18, 0.005.ltoreq.y.ltoreq.0.04 and
0.001.ltoreq.z.ltoreq.0.03.
[0079] Another embodiment of the invention is shown in FIG. 3A. In
this embodiment, there are four subcells, the two bottom subcells
being III-AsNV subcells. The band gap of the bottom subcell, J4, is
between 0.7-1.1 eV. The band gap of J3 is between 0.9-1.3 eV, or
between 1.0-1.3 eV, and is greater than the band gap of J4. The
subcell J2, directly above the two III-AsNV subcells, is an
(Al,In)GaAs subcell with a band gap in the range of 1.4-1.7 eV. The
top subcell J1 is an (Al)InGaP subcell with a band gap in the range
of 1.9-2.2 eV. Examples of band gaps for the subcells, from bottom
to top, are respectively 0.8-0.9 eV or 0.9-1.0 eV, 1.1-1.2 eV,
1.5-1.6 eV, and 1.9-2.0 or 2.0-2.1 eV. The band gaps and
thicknesses of the subcells are most optimal when the currents
produced by the four subcells are substantially the same. All of
the subcells are substantially lattice-matched to each other and
may be connected in series by tunnel junctions. In a similar
embodiment of the invention, the structure is the same except that
the subcell directly above the bottom subcell, J3, is a GaInNAs
subcell. In another related embodiment, the structure is the same
except that J2 is a (Al)GaInPAs subcell. In another, related
embodiment that is depicted in FIG. 3B, J4 is a SiGe(Sn)
subcell.
[0080] The specific band gaps of the subcells, within the ranges
given in the preceding as well as subsequent embodiments, are
dictated by the band gap of the bottom subcell, the thicknesses of
the subcell layers, and the incident spectrum of light. Although
there are numerous structures in the present disclosure that will
produce efficiencies exceeding those of three junction solar cells,
it is not the case that any set of subcell band gaps that falls
within the disclosed ranges will produce such an efficiency. For a
certain choice of bottom subcell band gap, or alternately the band
gap of another subcell, incident spectrum of light, subcell
materials and subcell layer thicknesses, there is a narrower range
of band gaps for the remaining subcells that will produce
efficiencies exceeding those of three junction solar cells. The
band gaps may be found from the simulation and/or from
experimentation. In general, the higher the band gap of the bottom
subcell, the higher the band gaps of the upper subcells, within the
specified ranges. FIGS. 3C and 3D illustrate this for the
embodiment of the invention depicted in FIG. 3A using the AM1.5D
spectrum. The band gap of the bottom subcell in FIG. 3C is higher
than the band gap of the bottom subcell in FIG. 3D. Accordingly,
the band gaps of the upper subcells in FIG. 3C are higher than the
band gaps of the upper subcells, respectively, in FIG. 3D.
[0081] FIG. 4 shows the efficiencies predicted by the simulation as
a function of the band gap of the bottom subcell for a specific
embodiment of the invention depicted in FIG. 3A, under an
illumination intensity of 100 W/cm.sup.2 or 1000 suns at 25.degree.
C. In this embodiment, J4 and J3 are GaInNAsSb subcells with total
subcell thicknesses of 2-3 microns, J2 is an (Al)GaAs subcell with
a thickness of 4-5 microns and J1 is an (Al)InGaP subcell with a
thickness of 1-2 microns. The trend in efficiency with bottom band
gap has two peaks, near band gaps of 0.75 eV and 0.92 eV, which is
largely a result of the variation in solar spectrum irradiance in
the energy range between 0.7 eV and 1 eV. In all cases shown, the
efficiency is higher than the simulated efficiency for the
state-of-the-art triple junction solar cell structure under the
same conditions (40.8%).
[0082] FIG. 5 shows the efficiencies predicted by the simulation as
a function of the band gap of the bottom subcell for a specific
embodiment of the invention depicted in FIG. 3A, under the AM0
spectrum at 25.degree. C. In this embodiment, the bottom two
subcells are GaInNAsSb subcells with total subcell thicknesses of
2-3 microns, J2 is an (Al)GaAs subcell with a thickness of 3-4
microns and J1 is an (Al)InGaP subcell with a thickness of 0.5-1.5
microns. The solid line shows the efficiencies predicted for solar
cells made with today's material parameters, and the dashed line
shows predicted future efficiencies with improvements in material
quality that increase minority carrier lifetime and improve
interface recombination velocities. The trend in today's
efficiencies with bottom subcell band gap peaks near 0.90 eV. The
efficiency increases as the bottom band gap decreases because more
of the solar spectrum is absorbed until the upper band gap limit
for the top subcell is reached. At this point, the simulation fixes
the top subcell band gap and decreases the top subcell thickness to
reach current matching between subcells. This causes the overall
efficiency to decrease. In all cases shown, the efficiency is
higher than the simulated efficiency for the triple junction solar
cell structure under the same conditions.
[0083] Another embodiment of the invention has four subcells, with
the bottom subcell being a III-AsNV subcell. The band gap of the
bottom subcell, J4, is between 0.9-1.1 eV. The subcell J3 is an
(Al,In)GaAs or an (Al)GaInPAs subcell with a band gap between
1.4-1.5 eV. The subcell J2 is an Al(In)GaAs or an (Al)GaInPAs
subcell with a band gap in the range of 1.6-1.8 eV. The top subcell
J1 is an (Al)InGaP subcell with a band gap in the range of 1.9-2.3
eV. Examples of band gaps for the subcells, from bottom to top, are
respectively 1.0 eV, 1.4 eV, 1.7 eV, and 2.1 eV.
[0084] FIG. 6 illustrates another embodiment of the invention with
four subcells. J3, directly above the bottom subcell, is a III-AsNV
subcell with a band gap in the range of 0.9 eV-1.3 eV. The bottom
subcell is a Ge subcell, incorporating the Ge substrate, which has
a band gap of 0.7 eV. J2 is an (Al)InGaAs or (Al)GaInPAs subcell
with a band gap in the range of 1.4 eV-1.7 eV. J1 is an (Al)InGaP
subcell with a band gap in the range of 1.9 eV-2.2 eV. Examples of
band gaps for the subcells above the Ge subcell, from bottom to
top, are respectively, 1.0 eV-1.1 eV, 1.4 eV-1.5 eV, and 1.9 eV-2.0
eV. All of the subcells are substantially lattice-matched to the Ge
substrate and may be connected in series by tunnel junctions.
[0085] FIG. 7 depicts an embodiment of the invention with five
subcells. The bottom two subcells are III-AsNV subcells. The band
gap of the bottom subcell is between 0.7 eV-1.1 eV. The band gap of
J4, the subcell second from the bottom, is between 0.9 eV-1.3 eV,
or between 1.0-1.3 eV, and is greater than or equal to the band gap
of the bottom subcell. J3, the subcell above the III-AsNV subcells,
is an (Al,In)GaAs subcell with a band gap in the range of 1.4
eV-1.6 eV. J2 is an Al(In)GaAs or an (Al)InGaP subcell with a band
gap in the range of 1.6 eV-1.9 eV. The top subcell is an (Al)InGaP
subcell with a band gap in the range of 1.9-2.2 eV. Examples of
band gaps for subcells from bottom to top are, respectively, 0.9
eV-1.0 eV, 1.1 eV-1.2 eV, 1.4 eV-1.5 eV, 1.7 eV-1.8, and 1.9 eV-2.1
eV. All of the subcells are substantially lattice-matched to each
other and may be connected in series by tunnel junctions. In a
related embodiment of the invention, the structure is the same
except that J4 is a GaInNAs subcell. In another related embodiment,
the structure is the same except that one or both of J2 and J3 is
an (Al)GaInPAs subcell. In an additional related embodiment, the
bottom subcell is a SiGe(Sn) subcell.
[0086] In another embodiment of the invention, the bottom three
subcells are III-AsNV subcells. The band gap of the bottom subcell,
J5, is between 0.7 eV-1.0 eV. The band gap of J4 is between 0.9
eV-1.2 eV and is greater than the band gap of J5. J3 is a III-AsNV
subcell with a band gap in the range of 1.2 eV-1.4 eV. J2 is an
Al(In)GaAs, (Al)InGaPAs or InGaP subcell with a band gap in the
range of 1.6 eV-1.8 eV. The top subcell, J1, is an (Al)InGaP
subcell with a band gap in the range of 1.9 eV-2.2 eV. In certain
embodiments, band gaps for the subcells, from bottom to top,
respectively, are 0.7 eV-0.8 eV, 0.9 eV-1.1 eV, 1.2 eV-1.3 eV. 1.6
eV-1.7 eV, and 2.0 eV-2.1 eV. The subcells may be substantially
lattice-matched to each other and connected in series by tunnel
junctions. In a related embodiment of the invention, the structure
is the same except J3 and/or J4 is a GaInNAs subcell.
[0087] FIG. 8 shows the efficiencies predicted by the simulation as
a function of the band gap of the bottom subcell for a specific
embodiment of the invention depicted in FIG. 7 under an
illumination intensity of 100 W/cm.sup.2 or 1000 suns of the AM1.5D
spectrum at 25.degree. C. In this embodiment, the bottom two
subcells are GaInNAsSb subcells with total subcell thicknesses of
2-3 microns, J3 and J2 are (Al)GaAs subcells with thicknesses of
4-5 microns and the top subcell is an (Al)InGaP subcell with a
thickness up to 1.5 microns. The trend in efficiency with bottom
band gap has two peaks, near 0.80 eV and 0.92 eV, which is a result
of at least two factors. One is the variation in solar spectrum
irradiance in the energy range between 0.7 eV and 1 eV. Another
factor is the limitation on composition (x.ltoreq.0.2 for
Al.sub.xIn.sub.0.5Ga.sub.1-xP) and thus band gap for the AlInGaP
subcell. When the band gap reaches the upper limit, the efficiency
begins to decrease because the limitation on band gap places a
limitation on the solar cell voltage. In all cases shown, the
measured efficiency is higher than the simulated efficiency for the
state-of-the-art triple junction solar cell structure under the
same conditions.
[0088] In FIG. 9 is shown another embodiment of the invention with
five subcells. The bottom subcell is a Ge subcell, incorporating
the Ge substrate. J4, the subcell directly above the Ge subcell, is
a III-AsNV subcell with a band gap between 1.0 eV-1.2 eV. J3, above
the III-AsNV subcell, is an (Al)InGaAs subcell with a band gap in
the range of 1.4 eV-1.5 eV. J2 is an AlInGaAs or an InGaP subcell
with a band gap in the range of 1.6 eV-1.8 eV. J1 is an (Al)InGaP
subcell with a band gap in the range of 1.9 eV-2.2 eV. Examples of
band gaps for the subcells above the Ge subcell, from the bottom to
the top are, respectively, 1.0 eV-1.1 eV, 1.4 eV, 1.6 eV-1.7 eV,
and 2.0-2.1 eV. All of the subcells are substantially
lattice-matched to each other and may be connected in series by
tunnel junctions. In a related embodiment, the structure is the
same except that one or both of J2 and J3 is an (Al)GaInPAs
subcell.
[0089] Yet another embodiment of the invention with five subcells
is depicted in FIG. 10. The bottom subcell is a Ge subcell,
incorporating the Ge substrate. J4 is a III-AsNV subcell with a
band gap between 0.9 eV-1.0 eV or between 1.0 eV-1.2 eV. J3 is a
III-AsNV subcell with a band gap in the range of 1.2 eV-1.4 eV. J2
is an AlInGaAs or an InGaP subcell with a band gap in the range of
1.6 eV-1.8 eV. The top subcell is an (Al)InGaP subcell with a band
gap in the range of 1.9 eV-2.2 eV. Examples of band gaps for the
subcells above the Ge subcell, from bottom to top, are,
respectively 1.0 eV-1.1 eV, 1.3 eV, 1.6 eV-1.7 eV and 2.0-2.1 eV.
All of the subcells are substantially lattice-matched to each other
and may be connected in series by tunnel junctions. In a related
embodiment of the invention, the structure is the same except that
J3 is a GaInNAs subcell. In another related embodiment, the
structure is the same except that J2 is a (Al)GaInPAs subcell.
[0090] Embodiments of the invention with six subcells are
illustrated in FIGS. 11, 12, 13A, and 13B.
[0091] In FIG. 11, the bottom three subcells are III-AsNV subcells.
The band gap of the bottom subcell, J6, is between 0.7 eV-1.1 eV.
The band gap of J5 is between 0.9-1.3 eV and is greater than or
equal to the band gap of the bottom subcell. The band gap of J4 is
between 1.1-1.4 eV and is greater than or equal to the band gap of
J5. Above the III-AsNV subcells is J3, which is an (Al,In)GaAs
subcell with a band gap in the range of 1.4-1.7 eV. J2 is an
Al(In)GaAs or an (Al)InGaP subcell with a band gap in the range of
1.7-2.0 eV. The top subcell is an (Al)InGaP subcell with a band gap
in the range of 1.9-2.2 eV or 2.2-2.3 eV. Examples of band gaps for
the subcells from bottom to top are, respectively, 0.9-1.0 eV,
1.1-1.2 eV, 1.3-1.4 eV, 1.5-1.6 eV, 1.8-1.9 eV, and 2.0-2.1 eV. As
another example, the band gaps of the subcells from bottom to top
are, respectively, 0.7-0.8 eV, 0.9-1.0 eV, 1.1-1.2 eV, 1.4-1.5 eV,
1.7-1.8 eV, and 2.1-2.2 eV. All of the subcells are substantially
lattice-matched to each other and may be connected in series by
tunnel junctions. In a related embodiment of the invention, the
structure is the same except that one or both of J4 and J5 are
GaInNAs subcell(s). In a related embodiment, the structure is the
same except that one or both J2 and J3 is a (Al)GaInPAs subcell. In
an additional related embodiment, the structure is the same except
the bottom subcell is a SiGe(Sn) subcell.
[0092] FIG. 14 shows the efficiencies predicted by the simulation
as a function of the band gap of the bottom subcell for a specific
embodiment of the invention depicted in FIG. 11 under an
illumination intensity of 100 W/cm.sup.2 or 1000 suns under the
AM1.5D spectrum at 25.degree. C. In this embodiment, the three
bottom subcells are GaInNAsSb subcells each with a total subcell
thicknesses of 2-3 microns, the J2 and J3 are (Al)GaAs subcells
each with a thickness of 4-5 microns, and J1 is an (Al)InGaP
subcell with a thickness up to 1.5 microns. In all cases shown, the
efficiency is higher than the simulated efficiency for the
state-of-the-art triple junction solar cell structure under the
same conditions.
[0093] In FIG. 12, the bottom two subcells are III-AsNV subcells.
The band gap of the bottom subcell, J6, is between 0.7-1.1 eV. The
band gap of J5 is between 0.9-1.3 eV and is greater than or equal
to the band gap of J6. The band gap of J4 is between 1.4-1.5 eV and
it is an (Al,In)GaAs subcell. J3 is an (Al,In)GaAs subcell with a
band gap in the range of 1.5-1.7 eV. J2 is an Al(In)GaAs or an
(Al)InGaP subcell with a band gap in the range of 1.7-2.0 eV. J1 is
an (Al)InGaP subcell with a band gap in the range of 1.9-2.2 eV or
2.2-2.3 eV. Examples of band gaps for the subcells, from bottom to
top, are, respectively, 0.9-1.0 eV, 1.1-1.2 eV, 1.4 eV, 1.6-1.7 eV,
1.8-1.9 eV, and 2.0-2.1 eV. All of the subcells are substantially
lattice-matched to each other and may be connected in series by
tunnel junctions. In a related embodiment of the invention, the
structure is the same except that J5 is a GaInNAs subcell. In a
related embodiment, the structure of a photovoltaic cell is the
same as in FIG. 12 except that one or more of J2, J3, and J4 is a
(Al)GaInPAs subcell. In an additional related embodiment, the
bottom subcell is a SiGe(Sn) subcell.
[0094] FIG. 13A depicts an embodiment of the invention where the
bottom subcell comprises the Ge substrate. J4 and J5 are III-AsNV
subcells. The band gap of J5 is between 0.9-1.1 eV. The band gap of
J4 is between 1.1-1.3 eV and is greater than or equal to the band
gap of the bottom subcell. J3 is an (Al)InGaAs subcell with a band
gap in the range of 1.4-1.6 eV. J2 is an AlInGaAs or an (Al)InGaP
subcell with a band gap in the range of 1.6-1.9 eV. The top subcell
is an (Al)InGaP subcell with a band gap in the range of 1.9-2.2 eV
or 2.2-2.3 eV. Examples of band gaps for J5 to J1 are,
respectively, 0.9-1.0 eV, 1.1-1.2 eV, 1.4-1.5 eV, 1.7-1.8 eV, and
2.0-2.1 eV. All of the subcells are substantially lattice-matched
to each other and may be connected in series by tunnel junctions.
In a related embodiment of the invention, the structure of a
photovoltaic cell is the same as depicted in FIG. 13A except that
J4 is a GaInNAs subcell. In a related embodiment to the
above-described certain embodiment, one or both of the J2 and J3 is
a (Al)InGaPAs subcell. This embodiment is depicted in FIG. 13B.
[0095] In certain of the embodiments described herein, including
any of the photovoltaic cells shown in FIG. 1A-B, 2A-B, 3A, 3C-D,
7, 11-12, 15, 16, or 17B, a bottom III-AsNV subcell is a GaInNAsSb
subcell with a base layer comprising
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y, and z are 0.ltoreq.x.ltoreq.0.24, 0.01.ltoreq.y.ltoreq.0.07
and 0.001.ltoreq.z.ltoreq.0.20; in certain embodiments,
0.02.ltoreq.x.ltoreq.0.24, 0.01.ltoreq.y.ltoreq.0.07 and
0.001.ltoreq.z.ltoreq.0.03; in certain embodiments,
0.02.ltoreq.x.ltoreq.0.18, 0.01.ltoreq.y.ltoreq.0.04 and
0.001.ltoreq.z.ltoreq.0.03; in certain embodiments,
0.08.ltoreq.x.ltoreq.0.18, 0.025.ltoreq.y.ltoreq.0.04 and
0.001.ltoreq.z.ltoreq.0.03; and in certain embodiments,
0.06.ltoreq.x.ltoreq.0.20, 0.02.ltoreq.y.ltoreq.0.05 and
0.005.ltoreq.z.ltoreq.0.02.
[0096] In certain of the embodiments described herein, including
any of the photovoltaic cells shown in FIG. 1A-B, 2A-B, 3A-D, 6, 7,
9-13, 15, 16, 17B, 18B, or 19B, a III-AsNV subcell directly above
the bottom subcell is a GaInNAsSb subcell with a base layer
comprising Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which
values for x, y, and z are 0.ltoreq.x.ltoreq.0.18,
0.001.ltoreq.y.ltoreq.0.05 and 0.001.ltoreq.z.ltoreq.0.15, and in
certain embodiments, 0.ltoreq.x.ltoreq.0.18,
0.001.ltoreq.y.ltoreq.0.05 and 0.001.ltoreq.z.ltoreq.0.03; in
certain embodiments, 0.02.ltoreq.x.ltoreq.0.18,
0.005.ltoreq.y.ltoreq.0.04 and 0.001.ltoreq.z.ltoreq.0.03; in
certain embodiments, 0.04.ltoreq.x.ltoreq.0.18,
0.01.ltoreq.y.ltoreq.0.04 and 0.001.ltoreq.z.ltoreq.0.03; in
certain embodiments, 0.06.ltoreq.x.ltoreq.0.18,
0.015.ltoreq.y.ltoreq.0.04 and 0.001.ltoreq.z.ltoreq.0.03; and in
certain embodiments, 0.08.ltoreq.x.ltoreq.0.18,
0.025.ltoreq.y.ltoreq.0.04 and 0.001.ltoreq.z.ltoreq.0.03.
[0097] In certain of the embodiments described herein, including
any of the photovoltaic cells shown in FIG. 1A-B, 2A-B, 10-11,
13A-B, 15, or 19B, a III-AsNV subcell that is the third from the
bottom (e.g., J4 in a six junction solar cell or J3 in a five
junction solar cell) is a GaInNAsSb subcell with a base layer
comprising Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which
values for x, y, and z are 0.ltoreq.x.ltoreq.0.12,
0.001.ltoreq.y.ltoreq.0.03 and 0.001.ltoreq.z.ltoreq.0.10; in
certain embodiments, 0.ltoreq.x.ltoreq.0.12,
0.001.ltoreq.y.ltoreq.0.03 and 0.001.ltoreq.z.ltoreq.0.03; in
certain embodiments, 0.02.ltoreq.x.ltoreq.0.10,
0.005.ltoreq.y.ltoreq.0.02 and 0.001.ltoreq.z.ltoreq.0.02; in
certain embodiments, 0.01.ltoreq.x.ltoreq.0.06,
0.005.ltoreq.y.ltoreq.0.015 and 0.001.ltoreq.z.ltoreq.0.02; and in
certain embodiments, 0.01.ltoreq.x.ltoreq.0.08,
0.005.ltoreq.y.ltoreq.0.025 and 0.001.ltoreq.z.ltoreq.0.02.
[0098] In each of the embodiments described herein, the tunnel
junctions are designed to have minimal light absorption. Light
absorbed by tunnel junctions is not converted into electricity by
the solar cell, and thus if the tunnel junctions absorb significant
amounts of light, it will not be possible for the efficiencies of
the multijunction solar cells to exceed those of the best triple
junction solar cells. Accordingly, the tunnel junctions must be
very thin (preferably less than 40 nm) and/or be made of materials
with band gaps equal to or greater than the subcells immediately
above them. An example of a tunnel junction fitting these criteria
is a GaAs/AlGaAs tunnel junction, where each of the GaAs and AlGaAs
layers forming a tunnel junction have a thickness of between 5- and
15 nm. The GaAs layer is doped with Te, Se, S and/or Si, and the
AlGaAs layer is doped with C.
[0099] In each of the embodiments described and illustrated herein,
additional elements are present in order to create a solar cell
device. Specifically, cap or contact layer(s), anti-reflection
coating (ARC) layers and electrical contacts (also denoted the
"metal grid") are typically formed above the top subcell, and
buffer layer(s), the substrate or handle, and bottom contacts are
typically formed or exist below the bottom subcell. In certain
embodiments, the substrate may be part of the bottom subcell, such
as in a Ge subcell. Other elements, such as additional tunnel
junctions, may also be present. Devices may also be formed without
all of the elements listed above, as known to those skilled in the
art. An example illustrating these typical additional elements, and
their typical positions relative to the top and bottom subcells, is
shown in FIG. 15.
[0100] A structural example depicting the individual layers that
may compose a multijunction solar cell with four subcells according
to the invention is shown in detail in FIG. 16 and described
herein. In operation, a multijunction cell is configured such that
the subcell having the highest bandgap faces the incident solar
radiation, with subcells characterized by increasingly lower band
gaps situated underneath.
[0101] In the embodiments disclosed herein, each subcell may
comprise several layers. For example, each subcell may comprise a
window layer, an emitter, a base, and a back surface field (BSF)
layer. As shown in FIG. 16, the window layer is above the emitter
layer, which is above the base, which is above the BSF.
[0102] In operation, the window layer is the topmost layer of a
subcell and faces the incident solar radiation. In certain
embodiments, the thickness of a window layer is from about 10 nm to
about 500 nm, from about 10 nm to about 300 nm, from about 10 nm to
about 150 nm, and in certain embodiments, from about 10 nm to about
50 nm. In certain embodiments, the thickness of a window layer is
from about 50 nm to about 350 nm, from about 100 nm to about 300
nm, and in certain embodiments, from about 50 nm to about 150
nm.
[0103] In certain embodiments, the thickness of an emitter layer is
from about 10 nm to about 300 nm, from about 20 nm to about 200 nm,
from about 50 nm to about 200 nm, and in certain embodiments, from
about 75 nm to about 125 nm.
[0104] In certain embodiments, the thickness of a base layer is
from about 0.1 .mu.m to about 6 .mu.m, from about 0.1 .mu.m to
about 4 .mu.m, from about 0.1 .mu.m to about 3 .mu.m, from about
0.1 .mu.m to about 2 .mu.m, and in certain embodiments, from about
0.1 .mu.m to about 1 .mu.m. In certain embodiments, the thickness
of a base layer is from about 0.5 .mu.m to about 5 .mu.m, from
about 1 .mu.m to about 4 .mu.m, from about 1.5 .mu.m to about 3.5
.mu.m, and in certain embodiments, from about 2 .mu.m to about 3
.mu.m.
[0105] In certain embodiments the thickness of a BSF layer is from
about 10 nm to about 500 nm, from about 50 nm to about 300 nm, and
in certain embodiments, from about 50 nm to about 150 nm.
[0106] In certain embodiments, an (Al)InGaP subcell comprises a
window comprising AlInP, an emitter comprising (Al)InGaP, a base
comprising (Al)InGaP, and a back surface field layer comprising
AlInGaP.
[0107] In certain embodiments, an (Al)InGaP subcell comprises a
window comprising AlInP having a thickness from 10 nm to 50 nm, an
emitter comprising (Al)InGaP having a thickness from 20 nm to 200
nm, a base comprising (Al)InGaP having a thickness from 0.1 .mu.m
to 2 .mu.m, and a BSF layer comprising AlInGaP having a thickness
from 50 nm to 300 nm.
[0108] In certain of such embodiments, an (Al)InGaP subcell is
characterized by a band gap from about 1.9 eV to about 2.2 eV.
[0109] In certain embodiments, an (Al,In)GaAs subcell comprises a
window comprising (Al)In(Ga)P or Al(In)GaAs, an emitter comprising
(Al)InGaP or (Al,In)GaAs, a base comprising (Al,In)GaAs, and a BSF
layer comprising Al(In)GaAs or (Al)InGaP. In certain embodiments,
an (Al,In)GaAs subcell comprises a window comprising (Al)InGaP
having a thickness from 50 nm to 400 nm, an emitter comprising
(Al,In)GaAs having a thickness from 100 nm to 200 nm, a base
comprising (Al,In)GaAs having a thickness from 1 .mu.m to 4 .mu.m,
and a BSF layer comprising Al(In)GaAs having a thickness from 100
nm to 300 nm.
[0110] In certain embodiments, an (Al,In)GaAs subcell comprises a
window comprising (Al)InGaP having a thickness from 200 nm to 300
nm, an emitter comprising (Al,In)GaAs having a thickness from 100
nm to 150 nm, a base comprising (Al,In)GaAs having a thickness from
2 .mu.m to 3.5 .mu.m, and a BSF layer comprising Al(In)GaAs having
a thickness from 150 nm to 250 nm.
[0111] In certain of such embodiments, an (Al,In)GaAs subcell is
characterized by a band gap from about 1.4 eV to about 1.7 eV.
[0112] In certain embodiments, an (Al)GaInPAs subcell comprises a
window comprising (Al)In(Ga)P, an emitter comprising (Al)InGaP or
(Al)GaInPAs, a base comprising (Al)GaInPAs, and a BSF layer
comprising Al(In)GaAs or (Al)InGaP. In certain embodiments, an
(Al)GaInPAs subcell comprises a window comprising (Al)In(Ga)P
having a thickness from 50 nm to 300 nm, an emitter comprising
(Al)InGaP or (Al)GaInPAs having a thickness from 100 nm to 200 nm,
a base comprising (Al)GaInPAs having a thickness from 0.5 .mu.m to
4 .mu.m, and a BSF layer comprising Al(In)GaAs or (Al)InGaP having
a thickness from 50 nm to 300 nm.
[0113] In certain of such embodiments, an (Al)GaInPAs subcell is
characterized by a band gap from about 1.4 eV to about 1.8 eV.
[0114] In certain embodiments, a III-AsNV alloy subcell comprises a
window comprising InGaP or (Al,In)GaAs, an emitter comprising
(In)GaAs or a III-AsNV alloy, a base comprising a III-AsNV alloy,
and a BSF layer comprising (In)GaAs.
[0115] In certain embodiments, a III-AsNV alloy subcell comprises a
window comprising InGaP or (In)GaAs, having a thickness from 0 nm
to 300 nm, an emitter comprising (In)GaAs or a III-AsNV alloy
having a thickness from 100 nm to 200 nm, a base comprising a
III-AsNV alloy having a thickness from 1 .mu.m to 4 .mu.m, and a
BSF layer comprising (In)GaAs having a thickness from 50 nm to 300
nm. In certain embodiments, a III-AsNV alloy subcell comprises an
emitter comprising InGaAs or a III-AsNV alloy having a thickness
from 100 nm to 150 nm, a base comprising a III-AsNV alloy having a
thickness from 2 .mu.m to 3 .mu.m, and a BSF layer comprising
(In)GaAs having a thickness from 50 nm to 200 nm.
[0116] In certain of such embodiments, a III-AsNV subcell is
characterized by a band gap from about 0.7 to about 1.1 eV, or
about 0.9 eV to about 1.3 eV. In certain of such embodiments, the
III-AsNV subcell is a GaInNAsSb subcell.
[0117] In certain of such embodiments, a III-AsNV subcell has a
compressive strain of less than 0.6%, meaning that the in-plane
lattice constant of the III-AsNV material in its fully relaxed
state is between 0.0% and 0.6% greater than that of the substrate.
In certain of such embodiments, the III-AsNV subcell contains Sb
and does not contain Bi.
[0118] In certain embodiments, a SiGe(Sn) subcell is characterized
by a band gap from about 0.7 eV to about 0.9 eV. In certain
embodiments, a SiGe(Sn) subcell comprises a window comprising InGaP
or (In)GaAs, having a thickness from 0 nm to 300 nm, an emitter
comprising (In)GaAs or a III-AsNV alloy having a thickness from 50
nm to 500 nm, and a base comprising SiGe(Sn) having a thickness
from 1 .mu.m to 20 .mu.m. In some embodiments, the subcell also
comprises a BSF layer comprising (In)GaAs or SiGe(Sn) having a
thickness from 50 nm to 300 nm.
[0119] In certain embodiments, a Ge subcell comprises a window
comprising InGaP or (In)GaAs, having a thickness from 0 nm to 300
nm, an emitter comprising (In)GaAs, (Al,Ga)InP, or a III-AsNV
alloy, having a thickness from 10 nm to 500 nm, and a base
comprising the Ge substrate. It is to be noted that solar cells
disclosed by the invention may also be formed on a Ge substrate
wherein the substrate is not part of a subcell.
[0120] In certain embodiments, one or more of the subcells has an
emitter and/or a base in which there is a graded doping profile.
The doping profile may be linear, exponential or with other
dependence on position. In certain of such embodiments, one or more
of the III-AsNV subcells has an exponential or linear doping
profile over part or all of the base, with the doping levels
between 1.times.10.sup.15 and 1.times.10.sup.19 cm.sup.-3, or
between 1.times.10.sup.16 and 5.times.10.sup.18 cm.sup.-3. Further,
the region of the III-AsNV base that is closest to the emitter may
have constant or no doping, as disclosed, for example, in U.S.
patent application Ser. No. 12/914,710, which is incorporated by
reference. Typical dopants include, for example, Be, Mg, Zn, Te,
Se, Si, C, and others known in the art.
[0121] As shown in FIG. 16, a tunnel junction may be disposed
between each of the subcells. Each tunnel junction comprises two or
more layers that electrically connect adjacent subcells. The tunnel
junction includes a highly doped n-type layer adjacent to a highly
doped p-type layer to form a p-n junction. Typically, the doping
levels in a tunnel junction are between 10.sup.18 cm.sup.-3 and
10.sup.21 cm.sup.-3.
[0122] In certain embodiments, a tunnel junction comprises an
n-type (In)GaAs or InGaP(As) layer and a p-type (Al,In)GaAs layer.
In certain embodiments the dopant of the n-type layer comprises Si
and the dopant of the p-type layer comprises C. A tunnel junction
may have a thickness less than about 100 nm, less than 80 nm, less
than 60 nm, less than 40 nm, and in certain embodiments, less than
20 nm. For example, in certain embodiments, a tunnel junction
between (Al)InGaP subcells, between an (Al)InGaP subcell and an
(Al,In)GaAs or (Al)GaInPAs subcell, or between (Al,In)GaAs subcells
may have a thickness less than about 30 nm, less than about 20 nm,
less than about 15 nm, and in certain embodiments, less than about
12 nm. In certain embodiments, a tunnel junction separating an
(Al,In)GaAs and III-AsNV alloy subcell, separating adjacent
III-AsNV alloy subcells, or separating a III-AsNV alloy and a
SiGe(Sn) or Ge subcell may have a thickness less than 100 nm, less
than 80 nm, less than 60 nm, and in certain embodiments, less than
40 nm.
[0123] A multijunction solar cell may be fabricated on a substrate
such as a Ge substrate. In certain embodiments, the substrate
comprises GaAs, InP, Ge, or Si. In certain embodiments, all of the
subcells are substantially lattice-matched to the substrate. In
certain embodiments, one or more of the layers that are included
within the completed solar cell but are not part of a subcell such
as, for example, anti-reflective coating layers, contact layers,
cap layers, tunnel junction layers, and buffer layers, are not
substantially lattice-matched to the subcells.
[0124] In certain embodiments, a buffer layer is fabricated
overlying the substrate. In certain embodiments, the buffer layer
comprises (In)GaAs.
[0125] As shown in FIG. 16, the multijunction solar cell comprises
subcells characterized by progressively higher band gaps overlying
the buffer layer, with each of the subcells typically separated by
a tunnel junction.
[0126] In certain embodiments, the multijunction solar cell
comprises an anti-reflection coating overlying the uppermost
subcell. The materials comprising the anti-reflection coating and
the thickness of the anti-reflection coating are selected to
improve the efficiency of light capture in the multijunction solar
cell. In certain embodiments, one or more contact layers overlie
the uppermost subcell in the regions underlying or near the metal
grid. In certain embodiments, the contact layers comprise (In)GaAs
and the dopant may be Si or Be.
[0127] In certain embodiments, a photovoltaic cell comprises at
least four subcells, wherein: the at least four subcells comprise
at least one subcell comprising a base layer, wherein the base
layer comprises an alloy of elements from group III on the periodic
table, nitrogen, arsenic, and at least one element selected from Sb
and Bi; and each of the at least four subcells is substantially
lattice matched to each of the other subcells.
[0128] In certain embodiments of a photovoltaic cell, each of the
at least four subcells is substantially lattice matched to a
material selected from Si, Ge, SiGe, GaAs, and InP.
[0129] In certain embodiments of a photovoltaic cell, the at least
one subcell is characterized by a bandgap selected from 0.7 eV to
1.1 eV, from 0.9 eV to 1.0 eV, from 0.9 eV to 1.3 eV, from 1.0 eV
to 1.1 eV from 1.0 eV to 1.2 eV, from 1.1 eV to 1.2 eV, from 1.1 eV
to 1.4 eV, and from 1.2 eV to 1.4 eV.
[0130] In certain embodiments of a photovoltaic cell, the base
layer of the at least one subcell comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.ltoreq.x.ltoreq.0.24, 0.001.ltoreq.y.ltoreq.0.07
and 0.001.ltoreq.z.ltoreq.0.20.
[0131] In certain embodiments of a photovoltaic cell, the base
layer of the at least one subcell comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.01.ltoreq.x.ltoreq.0.18,
0.005.ltoreq.y.ltoreq.0.05, and 0.001.ltoreq.z.ltoreq.0.03.
[0132] In certain embodiments of a photovoltaic cell, the at least
four subcells comprise at least two subcells, each of the at least
two subcells comprising a base layer comprising an alloy of
elements from group III on the periodic table, nitrogen, arsenic,
and at least one element selected from Sb and Bi.
[0133] In certain embodiments of a photovoltaic cell, one of the at
least two subcells is characterized by a first band gap of 0.7 to
1.1 eV; and a second of the at least two subcells is characterized
by a second band gap of 0.9 to 1.3 eV, wherein the second band gap
is greater than the first band gap.
[0134] In certain embodiments of a photovoltaic cell, each of the
at least two subcells comprise a base layer comprising a material
independently selected from GaInNAsSb, GaInNAsBi, GaInNAsSbBi,
GaNAsSb, GaNAsBi, and GaNAsSbBi.
[0135] In certain embodiments of a photovoltaic cell, one of the at
least two subcells comprises a base layer comprising
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.02.ltoreq.x.ltoreq.0.24,
0.015.ltoreq.y.ltoreq.0.07 and 0.001.ltoreq.z.ltoreq.0.03 and a
second of the at least two subcells comprises a base layer
comprising Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which
values for x, y and z are 0.ltoreq.x.ltoreq.0.18,
0.005.ltoreq.y.ltoreq.0.05 and 0.001.ltoreq.z.ltoreq.0.03.
[0136] In certain embodiments of a photovoltaic cell, the
photovoltaic cell comprises a first subcell comprising a first base
layer comprising a material selected from Ge, SiGe(Sn), and an
alloy of elements from group III on the periodic table, nitrogen,
arsenic, and at least one element selected from Sb and Bi, and
characterized by a band gap of 0.7 eV to 1.1 eV; a second subcell
comprising a second base layer overlying the first subcell, wherein
the second base layer comprises an alloy of elements from group III
on the periodic table, nitrogen, arsenic, and at least one element
selected from Sb and Bi, and characterized by a band gap of 0.9 eV
to 1.3 eV; a third subcell comprising a third base layer overlying
the second subcell, the third base layer comprising a material
selected from GaInPAs and (Al,In)GaAs and characterized by a band
gap from 1.4 eV to 1.7 eV; and a fourth subcell comprising a fourth
base layer overlying the third subcell, the fourth base layer
comprising (Al)InGaP and characterized by a band gap from 1.9 eV to
2.2 eV.
[0137] In certain embodiments of a photovoltaic cell, the first
base layer, the second base layer, or both the first and the second
base layer comprises the alloy
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.ltoreq.x.ltoreq.0.24, 0.001.ltoreq.y.ltoreq.0.07
and 0.001.ltoreq.z.ltoreq.0.20.
[0138] In certain embodiments of a photovoltaic cell, the band gap
of the first base layer is 0.7 to 0.9 eV, the band gap of the
second base layer is 1.0 to 1.2 eV, the band gap of the third base
layer is 1.5 to 1.6 eV, and the band gap of the fourth base layer
is 1.9 eV to 2.1 eV.
[0139] In certain embodiments of a photovoltaic cell, the first
base layer, the second base layer, or both the first and the second
base layer comprises the alloy
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.01.ltoreq.x.ltoreq.0.18,
0.005.ltoreq.y.ltoreq.0.05, and 0.001.ltoreq.z.ltoreq.0.03.
[0140] In certain embodiments of a photovoltaic cell, each of the
four subcells is substantially lattice matched to a material
selected from Ge and GaAs.
[0141] In certain embodiments of a photovoltaic cell, the
photovoltaic cell comprises a first subcell comprising a first base
layer comprising a material selected from the group consisting of
Ge, SiGe(Sn), and an alloy of elements from group III on the
periodic table, nitrogen, arsenic, and at least one element
selected from Sb and Bi, and characterized by a band gap of 0.7 eV
to 1.1 eV; a second subcell comprising a second base layer
overlying the first subcell, wherein the second base layer
comprises an alloy of elements from group III on the periodic
table, nitrogen, arsenic, and at least one element selected from Sb
and Bi, and characterized by a band gap of 0.9 eV to 1.3 eV; a
third subcell comprising a third base layer overlying the second
subcell, wherein the second base layer comprises a material
selected from GaInPAs, (Al,In)GaAs, and an alloy of elements from
group III on the periodic table, nitrogen, arsenic, and at least
one element selected from Sb and Bi, and characterized by a band
gap of 1.2 eV to 1.6 eV; a fourth subcell comprising a fourth base
layer overlying the third subcell, the fourth base layer comprising
a material selected from GaInPAs and (Al,In)GaAs and characterized
by a band gap from 1.6 eV to 1.9 eV; and a fifth subcell comprising
a fifth base layer overlying the fourth subcell, the fifth base
layer comprising (Al)InGaP and characterized by a band gap from 1.9
eV to 2.2 eV.
[0142] In certain embodiments of a photovoltaic cell comprising
five subcells, one or more of the first base layer, the second base
layer, and the third base layer comprise the alloy
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.ltoreq.x.ltoreq.0.24, 0.001.ltoreq.y.ltoreq.0.07
and 0.001.ltoreq.z.ltoreq.0.20. In certain of such embodiments, one
or more of the first base layer, the second base layer, and the
third base layer comprise the alloy
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.01.ltoreq.x.ltoreq.0.18,
0.005.ltoreq.y.ltoreq.0.05, and 0.001.ltoreq.z.ltoreq.0.03.
[0143] In certain embodiments of a photovoltaic cell, the
photovoltaic cell comprises a first subcell comprising a first base
layer comprising a material selected from Ge, SiGe(Sn), and an
alloy of elements from group III on the periodic table, nitrogen,
arsenic, and at least one element selected from Sb and Bi, and
characterized by a band gap of 0.7 eV to 1.1 eV; a second subcell
comprising a second base layer overlying the first subcell, wherein
the second base layer comprises an alloy of elements from group III
on the periodic table, nitrogen, arsenic, and at least one element
selected from Sb and Bi, and characterized by a band gap of 0.9 eV
to 1.3 eV; a third subcell comprising a third base layer overlying
the second subcell, wherein the third base layer comprises a
material selected from GaInPAs, (Al,In)GaAs and an alloy of
elements from group III on the periodic table, nitrogen, arsenic,
and at least one element selected from Sb and Bi, and characterized
by a band gap of 1.1 eV to 1.5 eV; a fourth subcell comprising a
fourth base layer overlying the third subcell, the fourth base
layer comprising a material selected from (Al,In)GaAs and
(Al)InGa(P)As, and characterized by a band gap from 1.4 eV to 1.7
eV; a fifth subcell comprising a fifth base layer overlying the
fourth subcell, the fifth base layer comprising a material selected
from (Al)InGaP and Al(In)Ga(P)As, and characterized by a band gap
from 1.6 eV to 2.0 eV; and a sixth subcell comprising a sixth base
layer overlying the fifth subcell, the sixth base layer comprising
(Al)InGaP, and characterized by a band gap from 1.9 eV to 2.3
eV.
[0144] In certain embodiments of a photovoltaic cell comprising six
subcells, one or more of the first base layer, the second base
layer, and the third base layer comprise the alloy
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.ltoreq.x.ltoreq.0.24, 0.001.ltoreq.y.ltoreq.0.07
and 0.001.ltoreq.z.ltoreq.0.20. In certain of such embodiments, one
or more of the first base layer, the second base layer, and the
third base layer comprise the alloy
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.01.ltoreq.x.ltoreq.0.18,
0.005.ltoreq.y.ltoreq.0.05, and 0.001.ltoreq.z.ltoreq.0.03.
[0145] In one embodiment of the invention, a photovoltaic power
system comprises one or more of a photovoltaic cell provided by the
present disclosure such as, for example, one or more photovoltaic
cells having at least four subcells, including one or more III-AsNV
subcells. In one specific embodiment, the one or more photovoltaic
cells has a III-AsNV subcell as the bottom subcell or the subcell
immediately above the bottom subcell. In certain embodiments, the
photovoltaic power system may be a concentrating photovoltaic
system, wherein the system may also comprise mirrors and/or lenses
used to concentrate sunlight onto one or more photovoltaic cells.
In certain embodiments, the photovoltaic power system comprises a
single or dual axis tracker. In certain embodiments, the
photovoltaic power system is designed for portable applications,
and in other embodiments, for grid-connected power generation. In
certain embodiments, the photovoltaic power system is designed to
convert a specific spectrum of light, such as AM1.5G, AM1.5D or
AM0, into electricity. In certain embodiments, the photovoltaic
power system may be found on satellites or other extra-terrestrial
vehicles and designed for operation in space without the influence
of a planetary atmosphere on the impinging light source. In certain
embodiments, the photovoltaic power system may be designed for
operation on astronomical bodies other than Earth. In certain
embodiments, the photovoltaic power system may be designed for
satellites orbiting about astronomical bodies other than Earth. In
certain embodiments, the photovoltaic power system may be designed
for roving on the surface of an astronomical body other than
Earth.
[0146] In certain embodiments of the invention, photovoltaic
modules are provided comprising one or more photovoltaic cells
provided by the present disclosure. A photovoltaic module may
comprise one or more photovoltaic cells provided by the present
disclosure to include an enclosure and interconnects to be used
independently or assembled with additional modules to form a
photovoltaic power system. A module and/or power system may include
power conditioners, power converters, inverters and other
electronics to convert the power generated by the photovoltaic
cells into usable electricity. A photovoltaic module may further
include optics for focusing light onto a photovoltaic cell provided
by the present disclosure such as in a concentrated photovoltaic
module.
[0147] In one embodiment of the invention, the semiconductor layers
composing the solar cell, excepting the substrate, are fabricated
using molecular beam epitaxy (MBE) or chemical vapor deposition. In
certain embodiments, more than one material deposition chamber is
used for the deposition of the semiconductor layers comprising the
solar cell. The materials deposition chamber is the apparatus in
which the semiconductor layers composing the solar cell are
deposited. The conditions inside the chamber may range from
10.sup.-11 Torr to 10.sup.3 Torr pressures. In certain embodiments
the alloy constituents are deposited via physical and/or chemical
processes. Each materials deposition chamber can have different
configurations which allow it to deposit different semiconductor
layers and can be independently controlled from other materials
deposition chambers. The semiconductor layers may be fabricated
using metal organic chemical vapor deposition (MOCVD), MBE, or by
other methods, including a combination of any of the foregoing.
[0148] The movement of the substrate and semiconductor layers from
one materials deposition chamber to another is defined as the
transfer. For example, a substrate is placed in a first materials
deposition chamber, and then the buffer layer(s) and the bottom
subcell(s) are deposited. Then the substrate and semiconductor
layers are transferred to a second materials deposition chamber
where the remaining subcells are deposited. The transfer may occur
in vacuum, at atmospheric pressure in air or another gaseous
environment, or in any environment in between. The transfer may
further be between materials deposition chambers in one location,
which may or may not be interconnected in some way, or may involve
transporting the substrate and semiconductor layers between
different locations, which is known as transport. Transport may be
done with the substrate and semiconductor layers sealed under
vacuum, surrounded by nitrogen or another gas, or surrounded by
air. Additional semiconductor, insulating or other layers may be
used as surface protection during transfer or transport, and
removed after transfer or transport before further deposition.
[0149] In one embodiment of the invention, a plurality of layers is
deposited on a substrate in a first materials deposition chamber.
The plurality of layers may include etch stop layers, release
layers (i.e., layers designed to release the semiconductor layers
from the substrate when a specific process sequence, such as
chemical etching, is applied), contact layers such as lateral
conduction layers, buffer layers, or other semiconductor layers. In
one specific embodiment, the sequence of layers deposited is buffer
layer(s), then release layer(s), and then lateral conduction or
contact layer(s). Next the substrate is transferred to a second
materials deposition chamber where one or more subcells are
deposited on top of the existing semiconductor layers. The
substrate may then be transferred to either the first materials
deposition chamber or to a third materials deposition chamber for
deposition of one or more subcells and then deposition of one or
more contact layers. Tunnel junctions are also formed between the
subcells.
[0150] In one embodiment of the invention, the III-AsNV subcells
are deposited in a first materials deposition chamber, and the
(Al)InGaP, (Al,In)GaAs and (Al)GaInPAs subcells are deposited in a
second materials deposition chamber, with tunnel junctions formed
between the subcells. In a related embodiment of the invention,
III-AsNV layers are deposited in a first materials deposition
chamber, and other semiconductor layers that contain Al are
deposited in a second materials deposition chamber. In another
embodiment of the invention, a transfer occurs in the middle of the
growth of one subcell, such that the said subcell has one or more
layers deposited in one materials deposition chamber and one or
more layers deposited in a second materials deposition chamber.
[0151] In one embodiment of the invention, some or all of the
layers composing the III-AsNV subcells and the tunnel junctions are
deposited in one materials deposition chamber by molecular beam
epitaxy, and the remaining layers of the solar cell are deposited
by chemical vapor deposition in another materials deposition
chamber. For example, a substrate is placed in a first materials
deposition chamber and layers that may include nucleation layers,
buffer layers, emitter and window layers, contact layers and a
tunnel junction are grown on the substrate, followed by one or more
III-AsNV subcells. If there is more than one III-AsNV subcell, then
a tunnel junction is grown between adjacent subcells. One or more
tunnel junction layers may be grown, and then the substrate is
transferred to a second materials deposition chamber where the
remaining solar cell layers are grown by chemical vapor deposition.
In certain embodiments, the chemical vapor deposition system is a
MOCVD system. In a related embodiment of the invention, a substrate
is placed in a first materials deposition chamber and layers that
may include nucleation layers, buffer layers, emitter and window
layers, contact layers and a tunnel junction are grown on the
substrate by chemical vapor deposition. Subsequently, the top
subcells, two or more, are grown on the existing semiconductor
layers, with tunnel junctions grown between the subcells. Part of
the topmost III-AsNV subcell, such as the window layer, may then be
grown. The substrate is then transferred to a second materials
deposition chamber where the remaining semiconductor layers of the
topmost III-AsNV subcell may be deposited, followed by up to three
more III-AsNV subcells, with tunnel junctions between them.
[0152] In certain embodiments of the invention, the solar cell is
subjected to one or more thermal annealing treatments after growth.
For example, a thermal annealing treatment includes the application
of a temperature of 400.degree. C. to 1000.degree. C. for between
10 seconds and 10 hours. Thermal annealing may be performed in an
atmosphere that includes air, nitrogen, arsenic, arsine,
phosphorus, phosphine, hydrogen, forming gas, oxygen, helium and
any combination of the preceding materials. In certain embodiments,
a stack of subcells and associated tunnel junctions may be annealed
prior to fabrication of additional subcells.
[0153] Thus, methods of manufacturing a photovoltaic cell are
provided, comprising: forming one or more semiconductor layers on a
substrate; forming four or more subcells overlying the one or more
semiconductor layers; and wherein at least one of the subcells has
a base layer formed of an alloy of one or more elements from group
III on the periodic table, nitrogen, arsenic, and at least one
element selected from the group consisting of Sb and Bi; wherein
the photovoltaic cell comprises at least four subcells and each of
the at least four subcells is substantially lattice matched to each
of the other subcells. In certain embodiments, the substrate is a
subcell having a base layer formed of a material selected from the
group consisting of Ge, SiGe(Sn), and an alloy of one or more
elements from group III on the periodic table, nitrogen, arsenic,
and at least one element selected from the group consisting of Sb
and Bi. In certain embodiments, the methods comprise forming tunnel
junctions between the four or more subcells.
[0154] In certain embodiments, methods of manufacturing a
photovoltaic cell comprise: forming a first subcell having a first
base layer formed of a material selected from the group consisting
of Ge, SiGe(Sn), and an alloy of one or more elements from group
III on the periodic table, nitrogen, arsenic, and at least one
element selected from the group consisting of Sb and Bi, wherein
the first subcell is characterized by a band gap from 0.7 eV to 1.1
eV; forming a second subcell having a second base layer, wherein
the second base layer is formed of an alloy of one or more elements
from group III on the periodic table, nitrogen, arsenic, and at
least one element selected from the group consisting of Sb and Bi,
wherein the second subcell is characterized by a band gap from 0.9
eV to 1.3 eV; and forming at least two additional subcells
overlying the second subcell; wherein the photovoltaic cell
comprises at least four subcells and each of the at least four
subcells is substantially lattice matched to each of the other
subcells. In certain methods, each of the at least four subcells is
substantially lattice matched to a material selected from the group
consisting of Si, Ge, SiGe, GaAs, and InP. In certain methods, the
first base layer formed of an alloy
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.02.ltoreq.x.ltoreq.0.24,
0.015.ltoreq.y.ltoreq.0.07 and 0.001.ltoreq.z.ltoreq.0.03; and the
second base layer formed of an alloy
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z, in which values for
x, y and z are 0.ltoreq.x.ltoreq.0.18, 0.005.ltoreq.y.ltoreq.0.05
and 0.001.ltoreq.z.ltoreq.0.03. In certain methods, forming at
least two additional subcells overlying the second subcell
comprises: forming a third subcell having a third base layer is
overlying the second subcell, wherein the third base layer is
formed of a material selected from the group consisting GaInPAs and
(Al,In)GaAs, and characterized by a band gap from 1.4 eV to 1.7 eV;
and forming a fourth subcell having a fourth base layer overlying
the third subcell, wherein the fourth base layer is formed of
(Al)InGaP, and characterized by a band gap from 1.9 eV to 2.2
eV.
[0155] In certain embodiments, methods of manufacturing a
photovoltaic cell comprise: forming at least two subcells on a
substrate; forming a first subcell having a first base layer,
wherein the first base layer is formed of an alloy of one or more
elements from group III on the periodic table, nitrogen, arsenic,
and at least one element selected from the group consisting of Sb
and Bi, wherein the first subcell is characterized by a band gap
from 0.9 eV to 1.3 eV; and forming a second subcell having a second
base layer formed of a material selected from the group consisting
of Ge, SiGe(Sn), and an alloy of one or more elements from group
III on the periodic table, nitrogen, arsenic, and at least one
element selected from the group consisting of Sb and Bi, wherein
the second subcell is characterized by a band gap from 0.7 eV to
1.1 eV; wherein the photovoltaic cell comprises at least four
subcells and each of the at least four subcells is substantially
lattice matched to each of the other subcells.
[0156] In certain embodiments, methods of manufacturing a
photovoltaic cell comprise: forming one or more subcells on a
substrate in a first materials deposition chamber; transferring the
substrate to a second materials deposition chamber; and forming one
or more additional subcells overlying the one or more subcells; and
wherein one or more of the subcells of the photovoltaic cell has a
base layer formed of an alloy of one or more elements from group
III on the periodic table, nitrogen, arsenic, and at least one
element selected from the group consisting of Sb and Bi; and
wherein each of the subcells is substantially lattice matched to
each of the other subcells. In certain embodiments, methods of
manufacturing a photovoltaic cell further comprise: forming one or
more layers selected from the group consisting of a buffer layer, a
contact layer, an etch stop layer, a release layer, and other
semiconductor layer on the substrate in a chamber selected from the
group consisting of a third materials deposition chamber and the
second materials chamber; and transferring the substrate to the
first materials deposition chamber.
[0157] A more specific example of the embodiment illustrated by
FIG. 7 is the five junction solar cell depicted in FIG. 17B. The
bottom subcell is a GaInNAsSb subcell with a band gap of 1.0 eV and
a total subcell thickness of 2-3 microns. J4 is a GaInNAsSb subcell
with a band gap of 1.2 eV and a total subcell thickness of 2-3
microns. J3 is an AlGaAs subcell with a band gap of 1.5 eV and a
total subcell thickness of 4-5 microns. J2 is an AlGaAs subcell
with a band gap of 1.7 eV and a subcell thickness of 4-5 microns.
J1 is an AlInGaP subcell with a band gap of 2.1 eV and a subcell
thickness of 0.3-1.0 microns. The upper two tunnel junctions
comprise GaAs:Si/AlGaAs:C, each with a total thickness of
approximately 15 nm to 25 nm. The lower two tunnel junctions are
GaAs:Si/GaAs:C, each with total thickness between 40 and 100 nm.
All of the subcells are substantially lattice-matched to a GaAs
substrate. The solar cell undergoes a thermal anneal after growth
and before device processing, and an additional thermal anneal
during device processing.
[0158] An I-V curve of the simulated current output as a function
of voltage of the multijunction solar cell shown in FIG. 17B at
1000 suns under the AM1.5D spectrum at 25.degree. C. is shown in
FIG. 17A, along with the performance of the state-of-the-art high
efficiency triple-junction solar cell described herein. The two
solar cells have bottom GaInNAsSb subcells with the same band gap.
The simulated efficiency of the five junction solar cell according
to certain embodiments is 45.5%, compared to 40.8% for the triple
junction solar cell. While the achievable current at this solar
concentration is lower for the five-junction solar cell compared to
the three junction solar cell, the voltage is substantially higher.
The higher efficiency is achieved because much less of the incident
light energy is being lost as heat. More photons are absorbed by
subcells with band gaps closer to their energies, allowing more of
the energy to be converted into electricity and less into heat.
[0159] An I-V curve of the simulated current output as a function
of voltage of the multijunction solar cell shown in FIG. 18B, a
four junction solar cell disclosed by the invention, under the AM0
spectrum at 1 sun at 25.degree. C. is shown in FIG. 18A, along with
the simulated performance of the typical InGaP/InGaAs/Ge triple
junction solar cell found on the market today for use in space. The
simulated efficiency of the four-junction solar cell disclosed by
the invention is 33.2%, compared to 30.6% for the triple junction
solar cell. While the achievable current is lower for the four
junction solar cell compared to the three junction solar cell, the
voltage is substantially higher. I-V curves for the six junction
solar cell disclosed by the invention and shown in FIG. 19B under
the AM0 spectrum at 25.degree. C. are shown in FIG. 19A, along with
the performance of the state-of-the-art triple junction solar cell
described above. Shown is both the data for a six junction cell
made today, as well as the data for a future cell with improved
minority carrier properties. The simulated efficiencies of the
current and future six junction solar cells of the invention are
33.3% and 39.7%, respectively, compared to 30.6% for today's triple
junction solar cell. While the achievable current is lower for the
six junction solar cells compared to the three junction solar cell,
the voltage is approximately double that of the triple junction
solar cell. In both FIGS. 18A and 19A, similar to the terrestrial
multijunction solar cell, the higher efficiency is achieved because
much less of the incident light energy is being lost as heat. More
photons are absorbed by subcells with band gaps closer to their
energies, allowing more of the energy to be converted into
electricity and less into heat.
[0160] Various values for band gaps have been recited in the
description and in the claims. It should be understood that these
values are not exact. However, the values for band gaps are
approximated to one significant figure to the right of the decimal
point, except where otherwise indicated. Thus, the value 0.9 covers
the range 0.850 to 0.949.
[0161] The invention has been explained with reference to specific
embodiments. Other embodiments will be evident to those of ordinary
skill in the art. It is therefore not intended for the invention to
be limited, except as indicated by the appended claims.
* * * * *