U.S. patent application number 10/515243 was filed with the patent office on 2006-07-27 for low bandgap, monolithic, multi-bandgap, optoelectronic devices.
Invention is credited to Jeffrey J. Carapella, Mark W. Wanlass.
Application Number | 20060162768 10/515243 |
Document ID | / |
Family ID | 36638980 |
Filed Date | 2006-07-27 |
United States Patent
Application |
20060162768 |
Kind Code |
A1 |
Wanlass; Mark W. ; et
al. |
July 27, 2006 |
Low bandgap, monolithic, multi-bandgap, optoelectronic devices
Abstract
Low-bandgap, monolithic, multi-bandgap, optoelectronic devices
(10), including PV converters, photodetectors, and LED's, have
lattice-matched (LM), double-heterostructure (DH), low-bandgap
GaInAs(P) subcells (22, 24) including those that are
lattice-mismatched (LMM) to InP, grown on an InP substrate (26) by
use of at least one graded lattice constant transition layer (20)
of InAsP positioned somewhere between the InP substrate (26) and
the LMM subcell(s) (22, 24). These devices are monofacial (10) or
bifacial (80) and include monolithic, integrated, modules (MIMs)
(190) with a plurality of voltage-matched subcell circuits (262,
264, 266, 270, 272) as well as other variations and
embodiments.
Inventors: |
Wanlass; Mark W.; (Golden,
CO) ; Carapella; Jeffrey J.; (Evergreen, CO) |
Correspondence
Address: |
Paul White;National Renewable Energy Laboratory
1617 Cole Blvd
Golden
CO
80401
US
|
Family ID: |
36638980 |
Appl. No.: |
10/515243 |
Filed: |
May 21, 2002 |
PCT Filed: |
May 21, 2002 |
PCT NO: |
PCT/US02/16101 |
371 Date: |
November 19, 2004 |
Current U.S.
Class: |
136/262 ;
257/E27.125; 438/93 |
Current CPC
Class: |
Y02E 10/544 20130101;
H01L 31/06875 20130101; H01L 31/046 20141201; H01L 31/0687
20130101; H01L 31/0475 20141201; H01L 31/043 20141201 |
Class at
Publication: |
136/262 ;
438/093 |
International
Class: |
H01L 31/00 20060101
H01L031/00 |
Goverment Interests
CONTRACTUAL ORIGIN OF INVENTION
[0001] The United States Government has rights in this invention
under Contract No. DE-AC36-99GO10337 between the United States D
Renewable Energy Laboratory, a Division of the Midwest Research
Institute.
Claims
1. A monolithic, multi-bandgap, photovoltaic converter, comprising:
a first subcell comprising GaInAs(P) with a first bandgap and a
first lattice constant; a second subcell comprising GaInAs(P) with
a second bandgap and a second lattice constant, wherein the second
bandgap is less than the first bandgap and the second lattice
constant is greater than the first lattice constant, and further,
wherein the second lattice constant is equal to a lattice constant
of a InAs.sub.yP.sub.1-y alloy with a bandgap greater than the
first bandgap; and a lattice constant transition material
positioned between the first subcell and the second subcell, said
lattice constant transition material comprising InAs.sub.yP.sub.1-y
alloy with a lattice constant that changes gradually from the first
lattice constant to the second lattice constant.
2. The monolithic, multi-bandgap, photovoltaic converter of claim
1, wherein the lattice constant transition material is grown
epitaxially on the first subcell with a gradually increasing value
for y.
3. The monolithic, multi-bandgap, photovoltaic converter of claim
w, wherein the second subcell is grown epitaxially on the lattice
constant transition material.
4. The monolithic, multi-bandgap, photovoltaic converter of claim
1, wherein the first subcell is a lattice-matched,
double-heterostructure, comprising homojunction layers of GaInAsP)
clad by InAs.sub.yP.sub.1-y cladding layers wherein the
InAs.sub.yP.sub.1-y cladding has a value for y in a range of
o.ltoreq.y<1, such the InAs.sub.yP.sub.1-y cladding layers of
the first subcell have a lattice constant equal to the first
lattice constant.
5. The monolithic, multi-bandgap, photovoltaic converter of claim
4, wherein the second subcell is a lattice-matched,
double-heterostructure comprising homojunction layers of GaInAs(P)
clad by InAs.sub.yP.sub.1-y cladding layers, wherein the
InAs.sub.yP.sub.1-y cladding has a value for y in a range of
o.ltoreq.y<1, such that the InAs.sub.yP.sub.1-y cladding layer
of the second subcell have a lattice constant equal to the second
lattice constant.
6. The monolithic, multi-bandgap, photovoltaic converter of claim
1, including a InP substrate, and wherein the first subcell is
grown epitaxially on the InP substrate.
7. The monolithic, multi-bandgap, photovoltaic converter of claim
1, including a tunnel junction positioned between the first subcell
and the second subcell.
8. The monolithic, multi-bandgap, photovoltaic converter of claim
7, wherein the tunnel junction is positioned between the first
subcell and the lattice constant transition layer.
9. The monolithic, multi-bandgap, photovoltaic converter of claim
1, wherein the lattice constant of the lattice constant transition
material is graded.
10. The monolithic, multi-bandgap, photovoltaic converter of claim
1, wherein the lattice constant of the lattice constant transition
material changes in steps of multiple, discrete, increments.
11. The monolithic, multi-bandgap, photovoltaic converter of claim
1, wherein the first bandgap is 0.74 eV.
12. The monolithic, multi-bandgap, photovoltaic converter of claim
6, wherein the first subcell is grown epitaxially on a front
surface of the InP substrate, the lattice constant transition layer
is grown epitaxially on a back surface of the InP substrate, and
the second subcell is grown epitaxially on the lattice constant
transition layer.
13. The monolithic, multi-bandgap, photovoltaic converter of claim
12, wherein the InP substrate is doped with deep acceptor atoms to
make the substrate more electrically insulating than InP, which is
not doped with deep acceptor atoms.
14. The monolithic, multi-bandgap, photovoltaic converter of claim
1, including a InP substrate positioned between the first subcell
and the second subcell.
15. The monolithic, multi-bandgap, photovoltaic converter of claim
14, wherein the InP substrate is positioned between the first
subcell and the lattice constant transition material.
16. The monolithic, multi-bandgap, photovoltaic converter of claim
1, including an isolation layer positioned between the first
subcell and the second subcell.
17. The monolithic, multi-bandgap, photovoltaic converter of claim
16, wherein the isolation layer is positioned between the first
subcell and the lattice constant transition material.
18. The monolithic, multi-bandgap, photovoltaic converter of claim
16, including a InP substrate positioned between the first subcell
and the second subcell.
19. The monolithic, multi-bandgap, photovoltaic converter of claim
18, wherein the InP substrate is positioned between the second
subcell and the isolation layer.
20. The monolithic, multi-bandgap, photovoltaic converter of claim
18, wherein the InP substrate is positioned between the first
subcell and the isolation layer.
21. The monolithic, multi-bandgap, photovoltaic converter of claim
14, including a first isolation layer positioned between the InP
substrate and the first subcell, and including a second isolation
layer positioned between the InP substrate and the second
subcell.
22. A monolithic, multi-bandgap, photovoltaic converter,
comprising: a InP substrate with a substrate lattice constant; a
first subcell comprising GaInAs(P) with a first bandgap and a first
lattice constant, wherein the first lattice constant is greater
than the substrate lattice constant; a lattice constant transition
material positioned between the InP substrate and the first
subcell, said lattice constant transition material comprising
InAs.sub.yP.sub.1-y alloy with a lattice constant that changes from
the substrate lattice constant to the first lattice constant; and a
second subcell comprising GaInAs(P) positioned behind the first
subcell, said GaInAs(P) of the second cell having a second bandgap,
which is less than the first bandgap, and a second lattice
constant.
23. The monolithic, multi-bandgap, photovoltaic converter of claim
22, wherein the second lattice second lattice constant is equal to
the first lattice constant.
24. The monolithic, multi-bandgap, photovoltaic converter of claim
23, including a tunnel junction positioned between the first
subcell and the second subcell.
25. The monolithic, multi-bandgap, photovoltaic converter of claim
23, including an isolation layer positioned between the first
subcell and the second subcell.
26. The monolithic, multi-bandgap, photovoltaic converter of claim
22, wherein the first subcell is a lattice-matched,
double-heterostructure comprising homojunction layers of GaInAs(P)
clad by InP cladding layers.
27. The monolithic, multi-bandgap, photovoltaic converter of claim
22, wherein the second subcell is a lattice-matched,
double-heterostructure comprising homojunction layers of GaInAs(P)
clad by InAs.sub.yP.sub.1-y cladding has a lattice constant equal
to the second lattice constant.
28. The monolithic, multi-bandgap, photovoltaic converter of claim
23, including: a third subcell behind the second subcell, said
third subcell having a third bandgap, which is less than the second
bandgap, and a third lattice constant, which is greater than the
second lattice constant; and a second lattice constant transition
material positioned between the second subcell and the first
subcell, said second lattice constant transition material
comprising InAs.sub.yP.sub.1-y alloy with a lattice constant that
changes from the second lattice constant to the third lattice
constant.
29. The monolithic, multi-bandgap, photovoltaic converter of claim
22, wherein the second lattice constant is greater than the first
lattice constant, and including a second lattice constant
transition material positioned between the first subcell and the
second subcell, said second lattice constant transition material
comprising InAs.sub.yP.sub.1-y alloy with a lattice constant that
changes from the first lattice constant to the second lattice
constant.
30. A monolithic, integrated, module (MIM), comprising: a plurality
of monolithic, multi-bandgap, photovoltaic converters, each of
which comprises: (i) a first subcell with a first bandgap and a
first lattice constant; (ii) a second subcell with a second bandgap
and a second lattice constant, wherein the second bandgap is less
than the first bandgap and the second lattice constant is greater
than the first lattice constant; and (iii) a lattice constant
transition material positioned between the first subcell and the
second subcell, said lattice constant transition material having a
bandgap at least as large as the first bandgap and a lattice
constant that changes from the first lattice constant to the second
lattice constant; and a common substrate with a substrate bandgap
and a substrate lattice constant, said common substrate being
positioned between the first subcell and the lattice constant
transition material of each of the monolithic, multi-bandgap,
photovoltaic converters, wherein the substrate bandgap is at least
as large as the first bandgap and the substrate lattice constant is
equal to the first lattice constant.
31. The monolithic, integrated, module (MIM) of claim 30, wherein
the first subcells are grown epitaxially on a front side of the
substrate, and wherein the lattice constant transition materials
and the second subcells are grown epitaxially on a back side of the
substrate.
32. The monolithic, integrated, module (MIM) of claim 30, wherein
the first subcell comprises GaInAs(P), the second subcell comprises
GaInAs(P), the lattice constant transition material comprises
InAs.sub.yP.sub.1-y, and the substrate comprises InP.
33. The monolithic, integrated, module (MIM) of claim 30, including
a tunnel junction positioned between the first subcell and the
second subcell of each of the monolithic, multi-bandgap,
photovoltaic converters.
34. The monolithic, integrated, module (MIM) of claim 33, wherein
the tunnel junction is positioned between the first subcell and the
substrate.
35. The monolithic, integrated, module (MIM) of claim 30, including
an isolation layer positioned between the first subcell and the
second subcell of each of the monolithic, multi-bandgap,
photovoltaic converters.
36. A monolithic, integrated, module (MIM, comprising: a plurality
of monolithic, multi-bandgap, photovoltaic converters, each of
which comprises: (i) a first subcell with a first bandgap and a
first lattice constant; (ii) a second subcell with a second bandgap
and a second lattice constant, wherein the second bandgap is less
than the first bandgap and the second lattice constant is greater
than the first lattice constant; and (iii) a lattice constant
transition material positioned between the first subcell and the
second subcell, said lattice constant transition material having a
bandgap at least as large as the first bandgap and a lattice
constant that changes from the first lattice constant to the second
lattice constant; and a common substrate with a substrate bandgap
and a substrate lattice constant, said common substrate being
positioned between the lattice constant transition material and the
second subcell of each of the monolithic, multi-bandgap,
photovoltaic converters, wherein the substrate bandgap is at least
as large as the first bandgap and the substrate lattice constant is
equal to the first lattice constant.
37. The monolithic, integrated, module (MIM) of claim 36, wherein
the lattice constant transition layers and the first subcells are
grown epitaxially on a front side of the substrate, and wherein the
second subcells are grown epitaxially on a back side of the
substrate.
38. The monolithic, integrated, module (MIM) of claim 36, wherein
the first subcell comprises GaInAs(P), the second subcell comprises
GaInAs(P), the lattice constant transition material comprises
InAs.sub.yP.sub.1-y, and the substrate comprises InP.
39. The monolithic, integrated, module (MIM) of claim 36, including
a tunnel junction positioned between the first subcell and the
second subcell of each of the monolithic, multi-bandgaps,
photovoltaic converters.
40. The monolithic, integrated, module (MIM) of claim 39, wherein
the tunnel junction is positioned between the substrate and the
second subcell.
41. The monolithic, integrated, module (MIM) of claim 36, including
an isolation layer positioned between the first subcell and the
second subcell of each of the monolithic, multi-bandgap,
photovoltaic converters.
42. The monolithic, integrated, module (MIM) of claim 41, wherein a
subcell circuit comprising the first subcells is voltage-matched to
a subcell circuit comprising the second subcells.
43. A method of fabricating a monolithic, multi-bandgap,
photovoltaic converter, comprising: growing a first subcell
comprising GaInAs(P) epitaxially on an InP substrate in a GaInAs(P)
formulation that provides a first bandgap that is less than 1.35 eV
and a first lattice constant, which equals in InP lattice constant;
growing a lattice constant transition layer on the first subcell,
wherein the lattice constant transition layer comprises
InAs.sub.yP.sub.1-y with an increasing proportion of As and a
decreasing proportion of P so that lattice constant of the
InAs.sub.yP.sub.1-y initially equals the first lattice constant and
changes to a second lattice constant, which is greater than the
first lattice constant, but also maintaining the proportions of As
and P at levels that keep the lattice constant transition layer
transparent to infrared radiation energy bands lower than infrared
radiation energy that can be absorbed by the first subcell; and
growing a second subcell comprising GaInAs(P) epitaxially on the
lattice constant transition layer in a GaInAs(P) formulation that
has a lattice constant equal to the second lattice constant and a
second bandgap that is less than the first bandgap.
43. A method of fabricating a monolithic, multi-bandgap,
photovoltaic converter, comprising: growing a first subcell
comprising GaInAs(P) epitaxially on a front side of an InP
substrate in a GaInAs(P) formulation that provides a first bandgap
that is less than 1.35 eV and a first lattice constant, which
equals an InP lattice constant; growing a lattice constant
transition layer comprising InAs.sub.yP.sub.1-y on a back side of
the InP substrate, including increasing proportion of As and
decreasing proportion of P so that lattice constant of the
InAs.sub.yP.sub.1-y initially equals the lattice constant of the
InP substrate and changes to a second lattice constant, which is
greater than the first lattice constant, but also maintaining the
proportions of As and P at levels that keep the lattice constant
transition layer transparent to infrared radiation energy levels
that are not absorbable by the first subcell.
Description
TECHNICAL FIELD
[0002] This invention relates to optoelectronic devices, and, more
specifically, to low bandgap, monolithic, multi-bandgap solar
photovoltaic (SPV) and thermophotovoltaic (TPV) cells for
converting solar and/or thermal energy to electricity as well as
for related photodetector devices for detecting light signals and
light emitting diode (LED) devices for converting electricity to
light and/or infrared (IR) radiant energy.
BACKGROUND OF THE INVENTION
[0003] It is well known that the most efficient conversion of
radiant energy to electrical energy with the least thermalization
loss in semiconductor materials is accomplished by matching the
photon energy of the incident radiation to the amount of energy
needed to excite electrons in the semiconductor material to
transcend the bandgap from the valence band to the conduction band.
However, since solar radiation and blackbody radiation usually
comprise a wide range of wavelengths, use of only one semiconductor
material with one bandgap to absorb such radiant energy and convert
it to electrical energy will result in large inefficiencies and
energy losses to unwanted heat.
[0004] Ideally, there would be a semiconductor material with a
bandgap to match the photon energy for every wavelength in the
radiation. That kind of device is impractical, if not impossible,
but persons skilled in the art are building monolithic stacks of
different semiconductor materials into devices commonly called
tandem converters and/or monolithic, multi-bandgap or multi-bandgap
converters, to get two, three, four, or more bandgaps to match more
closely to different wavelengths of radiation and, thereby, achieve
more efficient conversion of radiant energy to electrical energy.
Essentially, the radiation is directed first into a high bandgap
semiconductor material, which absorbs the shorter wavelength,
higher energy portions of the incident radiation and which is
substantially transparent to longer wavelength, lower energy,
portions of the incident radiation. Therefore, the higher energy
portions of the radiant energy are converted to electric energy by
the larger bandgap semiconductor materials without excessive
thermalization and loss of energy in the form of heat, while the
longer wavelength, lower energy portions of the radiation are
transmitted to one or more subsequent semiconductor materials with
smaller bandgaps for further selective absorption and conversion of
remaining radiation to electrical energy.
[0005] Semiconductor compounds and alloys with bandgaps in the
various desired energy ranges are known, but that knowledge alone
does not solve the problem of making an efficient and useful energy
conversion device. Defects in crystalline semiconductor materials,
such as impurities, dislocations, and fractures provide unwanted
recombination sites for photogenerated electron-hole pairs,
resulting in decreased energy conversion efficiency. Therefore,
high-performance, photovoltaic conversion cells comprising
semiconductor materials with the desired bandgaps, often require
high quality, epitaxially grown crystals with few, if any, defects.
Growing the various structural layers of semiconductor materials
required for a multi-bandgap, tandem, photovoltaic (PV) conversion
device in a monolithic form is the most elegant, and possibly the
most cost-effective, approach.
[0006] Epitaxial crystal growth of the various compound or alloy
semiconductor layers with desired bandgaps is most successful, when
all of the materials are lattice-matched (LM), so that
semiconductor materials with larger crystal lattice constants are
not interfaced with other materials that have smaller lattice
constants or vice versa. Lattice-mismatching (LMM) in adjacent
crystal materials causes lattice strain, which, when high enough,
is usually manifested in dislocations, fractures, wafer bowing, and
other problems that degrade or destroy electrical characteristics
and capabilities of the device. Unfortunately, the semiconductor
materials that have the desired bandgaps for absorption and
conversion of radiant energy in some energy or wavelength bands do
not always lattice match other semiconductor materials with other
desired bandgaps for absorption and conversion of radiant energy in
other energy or wavelength bands. Therefore, fabrication of device
quality, multi-bandgap, monolithic, converter structures is
difficult, if not impossible, for some portions of the radiation
frequency or wavelength spectrum.
[0007] This problem has been particularly difficult to solve in the
infrared (IR) portion of the spectrum, where options for suitable,
commercially available substrates on which to grow thin films with
the necessary bandgaps for absorption and conversion of the
infrared radiation to electrical energy are very limited, and where
compatible, i.e., lattice-matched, semiconductor materials with the
different bandgaps needed to absorb and convert different portions
of the infrared spectrum efficiently are also quite limited.
[0008] For example, the group III-V family of semiconductor alloys
include some of the best materials for fabricating photovoltaic
converters with bandgaps in a range of about 0.35 eV to 1.65 eV to
absorb and convert infrared (IR) radiation with wavelengths in a
range of about 3.54 .mu.m to 0.75 .mu.m. Group III-V alloys
comprise combinations of binary compounds formed from Groups III
and V of the Periodic Table. These binary compounds can be alloyed
together into various ternary or quaternary compositions to obtain
any desired bandgap in the range of 0.35 eV to 1.65 eV. These
alloys also have direct bandgaps (i.e., no change in momentum is
required for an electron to cross the bandgap between the valance
band and the conduction band), which facilitate efficient
absorption and conversion of radiant energy to electricity.
However, InP, which has a lattice constant of 5.869 .ANG.
(sometimes rounded to 5.87 .ANG.) and a bandgap of 1.35 eV, is one
of only a few feasible, commercially available substrate materials
with a lattice constant even close to those lower bandgap Group
III-V alloys i.e., InP-based or related ternary and quaternary
compounds. The lowest bandgap Group III-V alloy that can be
lattice-matched to the 5.869 .ANG. lattice constant of an InP
substrate is Ga.sub.0.47In.sub.0.53As, which has a bandgap of about
0.74 eV, which leaves a significant range of lower frequency,
longer wavelength (>1.67 .mu.m), infrared (IR) radiation that
cannot be absorbed and converted to electricity in monolithic
converters in which the semiconductor absorption materials are
lattice-matched to the substrate.
[0009] While the current unavailability of efficient and
cost-effective solar photovoltaic (SPV) converters, especially
multi-bandgap, monolithic, converter devices, capable of absorbing
and converting infrared (IR) radiation in wavelengths greater than
1.67 .mu.m leaves substantial amounts of energy in the solar
spectrum to remain unconverted to electricity, in state-of-the-art
SPV's, it is an even greater problem for thermophotovoltaic (TPV)
devices. Infrared (IR) radiation of wavelengths greater than 1.67
.mu.m comprises a substantial amount of the energy radiated from
blackbodies, and thermophotovoltaic (TPV) converters are intended
to absorb and convert as much radiant energy from blackbodies to
electric power as possible. Therefore, solutions to these problems,
especially if such solutions could enable fabrication of monolithic
converters with multiple bandgaps in infrared (IR) energy ranges,
they would facilitate capture of more electric energy from solar
and/or blackbody radiation.
[0010] U.S. Pat. No. 5,479,032 issued to S. Forrest et al., teaches
that one or more ternary In.sub.xGa.sub.1-xAs alloys with
x>0.53, i.3., with band-gaps less than 0.75 eV, can be grown
epitaxially on an InP substrate by using intervening, graded layers
of InAs.sub.yP.sub.1-y between the InP substrate and the
In.sub.xGa.sub.1-xP (x>0.53) layers. However, those Forrest et
al., patent teachings, which were directed to pixel detection of
near infrared radiation incident on a focal plane for
telecommunications applications, are not useful in SPV and TPV
applications.
SUMMARY OF THE INVENTION
[0011] Accordingly, a general object of the present invention is to
provide a monolithic, multi-bandgap, photovoltaic converter for
absorbing and converting infrared (IR) radiation of multiple
wavelengths to electricity.
[0012] A more specific object of this invention is to provide a
photovoltaic converter with at least one bandgap less than 0.74 eV
to absorb infrared radiation in wavelengths longer than 1.67 .mu.m
and convert it to electricity.
[0013] An even more specific object of this invention is to provide
a electric device quality, multi-bandgap, monolithic, photovoltaic
converter that has at least one lattice-matched (LM),
double-heterostructure (DH) with a bandgap less than 0.74 eV to
absorb infrared (IR) energy in wavelengths longer than 1.67 .mu.m
and convert it to electricity.
[0014] Another specific object of the invention is to provide a
device quality, multi-bandgap, monolithic, photovoltaic device with
at least one lattice-matched (LM), double-heterostructure (DH) with
a bandgap less than 0.74 eV, which is not lattice-matched to an InP
substrate, but including a lattice constant transition layer or
layers, which is transparent to infrared radiation wavelengths
longer than about 1.67 .mu.m, positioned somewhere between such
lattice-matched (LM), double-heterostructure (DH) and the InP
substrate.
[0015] Still another of this invention object is to provide a
lattice constant transition layer or layers, which is transparent
to infrared (IR) radiation wavelengths longer than about 1.67
.mu.m, positioned between two subcells in a multi-bandgap,
monolithic device, where the two subcells are not lattice-matched
to each other and at least one of the subcells has a bandgap, which
is less than the bandgap of the other subcell and is less than 0.74
eV.
[0016] Another object of the present invention is to provide one or
more subcells with bandgaps less than 0.74 eV on an InP
substrate.
[0017] Another object of the present invention is to provide a
bifacial, monolithic, integrated, module QI) comprising multiple
subcells, at least one subcell of which absorbs and converts
radiation wavelengths less than 0.92 .mu.m to electricity.
[0018] Another object of the present invention is to provide a
bifacial, monolithic, integrated, module (MIM) comprising multiple
subcells, at least one subcell of which absorbs and converts
radiation wavelengths less than 1.67 .mu.m to electricity.
[0019] Another specific object of this invention is to provide a
method of voltage-matching a plurality of subcell circuits that
have subcells with different bandgaps less than or equal to 1.35
eV.
[0020] Additional objects, advantages, and novel features of the
invention are set forth in part in the description that follows and
will become apparent to those skilled in the art upon examination
of the following description and figures or may be learned by
practicing the invention. Further, the objects and the advantages
of the invention may be realized and attained by means of the
instrumentalities and in combinations particularly pointed out in
the appended claims.
[0021] To achieve the foregoing and other objects and in accordance
with the purposes of the a present invention, as embodied and
broadly described herein, a method of this invention may comprise
growing one or more subcell(s) that has a lattice constant greater
than 5.869 .ANG., either alone or in combination with other
subcells, on an InP substrate by using a lattice constant
transition material between the InP substrate and the subcell(s)
that have the lattice constants greater than 6.869 .ANG.. The
lattice constant transition material can be InAs.sub.yP.sub.1-y,
where y is graded either continuously or in discrete stepped
increments from one (1) to a value at which the InAs.sub.yP.sub.1-y
has a lattice constant that matches the lattice constant of at
least one of the subcells with a lattice constant greater than
5.869 .ANG.. The subcell bandgap is lower than the bandgap of the
InP substrate and lower than the bandgap of the
InAs.sub.yP.sub.1-y, lattice constant transition material.
Additional subcells with even lower bandgaps can also be added,
and, if any of such additional subcells has an even greater lattice
constant that cannot be matched to the first subcell, then one or
more additional lattice constant transition layers can also be
added. All of the subcells can be grown on only one side of the
substrate (monofacial) or one or more subcells can be grown on the
front side of the substrate while one or more other subcells can be
grown on the back side (bifacial), using whatever lattice constant
transition layers are necessary to accommodate the subcell(s) on
each side of the substrate.
[0022] Isolation layers can be used between subcells for
independent electrical connection of the subcells, although, in
bifacial embodiments, the substrate can be insulating or
semi-insulating to serve as an isolation layer. Alternately, tunnel
junctions can be used for intra-cell current flow between subcells.
Either the monofacial or bifacial subcell structures can be made in
monolithic, integrated, modules (MIMs), which are particularly
useful for voltage-matching a plurality of such subcells, although
the bifacial embodiments are particularly suitable for such MIM
structures and voltage matching. On the other hand, the monofacial
embodiments are particularly useful in ultra-thin devices in which
the substrate is removed.
[0023] To achieve the foregoing and other objects and in accordance
with the purposes of the a present invention, as embodied and
broadly described herein, of this invention may also comprise a
monolithic, multi-bandgap, photovoltaic converter that has a first
subcell comprising GaInAs(P) with a first bandgap and a first
lattice constant, a second subcell comprising GaInAs(P) with a
second bandgap and a second lattice constant, wherein the second
bandgap is less than the first bandgap and the second lattice
constant is greater than the first lattice constant, and further,
wherein the second lattice constant is equal to a lattice constant
of a InAs.sub.yP.sub.1-y alloy with a bandgap greater than the
first bandgap, and a lattice constant transition material
positioned between the first subcell and the second subcell, said
lattice constant transition material comprising InAs.sub.yP.sub.1-y
alloy with a lattice constant that changes gradually from the first
lattice constant to the second lattice constant.
[0024] The first subcell is preferably a lattice-matched,
double-heterostructure, comprising homojunction layers of GaInAs(P)
clad by InAs.sub.yP.sub.1-y cladding layers wherein the
InAs.sub.yP.sub.1-y cladding has a value for y in a range of
o.ltoreq.y<1, such the InAs.sub.yP.sub.1-y cladding layers of
the first subcell have a lattice constant equal to the first
lattice constant. The second subcell is is also preferably a
lattice-matched, double-heterostructure comprising homojunction
layers of GaInAs(P) clad by InAs.sub.yP.sub.1-y cladding layers,
wherein the InAs.sub.yP.sub.1-y cladding has a value for y in a
range of o.ltoreq.y<1, such that the InAs.sub.yP.sub.1-y
cladding layer of the second subcell have a lattice constant equal
to the second lattice constant. Either a tunnel junction or an
isolation layer is also positioned between subcells. The InP
substrate can be doped with deep acceptor atoms to make the
substrate more electrically insulating, and, in bifacial
structures, this feature allows the substrate to serve as an
electrical isolation between subcells positioned on opposite sides
of the substrate.
[0025] A plurality of the monolithic, multi-bandgap, photovoltaic
converters can also be grown on a common substrate in a monolithic,
integrated, module (MIM), comprising the plurality of monolithic,
multi-bandgap, photovoltaic converters, each of which comprises:
(i) a first subcell with a first bandgap and a first lattice
constant; (ii) a second subcell with a second bandgap and a second
lattice constant, wherein the second bandgap is less than the first
bandgap and the second lattice constant is greater than the first
lattice constant; and (iii) a lattice constant transition material
positioned between the first subcell and the second subcell, said
lattice constant transition material having a bandgap at least as
large as the first bandgap and a lattice constant that changes from
the first lattice constant to the second lattice constant. Either
monofacial structures or bifacial structures can be grown in MIM
configurations, but the bifacial structure is particularly suited
to MIM applications. The subcells in MIM structures can be isolated
for independent electrical connection, or tunnel juctions can be
provided. Isolated, independently connected, subcells are
particularly adaped for voltage-matching in MIM structures. There
can be more subcell stacks on one side of the substrate than the
other to facilitate such voltage-matching, where the subcells on
one side of the substrate are lower bandgap than subcell on the
other side of the substrate.
[0026] The substrates can also be removed to provide ultra-thin
photovoltaic devices.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The accompanying drawings, which are incorporated in and
form a part of the specification, illustrate the preferred
embodiments of the present invention, and together with the
descriptions serve to explain the principles of the invention.
In the drawings:
[0028] FIG. 1 is a diagrammatic illustration of the general,
significant components of a monofacial, inverted, multi-bandgap,
monolithic, photovoltaic device with two lattice-matched (LM),
double-heterostructure (DH) subcells grown on an InP substrate and
series connected, wherein the second subcell, which has a bandgap
that is less than the bandgap of the first subcell and is less than
about 0.74 eV, is lattice-mismatched (LMM) to the InP substrate,
but is grown on a transparent, lattice constant transition layer
positioned between the two subcells to accommodate the lattice
mismatch;
[0029] FIG. 2 is a more detailed cross-sectional view of the device
of FIG. 1 showing some of the auxiliary structures and components
useful in an embodiment of the device;
[0030] FIG. 3 is a bandgap versus lattice parameter chart showing
bandgap and lattice constant parameters of semiconductor materials
used as examples in the embodiments of FIGS. 1 and 2 according to
this invention;
[0031] FIG. 4 is a detailed cross-sectional view of a monofacial,
inverted, multi-bandgap, monolithic, photovoltaic device similar to
FIGS. 1 and 2, but with the subcells isolated electrically for
independent connection;
[0032] FIG. 5 is a simplified cross-sectional view of a
photovoltaic device illustrating more subcells and graded
transparent layers according to this invention;
[0033] FIG. 6 is a diagrammatic illustration of a variation of the
monolithic, multi-bandgap, photovoltaic converter similar to FIG.
1, but with the lattice constant transition layer positioned
between the substrate and the first subcell;
[0034] FIG. 7 is a bandgap versus lattice parameter chart showing
bandgap and lattice constant parameters of the semiconductor
materials used as examples in the embodiment of FIG. 6;
[0035] FIG. 8 is a diagrammatic illustration of a variation of the
monolithic, multi-bandgap, photovoltaic converter similar to FIG.
6, but with an additional lattice constant transition layer and an
additional subcell added to the structure;
[0036] FIG. 9 is a diagrammatic illustration of the bifacial,
buried substrate, embodiment of the invention in which subcells are
grown epitaxially on opposite faces of the substrate;
[0037] FIG. 10 is a bandgap versus lattice parameter chart showing
bandgap and lattice constant parameters of the semiconductor
materials used as examples in the embodiment of FIGS. 9 and 11;
[0038] FIG. 11 is an illustration of a more complex, bifacial,
monolithic, multi-bandgap, photovoltaic device according to this
invention;
[0039] FIG. 12 is an illustration of another more complex,
bifacial, monolithic, multi-bandgap, photovoltaic device according
to this invention that is particularly useful for solar
photovoltaic (SPV) converter applications;
[0040] FIG. 13 is a bandgap versus lattice parameter chart showing
bandgap and lattice constant parameters of the semiconductor
materials used as examples in the embodiment of FIG. 12;
[0041] FIG. 14 is a cross-sectional view of a bifacial, monolithic
integrated module (WM) according to this invention;
[0042] FIG. 15 is a schematic diagram of an equivalent electric
circuit showing the voltage-matched electric subcell circuits of
the bifacial MIM in FIG. 14;
[0043] FIG. 16 is a diagrammatic illustration of a monolithic,
multi-bandgap, photovoltaic device similar to FIG. 2, but with an
added stop-etch layer and with the structure mounted on a panel,
heat sink, printed circuit board, or other object; and
[0044] FIG. 17 is a diagrammatic view similar to FIG. 16, but with
the substrate removed.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0045] A schematic diagram of principle components of a monofacial
embodiment of a low-bandgap, monolithic, multi-bandgap (tandem)
photovoltaic (PV) converter 10 according to this invention is shown
in FIG. 1 juxtaposed to a corresponding bandgap energy (E.sub.g)
profile. The diagram in FIG. 1 illustrates a cross-section of the
PV converter 10 profile in a manner that is conventional in the
industry, i.e., not necessarily in proportion to actual sizes,
because actual layer thicknesses are too small to illustrate in
actual proportions. Additional structural components used to
fabricate an example of the PV converter 10 of FIG. 1 are
illustrated in FIG. 2, which will be described in more detail
below.
[0046] In the monofacial embodiment or approach of this invention
illustrated in FIG. 1, all of the subcells, for example, subcells
22, 24 in FIG. 1, are grown epitaxially on only one side or face 25
of a substrate 26. Bifacial embodiments or approaches of this
invention (not shown in FIG. 1), in which subcells are grown
epitaxially on opposite sides or faces of a substrate, will be
illustrated in other figures and described below.
[0047] The monofacial PV converter 10 illustrated in FIG. 1 is
designed according to this invention with low bandgap, Group III-V
semiconductor alloy materials, especially for bandgaps below about
0.74 eV, where ternary GaInAs or AlInAs and quaternary (GaInAsP or
AlGaInAs semiconductor alloys do not match the crystal lattice
constant of InP substrates 26. A quick reference to the lattice
parameter versus bandgap chart in FIG. 3 shows that the crystal
lattice constant of InP is about 5.87 .ANG., as indicated by broken
line 12, while the lowest possible bandgap for a Group III-V alloy
with that same lattice constant of 5.87 .ANG. is about 0.74 eV,
which is provided by the ternary alloy Ga.sub.0.47In.sub.0.53As, as
indicated by the broken line 14. Lattice-matched (LM) materials
refers to materials with lattice constants that are either equal or
similar enough that when the materials are grown expitaxially, one
or the other adjacent each other in a single crystal, any
difference in size of crystalline structures of the respective
materials is resolved substantially by elastic deformation and not
by inelastic relaxation, separation, dislocations, or other
undesirable inelastic effects. A lattice-mismatch (LMM) is
generally considered to occur when a second crystalline material
being grown on a first crystalline material has a lattice constant
that is not equal to the lattice constant of the first material and
is not lattice-matched as described above. (The terms "lattice
parameter" and "lattice constant" mean substantially the same thing
and are often used interchangeably in the art and in this
description.) Therefore, as shown by broken lines 16, 18 in FIG. 3,
any Group III-V alloy with a bandgap less than about 0.74 eV will
be a lattice-mismatched (LW with an InP substrate. Since a
significant feature of this invention is to provide a monolithic,
multi-bandgap, photovoltaic (PV) converter with at least one
bandgap less than about 0.74 eV, such a lattice-mismatch has to be
mitigated in order to avoid the adverse manifestations of lattice
strain and stresses caused by such lattice-mismatch, such as
dislocations, fractures, wafer bowing, rough surface morphologies,
and the like.
[0048] Referring again to the exemplary monofacial, monolithic,
multi-bandgap, photovoltaic (PV) converter 10 illustrated in FIG.
1, such mitigation of lattice-mismatch between a first Group III-V
semiconductor subcell 22 and a second Group III-V semiconductor
subcell 24 with a different bandgap below about 0.74 eV is provided
by a lattice constant transition layer 20 that: (i) has graded
(either distinctly stepped increments or continuously increasing)
lattice constants, which span the difference between the respective
lattice constants of the first and second subcells 22, 24; and (ii)
is transparent to infrared wavelengths longer than those absorbed
by the first subcell 22. While a transparent lattice constant
transition layer 20, which is graded to have lattice constants that
vary continuously from the lattice constant of the first subcell 22
to the lattice constant of the second subcell 24, is satisfactory
for this purpose, a transparent lattice constant transition layer
20 comprising, discrete or stepped changes in lattice constants is
preferred. Dislocations in semiconductor crystals are undesirable,
because they facilitate recombination of charge carriers (electron
hole pairs), which is deleterious to the electrical performance of
a semiconductor device.
[0049] A preferred lattice constant transition layer 20 according
to this invention is a ternary InAs.sub.yP.sub.1-y material in
which the proportion of As is gradually increased, either
continuously or in discrete increments as will be discussed in more
detail below. A significant feature of this invention is that the
InAs.sub.yP.sub.1-y lattice constant transition layer 20 is
transparent to infrared (IR) radiation wavelengths longer than
those absorbed by the ternary Ga.sub.xIn.sub.1-xAs or optional
quaternary Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y in the first subcell
22, so there is virtually no loss of energy, or production of heat,
in the lattice constant transition layer 20.
[0050] The monofacial, monolithic, multi-bandgap, photovoltaic (PV)
converter 10 illustrated in FIG. 1 has an inverted structure with
the active subcell layers 22, 24, transparent lattice constant
transition layer 20, and preferred or optional other layers (shown
in FIG. 2, which will be described in more detail below), grown
epitaxially on one side 25 of a substrate 26. This structure is
called inverted, because the radiation energy R enters the
converter 10 through the substrate 26, so it has to be transmitted
through the substrate 26 before being absorbed and converted to
electricity by the subcells 22, 24. Therefore, as will be explained
in more detail below, the substrate 26 has to be transparent to all
the incident radiation R so that none of the incident radiation R
is absorbed and thermalized or lost as heat before it reaches the
subcells 22, 24, where it can be converted to electricity.
Likewise, the incident radiation R transmitted by the substrate 26
should encounter the subcell 22 with the highest bandgap before it
encounters the subcell 24 with the lowest bandgap, because higher
bandgap subcells will absorb only the higher energy radiation
(higher frequency and shorter wavelength) and convert it to
electricity while transmitting unabsorbed, lower energy radiation
(lower frequency and longer wavelength). Therefore, any of the
remaining lower energy incident radiation R that is not absorbed
and converted to electricity by the higher bandgap, first subcell
22 will be transmitted to the lower bandgap, second subcell 24,
where at least some, if not all, of it can be absorbed and
converted to electricity. The amount of the remaining, lower
energy, incident radiation that can be absorbed and converted to
electricity by the second subcell 24 will depend on the particular
bandgap of the subcell 24 and the particular radiation wavelengths
in such remaining, lower energy, incident radiation. Of course, the
lattice constant transition layer 20 has to be transparent to, and
not absorptive of, the remaining, lower energy, incident radiation
that is not absorbed by the first subcell 22 so that all of such
remaining, lower energy, incident radiation can reach the second
subcell 24. Further, as will be explained in more detail below,
additional subcells with different bandgaps can also be included in
order to optimize absorption and conversion of various incident
radiation energy levels or bands to electricity.
[0051] The back-surface reflector (BSR) or other spectral control
element 28, which can also function as an electrode contact or
lateral current flow element, is deposited on the second subcell
24, as will be described in more detail below. A spectral control
layer 30 would usually be deposited on the front side 27 of the
substrate 26 either to minimize reflection of incident radiation R,
e.g., an anti-reflective coating (ARC), as is well-known to persons
skilled in the art, especially for SPV converter applications, or
to reflect all incident radiation R with wavelengths lower than
those absorbable by the lowest bandgap subcell 24, especially for
the TPV converter applications used for generating electricity and
not heat. These structures and functions will be discussed in more
detail below. The terms front and back, as used in this
description, relate to the direction in which incident radiation
propagates into and through a device or layers in a device.
Therefore, radiation is incident first on the front of a device or
layer and propagates toward the back of the device or layer.
[0052] In converter 10, the preferred substrate 26 comprises InP,
because, as explained above: (i) InP has a lattice constant (5.87
.ANG.), which is one of a few commercially available bulk, single
crystal materials that are close to the lattice constants of Group
III-V alloys that have bandgaps less than 0.74 eV (for absorbing
infrared radiation wavelengths longer than about 1.67 .mu.m); (ii)
InP has a bandgap of about 1.35 eV (see FIGS. 1 and 3), thus does
not absorb, and is transparent to, infrared radiation wavelengths
longer than 0.93 .mu.m; (iii) InP can be doped to be highly
resistive and thereby function as an insulator or semi-insulator,
as described in more detail below; (iv) Lattice-mismatch between
InP and InAs.sub.yP.sub.1-y or GaInAs(P) materials, which have
lower bandgaps than InP and are used extensively in this invention
as explained below, is in compression rather than tension, so
lattice-mismatched InAs.sub.yP.sub.1-y or GaInAs(P) grown on InP
are not so likely to develop fissures or crack; and (v) Bulk InP
crystals are less expensive than InAs and GaSb. Therefore, the InP
substrate 26 is suitable in a monofacial, inverted PV converter 10
structure for any application in which the incident radiation R to
be converted to electric energy has 0.93 .mu.m and longer
wavelengths, such as thermophotovoltaic (TPV) cells and some solar
photovoltaic (SPV) cells as well as infrared detector devices and
multi-bandgap infrared (IR) LID's. However, InP is susceptible to
free carrier absorption of energy, which results in energy being
lost in the form of heat. To minimize or prevent such free carrier
absorption of energy, the InP substrate can be doped with deep
acceptor atoms, such as iron (Fe) or chromium (Cr), to pin the
Fermi level deeply within the bandgap, which makes the InP act more
like an insulator or semi-insulator.
[0053] Subject to accommodations for a contact, buffer, cladding,
optical control elements, and/or other auxiliary layers (not shown
in FIG. 1), which will be described in more detail below, the first
subcell 22 is deposited on substrate 26 with a bandgap E.sub.g1
designed to absorb the first desired wavelength or frequency band
of the incident radiation R and convert such absorbed radiation to
electricity. In the preferred embodiment, this first subcell 22 is
a lattice-matched (LM), double-heterostructure (DH),
InP/Ga.sub.xIn.sub.1-xAs or InP/Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y
with a desired bandgap E.sub.g1, somewhere in a range that is less
than the 1.35 eV bandgap of the InP substrate 26. This first
subcell 22 in FIG. 1 is preferably grown epitaxially and is
lattice-matched to the InP substrate 26. Please note that
"lattice-matched" when used in the context of a "lattice-matched,
double-heterostructure" for a subcell generally means that the
semiconductor materials within the subcell itself are
lattice-matched to each other. Therefore, a subcell can be a
lattice-matched, double-heterostructure, while such subcell may or
may not be lattice-matched to a substrate or to another layer or
material in the device that is not part of the subcell.
[0054] A particularly preferred subcell 22 lattice-matched to the
InP substrate 26 comprises InP/Ga.sub.0.47In.sub.0.53As with a
bandgap of about 0.74 eV, because, as shown by the lines 12, 14 in
FIG. 3, Ga.sub.0.47In.sub.0.53As is the lowest bandgap Group III-V
alloy that has the same lattice constant as the InP substrate 26.
Therefore, a InAs.sub.yP.sub.1-y lattice constant transition layer
20 can make a transition from the lattice constant of InP (about
5.87 .ANG.) to a lattice constant matching a Ga.sub.xIn.sub.1-xAs
alloy with a bandgap as low as about 0.52 eV, i.e., to a lattice
constant as high as about 5.968 .ANG. (see lines 15, 17 in FIG. 3),
and still be transparent to all infrared wavelengths that are
longer than those absorbed by the 0.74 eV bandgap of the first
subcell 22 (see line 14 in FIG. 3). Of course, the desired bandgap
for the second subcell 24 could also be anywhere between 0.74 eV
and 0.52 eV, in which case the InAs.sub.yP.sub.1-y lattice constant
transition layer 20 can be formulated to provide a back surface
with whatever lattice constant is needed on which to grow the
desired Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y that has such a desired
bandgap.
[0055] An example second subcell 24 for use in conjunction with a
first subcell 22 described above, therefore, can be a quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y or a ternary
Ga.sub.xIn.sub.1-xAs with a bandgap as low as 0.52 eV. A particular
preferred example second cell 24 comprises a lattice-matched,
double-heterostructure InAs.sub.yP.sub.1-y/Ga.sub.xIn.sub.1-xAs
with a bandgap 19 of 0.55 eV and a lattice constant 21 of about
5.952 .ANG..
[0056] The lattice constant transition layer 20, as mentioned
above, gradually makes a transition from the lattice constant of
the first subcell 22 to the lattice constant of the second subcell
24, preferably while remaining substantially transparent to all
infrared radiation wavelengths that are not absorbed by the first
subcell 22, as illustrated by the example PV converter 10 of FIG.
1. In that specific example, the first subcell 22 has a bandgap
E.sub.g1) 14 of about 0.74 eV and lattice constant of about 5.87
.ANG., while the second subcell 24 has a bandgap (E.sub.g2) 19 of
about 0.55 eV and lattice constant of about 5.952 .ANG., as
explained above. Therefore, the lattice constant transition layer
20 has to make a transition of lattice constants gradually from
about 5.87 .ANG. to about 5.952 .ANG.. As shown in FIGS. 1 and 3,
adding As to InP to produce InAs.sub.yP.sub.1-y increases the
lattice constant of the InAs.sub.yP.sub.1-y from about 5.87 .ANG.
to about 5.952 .ANG. without decreasing the bandgap of the
InAs.sub.yP.sub.1-y to a level below the 0.74 eV
Ga.sub.xIn.sub.1-xAs (x=0.47) of the first subcell 22. Therefore,
the InAs.sub.yP.sub.1-y lattice constant transition layer 20
remains transparent to all of the remaining infrared radiation R
that is not absorbed in the first subcell 22 so that it allows all
of such remaining infrared radiation to reach the second subcell
24.
[0057] As also mentioned above, such graded transition of the
InAs.sub.yP.sub.1-y lattice constant transition layer 20 from the
lattice constant of the first subcell 22 (e.g., 5.87 .ANG.) to the
lattice constant of the second subcell 24 (e.g., 5.952 .ANG.) can
be done by increasing the proportion of As on a gradual continuous
basis or, preferably, in incremental discrete steps as illustrated
by line 23 in the bandgap chart in FIG. 1. The stepped lattice
constant transition 23 illustrated in FIG. 1 seems to provide
better experimental results than gradual, continuous grading.
[0058] A more specific example of the monofacial PV converter 10 of
FIG. 1 with auxiliary layers useful in actual implementation of
such a device for high quality performance characteristics is shown
diagrammatically in FIG. 2. Again, the thicknesses of the various
layers are not illustrated in actual size or thickness proportions
in relation to each other.
[0059] The substrate 26 is preferably InP doped with a deep
acceptor element, such as Fe, (sometimes denoted as InP:Fe or as
(Fe) InP) to trap electrons and thereby suppress or prevent free
carrier absorption. The substrate 26 can be semi-insulating for
isolation or p-type for conducting, as desired for a particular
application, and other layers and components are designated as
either n-type or p-type, accordingly to provide the n/p junctions
34, 48 needed to convert the incident radiation R to electricity in
the subcells 22, 24, respectively. However, p/n junctions would
also work, as is understood by persons skilled in the art, so these
n-type and p-type designations could be reversed by substituting
donor dopants, for acceptor dopants and vice versa, which would be
considered equivalent for purposes of the invention.
[0060] While the subcells 22, 24 can be simple shallow
homojunctions, this invention is particularly conducive to the more
efficient, lattice-matched, double-heterostructure subcells 22, 24
illustrated in FIG. 2. Specifically, the example first subcell 22
illustrated in FIG. 2 has a n/p homojunction 34 formed by a p-type
Ga.sub.xIn.sub.1-xAs base layer 38 grown epitaxially on an n-type
Ga.sub.xIn.sub.1-xAs emitter layer 36, all of which is sandwiched
between front and back cladding layers 40, 42 of n-type InP and
p-type InP, respectively. The InP in the cladding layers 40, 42 is
a different compound than the Ga.sub.xIn.sub.1-xAs in the
homojunction layers 36, 38, but it has the same lattice constant as
the Ga.sub.xIn.sub.1-xAs. Therefore, the first subcell 22 is a
lattice-matched, double-heterostructure. The cladding layers 40, 42
passivate dangling bonds at terminated Ga.sub.xIn.sub.1-xAs crystal
structures at the front of layer 36 and at the rear of layer 38,
which otherwise function, at least to some extent, as unwanted
recombination sites for minority carriers in the
Ga.sub.xIn.sub.1-xAs. Also, the band offsets between the InP
(bandgap of 1.35 eV) and the Ga.sub.xIn.sub.1-xAs (bandgap of 0.74
eV in this example) repel minority carriers away from the
InP/Ga.sub.xIn.sub.1-xAs interface, which further reduces such
unwanted recombination of minority carriers. Therefore, a clad
subcell, such as the lattice-matched (LM), double-heterostructure
(DH), InP/Ga.sub.0.77In.sub.0.53As or optional
InP/Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y subcell 22 described above,
is more efficient in converting radiant energy to electricity than
non-clad subcells. This passivation and confinement of minority
carriers by the cladding layers 40, 42 is possible in the
monolithic, multi-bandgap PV converter structures of this
invention, because the cladding material, InP, has the same lattice
constant (5.869 .ANG.) as, and a higher bandgap than, the
homojunction cell material, Ga.sub.xIn.sub.1-xAs or
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y
(Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y is lattice-matched to InP when
y.apprxeq.2.2x).
[0061] The second subcell 24 is also preferably a lattice-matched,
double-heterostructure comprising a homojunction 48 formed by
n-type and p-type layers 50, 52 of either ternary
Ga.sub.xIn.sub.1-xAs or quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y, but its lattice constant is
larger than the lattice constant of the first subcell 22 and of the
InP substrate 26, as explained above. Consequently, the second
subcell 24 is lattice-mismatched (LMM) in relation to the InP
substrate 26 and first cell 22, and it cannot be clad with InP.
However, as explained above in relation to the lattice constant
transition layer 20, InAs.sub.yP.sub.1-y can be formulated to have
the same lattice constant as the Ga.sub.xIn.sub.1-xAs or
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y homojunction layers 50, 52.
Therefore, the passivation and confinement cladding layers 54, 56
of the second subcell 24 comprise InAs.sub.yP.sub.1-y that is
lattice-matched to the Ga.sub.xIn.sub.1-xAs or
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y homojunction layers 50, 52 to
form the lattice-matched, double-heterostructure of that subcell
24.
[0062] Prior to growing the first subcell 22, a buffer layer 32 of
n-InP about 300 .ANG. thick is deposited first on a surface 25 of
the InP substrate 26 to begin an epitaxial InP growth layer, if
needed. If the InP substrate 26 is doped with a deep acceptor to be
electrically insulating or semi-insulating as explained above, then
provisions have to be made for a front electrical contact 29 and a
conductive layer 33 for accommodating lateral flow of current
produced by the subcells 22, 24 to or from the contact 29. Such a
conductive layer 33 could be, for example, heavily n-doped InP or
any other heavily doped material that is lattice-matched to the InP
substrate 26 as well as transparent to all radiation wavelengths
that are transmitted by the InP substrate 26. Then, the first
subcell 22 comprising the lattice-matched, double-heterostructure
of n-Ga.sub.0.47In.sub.0.53As/p-Ga.sub.0.47In.sub.0.53As
homojunction layers 36, 38 between the two cladding layers 40, 42
of n-InP and p-InP, respectively. As is well-known in the art,
semiconductor materials are usually doped with small amounts of
elements from an adjacent group of the Periodic Table of the
Elements to provide the majority carriers. Therefore, an
appropriate donor dopant for the Group III-V semiconductor alloy
used in this invention can be, for example, sulphur (S) from Group
VI, and appropriate acceptor dopant can be, for example, zinc (Zn)
from Group II. The InP buffer layer 32 grown epitaxially on the InP
substrate 26 in this example is preferably heavily
(10.sup.-18-10.sup.-20 cm.sup.-3) n-type doped with sulfur (S).
Then, the InP front cladding layer 40 is grown epitaxially on the
buffer layer 32 to a thickness of about 0.01-0.1 .mu.m, but it is
more lightly doped n-type with, for example, S to a dopant level of
about 10.sup.16-10.sup.20 cm.sup.-3. The Ga.sub.0.47In.sub.0.53As
homojunction layers 36, 38, which lattice-match the InP substrate
26, buffer layer 32, and cladding layer 40, are grown epitaxially.
Therefore, the bandgap of the first subcell 22 is about 0.74 eV,
which absorbs portions of the incident radiation R with wavelengths
of about 1.67 .mu.m and less, as explained above, although other
values of x and other formulations would also work in this
invention. Lattice-matching quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y is also possible. The emitter
layer 36 of subcell 22 is grown epitaxially to a thickness,
preferably in a range of about 0.1-10 .mu.m, and is doped n-type
with, for example, S to a dopant level in a range of about
10.sup.16-10.sup.20 cm.sup.-3. The base layer 38 is then grown
epitaxially to a thickness of about 0.01-10 .mu.m, and doped p-type
to create the n/p junction 34. The p-type dopant, such as Zn in
this example, is at a dopant level of about 10.sup.16-10.sup.20
cm.sup.-3. To complete the lattice-matched, double-heterostructure,
first subcell 22, the back cladding layer 42 is grown epitaxially
on the base layer 38 to a thickness, preferably of about 0.01-0.1
.mu.m, and is p-type doped, for example, with Zn, to a dopant level
of about 10.sup.16-10.sup.20 cm.sup.-3 Each of the buffer layer 32,
conductive layer 33, and/or cladding layer 40 can all serve any one
or more of these functions, individually or together. Therefore,
instead of the three distinct layers 32,33,40 shown in FIG. 2, one
or two layers could serve those same functions, if desired.
[0063] The subcells 22, 24 can be electrically connected together
in series, or they can be electrically isolated from each other, as
will be described in more detail below. For a monolithic,
multi-bandgap, PV device 10 in which the subcells 22, 24 are series
connected, a tunnel junction comprising a layer 44 of heavily
p-doped Ga.sub.0.47In.sub.0.53As or
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y followed by a heavily n-doped
Ga.sub.0.47In.sub.0.53As or Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y
layer 46 is deposited and grown epitaxially on the back cladding
layer 42 of the first subcell 22 to facilitate low-resistive
current flow in an ohmic manner between the first subcell 22 and
the second subcell 24. Again, if the preferred homojunction layers
36, 38 of subcell 22 comprise Ga.sub.0.47In.sub.0.53As, as
discussed above, then it is also preferable that x=0.47 in the
Ga.sub.xIn.sub.1-xAs of the tunnel junction layers 44, 46 in order
to lattice-match them with the underlaying InP and
Ga.sub.0.47In.sub.0.53As layers described above, although other
values of x and other formulations would also work in this
invention. Tunnel junctions are well-known in the art, but, for
purposes of this invention, each tunnel junction layer 44, 46 can
be about 0.01-0.1 .mu.m thick and doped to a level of about
10.sup.-18-10.sup.-20 cm.sup.-3. Alternative monolithic,
multi-bandgap, PV converters with the subcells 22, 24 isolated
electrically from each other will be described below.
[0064] The transparent, lattice constant transition layer 20 of
this invention comprising gradually increasing lattice constants is
deposited and grown epitaxially on the Ga.sub.xIn.sub.1-xAs or
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y tunnel junction layer 46 in
order to make the transition from the lattice constant of the InP
substrate 26 and intervening layers described above to a lattice
constant that matches the Ga.sub.xIn.sub.1-xAs or
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y of the second subcell 24, which
is formulated to provide a desired bandgap E.sub.g2, as described
above. According to this invention, the bandgap E.sub.g2 is less
than the bandgap E.sub.g1 of the first subcell 24 in the
monofacial, inverted PV converter embodiment 10 of FIGS. 1 and 2
for the reasons explained above. InAs.sub.yP.sub.1-y is used for
this lattice constant transition layer 20, because it can be
formulated to lattice match the lower bandgap E.sub.g2 of the
Ga.sub.xIn.sub.1-xAs of Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y of the
second subcell 24, while it also remains transparent to the longer
infrared radiation R wavelengths that are not absorbed by the
higher bandgap E.sub.g1 material of the first subcell 22. This
feature is important in order to ensure that substantially all of
the longer wavelength radiation R, which is not absorbed in the
first subcell 22, reaches the second subcell 24.
[0065] To form the lattice constant transition layer 20, (As) is
added to a growing layer of InP in increasing proportions so that
the proportion of arsenic (As) increases in the resulting
InAs.sub.yP.sub.1-y material, which increases the lattice constant
of the InAs.sub.yP.sub.1-y. As mentioned above, this change can be
accomplished continuously, but it is preferred that the changes in
proportions be made in incremental steps. In the
InAs.sub.yP.sub.1-y of the lattice constant transition layer 20 of
this example PV converter 10, y varies from zero (where it
lattice-matches the Ga.sub.0.47In.sub.0.53As of the first subcell
22) to about 0.44, where it lattice-matches to the
Ga.sub.xIn.sub.1-xAs of the second subcell 24, in which
x.apprxeq.0.26 and the consequent bandgap E.sub.g2 is about 0.55
eV. That example bandgap Eg.sub.2=0.55 eV enables the second
subcell 24 to absorb infrared radiation R with wavelengths up to
about 2.25 .mu.m. In general, the lattice-matching condition of
Ga.sub.xIn.sub.1-xAs to InAs.sub.yP.sub.1-y occurs when the crystal
lattices of the epi-layers are fully relaxed, which is where
y.apprxeq.2.143x.
[0066] Of course, as mentioned above, the Ga.sub.xIn.sub.1-xAs of
the second subcell 24 can have x equal to some other value for a
different desired bandgap E.sub.g2, and the y in the
InAs.sub.yP.sub.1-y of the lattice constant transition layer 20 can
be varied or customized accordingly to make the necessary
corresponding lattice constant transition. Also, as mentioned
above, either or both of the subcell materials and/or the lattice
constant transition materials could be quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y with the x and y values
customized to desired bandgaps and lattice constants within the
physical constraints illustrated by the bandgap vs. lattice
parameter chart of FIG. 3.
[0067] As explained above and shown in FIG. 2, the second subcell
24 comprises a lattice-matched, double-heterostructure of n-type
InAs.sub.yP.sub.1-y front cladding layer 54, n-type
Ga.sub.xIn.sub.1-xAs or Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y emitter
layer 50 and p-type Ga.sub.xIn.sub.1-xAs or
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y base 52 to form the
homojunction 48, and p-type InAs.sub.yP.sub.1-y back cladding layer
56, all grown epitaxially on the lattice constant transition layer
20. As explained above for the first subcell 22, the cladding
layers 54, 56 confine and passivate the front and back surfaces of
the homojunction layers 50, 52 to prevent recombination of minority
carriers. The thicknesses and doping levels of the subcell 24
layers 50, 52, 54, 56 can be similar to those described above for
the first subcell 22.
[0068] A back surface spectral control element 28, which can also
be used as a back electrical contact layer, can be deposited onto
the back cladding layer 56 or onto an additional contacting layer
(not shown) disposed atop the back cladding layer 56. The nature of
the back surface spectral element 28 may depend on the application
of the device 10. For example, if the device 10 is a SPV or TVP,
the sole purpose of which is to convert radiation to electricity,
then the back surface spectral element may comprise a reflector to
reflect any remaining, unabsorbed radiation from the second subcell
24 back through the subcells 24, 22. Some of such reflected
radiation could be absorbed in this second pass through the
subcells, but most of it will continue propagating all the way back
through the substrate 26 toward whatever radiator source (not
shown) produces the incident radiation R in the first place. Adding
such unabsorbed, back-reflected, radiation energy back into the
radiator source may enable the radiator source to use such
back-reflected energy in the production of new incident radiation R
for conversion to electricity in the converter 10. This feature is
particularly appropriate for TPV configurations of converter 10
that are applied to convert infrared radiation R produced by a
blackbody infrared radiation source (not shown) to electricity. Any
radiation reflected back into the blackbody radiator adds energy to
the blackbody radiator and thereby tends to raise the temperature
of the blackbody radiator, which causes the blackbody radiator to
produce more blackbody infrared radiation for the converter 10.
Therefore, such back-reflected radiation can help the blackbody
radiator to produce more incident radiation R for the device 10
without having to use so much fuel.
[0069] On the other hand, some devices 10 are used both for
producing electricity and gathering heat for an environment. In
those applications, the back surface spectral control element 28
may be a material that is transparent to remaining infrared
radiation that is not absorbed by the second subcell 24 so that
such remaining infrared can be used as heat someplace behind the
device 10.
[0070] If the layer 28 is a back surface reflector (BSR), there can
be several advantages to designing the last (second) subcell 24
with only one-half of its normal thickness, i.e., one-half the
thickness that would be required for full absorption of radiation
in the wavelengths that correspond to the bandgap, because any
unabsorbed radiation will be reflected by the BSR 28 back into the
last subcell 24. The advantages of this kind of design include an
enhanced photocurrent, higher operating voltage, and thinner
structure that requires less growth time and provides easier device
processing. Regardless of its optical characteristics, as described
above, the layer 28 can also be a back surface electrical contact.
Therefore, it is preferably electrically conductive. An optional,
additional metallic contact 45 can also be used on the conductive
layer 28 for making an electrical connection, if desired.
[0071] The design of the front surface spectral control element 30
on the front surface 27 of the substrate 26 may also depend on
usage of the device 10. For example, if the device 10 is to be used
only for producing electricity from blackbody radiation, the front
surface spectral control element 30 may be a coating layer that
transmits only shorter wavelength incident radiation R that can be
absorbed and converted to electricity by the subcells 22, 24 and
that reflects all longer wavelength incident radiation R back into
the blackbody radiator (not shown) for recovery and re-use. On the
other hand, if the device 10 is to be used both for producing
electricity and heat for an environment, then the front surface
spectral control element 30 may be an antireflective coating to
enhance transmission of all the incident radiation R into the
device 10.
[0072] As mentioned above, the monofacial PV converter 10 described
above and illustrated in FIGS. 1 and 2 is configured with the
subcells 22, 24 connected electrically in series facilitated by the
tunnel junction layers 44, 46. An alternate embodiment monofacial,
low-bandgap, monolithic, multi-bandgap, PV converter 110 is shown
in FIG. 4 with much the same first subcell 22, second subcell 24,
and substrate 26 structures and materials described above for the
PV converter 10, but with the subcells 22, 24 isolated electrically
from each other. The electrical isolation instrumentality in the PV
converter 110 is illustrated as a discrete electrical isolation
layer 39 positioned between the first subcell 22 and the second
subcell 24. However, such electrical isolation function could be
incorporated into other components, such as into the graded lattice
constant transition layer 20, as will be explained below.
[0073] There are a number of reasons that such electrical isolation
of the subcells 22, 24 may be desirable in some applications. For
example, as mentioned above, current flow through series connected
subcells 22, 24 is limited by the lowest photocurrent producing
subcell. Therefore, for series connected subcells, a number of
subcell design factors, such as bandgaps, thicknesses, doping
concentrations, and the like are used to optimize the operating
characteristics of the series connected subcells 22, 24, so that
electric power production from the tandem combination is maximized.
In some designs and applications, however, more efficient
conversion of radiant energy to electricity can be accomplished by
extracting electric power from the individual subcells 22, 24
separately or independently, or, in some applications, to design
the subcells 22, 24 for voltage matching. Such voltage matching
techniques with subcells in other devices will be discussed in more
detail below in relation to monolithic, integrated module (MIM)
devices.
[0074] To isolate the subcells 22, 24 electrically from each other,
there has to be some material between them that inhibits electric
current flow between the subcells 22, 24. However, such electrical
isolation material cannot interfere with radiation transmission
from one subcell 22 to the other subcell 24. In the PV converter
110 of FIG. 4, a discrete isolation layer 39 is shown positioned
between the first subcell 22 and the lattice constant transition
layer 20, although it could be positioned between the lattice
constant transition layer 20 and the second subcell 24.
[0075] An isolation material for isolation layer 39 can be
fabricated in a number of ways. One such approach is to fabricate
the isolation layer 39 with a high-resistivity semiconductor
material that has a high enough bandgap to be transparent to the
longer wavelength radiation that is not absorbed in the first
subcell 22 and is being transmitted to the second subcell 24.
Another such approach is to form the isolation layer 39 as an
isolation diode, which, of course, must also be transparent to the
radiation being transmitted from the first subcell 22 to the second
subcell 24. Also, such high-resistivity material or isolation diode
material has to be lattice-matched to the materials in front and in
back of it, which, in the position of isolation layer 39 shown in
FIG. 4, has to be lattice-matched to the first cell 22 and
substrate 26.
[0076] As mentioned above, InP doped with a deep acceptor element,
such as Fe or Cr, is a high-resistivity material and has a bandgap
(1.35 eV) that makes it transparent to all radiation that is not
absorbed by the first subcell 22. It is also lattice-matched to the
InP substrate 26 and to the ternary Ga.sub.0.47In.sub.0.53As or
quaternary Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y of the first subcell
22. Therefore, deep acceptor-doped InP can be used as the
high-resistivity, isolation layer 39. Such deep acceptor-doping of
other lattice-matched semiconductor materials, such as ternary
Ga.sub.xIn.sub.1-xAs, quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y, or even AlGaInAs in some
circumstances, with high enough bandgaps to be transparent to the
radiation being transmitted, could also be used to provide suitable
high-resistivity materials for the isolation layer 39.
[0077] An isolation diode for isolation layer 39 can be provided by
one or more doped junctions, such as an n-p junction or n-p-n
junctions with high enough reverse-bias breakdown characteristics
to prevent current flow between the subcells 22, 24. Again,
lattice-matched semiconductor materials, such as InP,
Ga.sub.xIn.sub.1-xAs or Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y, or
even AlGaInAs, can be doped to provide an isolation diode structure
for isolation layer 39.
[0078] While a discrete isolation layer 39 is shown in the PV
converter 110 of FIG. 4, it is possible to dope the lattice
constant transition layer 20 to also function as an isolation layer
between the two subcells 22, 24. The InAs.sub.yP.sub.1-y of the
lattice constant transition layer 20 can also be doped with a deep
acceptor element, such as Fe or Cr, to make it highly-resistive, or
discrete sublayers of the InAs.sub.yP.sub.1-y can be n-p or n-p-n
doped to form an isolation diode structure.
[0079] Of course, with each subcell 22, 24 isolated electrically
from each other, some additional provisions for electrical contacts
are necessary to extract electric power independently from each
subcell 22, 24. Persons skilled in the art will be able to design
myriad structures for such contacts, once they understand the
principles of this invention. The example additional contacts 27,
42 for this purpose are shown fabricated on lateral current flow
layers 39, 41 respectively. Such lateral current flow layers 39, 41
are lattice-matched to their respective subcells 22, 24 and must be
transparent to radiation being transmitted from the first subcell
22 to the second subcell 24. Heavily doped
GaIn.sub.1-xAs.sub.yP.sub.1-y with 0.ltoreq.x.ltoreq.1 and
0.ltoreq.y.ltoreq.1 as necessary for lattice matching and
transparency can be used for these lateral current flow layers 39,
41.
[0080] While the series connected PV converter 10 and isolated or
independently connected PV converter 110 described above are
illustrated with only two subcells 22, 24, and only one lattice
constant transition layer 20 between them, any number of subcells
with any number of lattice constant transition layers can be
included in a monolithic, multi-bandgap, optoelectronic device
according to this invention. To illustrate this principle, a more
complex monolithic, multi-bandgap, PV converter 112 is illustrated
in FIG. 5.
[0081] In the PV converter 112, an arbitrary number (five) subcells
114, 116, 118, 120, 122 are illustrated with arbitrary bandgaps
E.sub.g1>E.sub.g2>E.sub.g3>E.sub.g4>E.sub.g5. The
substrate 124 is InP, and the first and second subcells 114, 116
both have bandgaps E.sub.g1, E.sub.g2 that can be ternary GaInAs or
quaternary GaInAsP and are lattice-matched to the InP substrate 124
(see, e.g., lines 130, 12, 14 in FIG. 3). These first and second
subcells 114, 116 are preferably both lattice-matched (LM),
double-heterostructures (DH) with junctions comprising n-type and
p-type ternary Ga.sub.xIn.sub.1-xAs or quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y clad with n-type and p-type
layers of InP, as described above for the other PV converter
embodiment 10. The third and fourth subcells 118, 120 are LM, DH
ternary or quaternary GaInAs(P) with bandgaps E.sub.g3, E.sub.g4
that can be lattice-matched to each other, but not to the InP
substrate (see, e.g., lines 12, 132, 134, 136 in FIG. 3).
Therefore, a lattice constant transition layer 126 of graded
InAs.sub.yP.sub.1-y is used to make the transition from the second
subcell 116 to the lattice-mismatched subcell 118. A fifth LM, DH
subcell 122 of ternary or quaternary GaInAs(P) has a bandgap
E.sub.g5 that cannot be lattice-matched to the fourth subcell 120.
Therefore, another lattice constant transition layer 128 is
provided to make the transition from the fourth subcell 120 to the
lattice-mismatched fifth subcell 122.
[0082] As mentioned above, the numbers and combinations of subcells
and lattice constant transition layers as well as the specific
example bandgap values shown in the PV converter 112 of FIG. 5 are
selected arbitrarily to illustrate the principles of this
invention. The only requirement is that the incident radiation
reaches the subcells in order of decreasing bandgaps, so that the
shorter wavelength radiation is absorbed and converted to
electricity by higher bandgap subcells that will transmit
unabsorbed, longer wavelength radiation to the next subcell(s).
Other details, such as buffer layers, tunnel junction or isolation
layers, contacts, optic control layers, etc., for fabricating a
working PV converter can be similar to those described above for
either the series connected subcell embodiments 10 of FIGS. 1 and 2
or the independently connected subcell embodiment 110 of FIG.
4.
[0083] Now, as illustrated in another alternative inverted,
monofacial, multi-bandgap, PV converter 140 in FIG. 6, the
positions of the transparent lattice constant transition layer 20
and the first subcell 22 positions can be reversed from their
positions shown in the FIG. 1 embodiment 10. Specifically, the
lattice constant transition layer 20 can be grown expitaxially on
the InP substrate 26 by gradually adding more and more As to the
growing InAs.sub.yP.sub.1-y lattice constant transition layer 20,
as described above, until a desired lattice constant is attained
for a desired Ga.sub.xIn.sub.1-xAs or
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y semiconductor material with a
desired bandgap to be grown on the InP substrate 26. As explained
above, the desire bandgap is chosen for absorbing and converting
infrared radiation R of a desired wavelength or frequency band to
electricity.
[0084] For example, but not for limitation, if it is desired to
have the first subcell 22 in the PV converter 140 of FIG. 6 absorb
and convert infrared radiation of at most 1.77 .mu.m wavelength to
electricity and to have the second subcell 24 absorb and convert
infrared radiation in the range of 1.77 .mu.m to 2.14 .mu.m to
electricity, the first subcell 22 would need a bandgap of about
0.70 eV, and the second subcell 24 would need a bandgap of about
0.58 eV. Therefore, an appropriate lattice constant transition
layer 20 can be InAs.sub.yP.sub.1-y with a gradually increasing
proportion of As until an InAs.sub.yP.sub.1-y semiconductor
material having a lattice constant of about 5.94 .ANG. and a
bandgap of about 0.90 eV, as illustrated in FIG. 7 by broken lines
60, 62, respectively. Therefore, a lattice constant transition
layer 20 with those criteria will provide a transition of lattice
constant 12 from the 5.87 .ANG. of the in P substrate to the 5.94
.ANG. of the terminal InAs.sub.yP.sub.1-y material in the lattice
constant transition layer 20. With a terminal bandgap of 0.90 eV,
the lattice constant transition layer 20 is transparent to infrared
radiation (IR) with wavelengths longer than about 1.38 .mu.m.
[0085] The first subcell 22 with the example desired 0.70 eV
bandgap can then be a lattice-matched (LM), double-heterostructure
(DH) of, for example,
InAs.sub.yP.sub.1-y/Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y with the
same lattice constant, 5.94 .ANG., as the terminal
InAs.sub.yP.sub.1-y of the lattice constant transition layer 20
(see broken line 60 in FIG. 7). The
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y base of the subcell 22 can be
formulated to have the desired bandgap of 0.70 eV (see broken line
64 in FIG. 7 and corresponding line 64 in FIG. 6), so it will
absorb and convert 1.77 .mu.m and shorter radiation R to
electricity, but it will transmit and not absorb virtually all the
incident infrared radiation (IR) that is longer wavelength than
1.77 .mu.m. Such formulation of appropriate proportions of Group
III-V elements in quaternary alloys, such as the
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y in this example, to achieve
certain desired bandgap characteristics, such as the 0.70 eV in
this example, is well-known and within the capabilities of persons
skilled in the art, thus need not be explained in detail here to
enable persons skilled in the art to understand and practice this
invention.
[0086] The second subcell 24 in this example can be formulated with
a lattice-matched (LM), double-heterostructure (DH) with the same
lattice constant of 5.94 .ANG. as the first subcell 22 and still
have a bandgap as low as 0.58 eV (see broken lines 60, 66 in FIG.
7). Therefore, if it is desired to formulate the second cell 24 to
absorb and convert as much of the infrared radiation R, which
passed through the first cell 22, as possible, and still be
lattice-matched to the first cell 22, then Ga.sub.xIn.sub.1-xAs
with a bandgap of 0.58 eV can be used. This example second cell 24,
with its 0.58 eV bandgap 66, would absorb and convert infrared
radiation R of 2.14 .mu.m wavelength and shorter to electricity. Of
course, other auxiliary layers, such as buffers, cladding, tunnel
junction or isolation layers, contacts, and antireflective or
optical control layers can be used to make this structure a
functioning device, as explained above, and as would be understood
by persons skilled in the art.
[0087] While two lattice-mismatched (LM) subcells 22, 24 and one
lattice constant transition layer 20 in any of a variety of ternary
and/or quaternary formulations comprising Ga, In, As, and/or P
provide wide flexibility in low bandgap designs for efficient
absorption and conversion of desired infrared radiation wavelength
bands to electricity, this invention also extends to three, four,
five, or more subcells and bandgaps with one or more lattice
constant transition layers, as needed. For example, there is no
theoretical limit to the number of quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y formulations for different
bandgaps between lines 13 and 14 (0.74 eV to 1.35 eV) in FIG. 3,
which can be lattice-matched on line 12 (5.869 .ANG.). Likewise,
there is no theoretical limit to the number of quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y formulations for different
bandgaps between lines 14 and 17 (0.74 eV to 0.52 eV) in FIG. 3,
which can be lattice-matched on line 15 (5.968 .ANG.). Further,
there is no theoretical limit to the number of ternary and
quaternary Ga, In, As, and/or P formulations for possible lattice
constants between those of InP (5.869 .ANG.) and InAs (6.059
.ANG.), lines 12 and 23, respectively, in FIG. 3.
[0088] In other words, every ternary Ga.sub.xIn.sub.1-xAs or
quaternary Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y with a bandgap in
the range between 0.74 eV and 0.355 eV (lines 14 and 31 in FIG. 3)
can be lattice-matched to some higher bandgap InAs.sub.yP.sub.1-y,
which is transparent to at least some infrared radiation that can
be absorbed and converted to electricity by such ternary
Ga.sub.xIn.sub.1-xAs or quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y DH subcells. Further, any
InAs.sub.yP.sub.1-y, which is used to make a transition between the
lattice constant of such ternary Ga.sub.xIn.sub.1-xAs or quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y to a larger lattice constant,
also has a higher bandgap than such Ga.sub.xIn.sub.1-xAs or
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y thus is transparent to at least
some infrared radiation that can be absorbed and converted to
electricity by such ternary Ga.sub.xIn.sub.1-xAs or quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y DH subcells. This invention
utilized these characteristics of
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1) for the design, formulation, and fabrication
of low bandgap (less than 1.35 eV, and preferably less than 0.74
eV), monolithic, multi-bandgap, photovoltaic converters, as
described above, and as will be further described below.
[0089] This invention, as mentioned above, also extends to low
bandgap, monolithic, multi-bandgap PV converters with more than one
lattice constant transition layer. For example, referring again to
FIG. 6, one or more additional subcells with even lower bandgap(s)
than the 0.58 eV bandgap of the second subcell 24 can be grown on
top of subcell 24. Such an example PV converter 150 with three
subcells 22, 24, 72 is illustrated diagrammatically in FIG. 8. This
example three-bandgap PV converter 150 is illustrated for
convenience with the same substrate 26, first lattice constant
transition layer 20, first subcell 22, and second subcell 24 as the
two-bandgap embodiment 140 of FIG. 6, but it has a second lattice
constant transition layer 70 positioned between the second subcell
24 and a third subcell 72.
[0090] As was explained above in relation to the inverted tandem
(two-subcell) PV converter 140 in FIG. 6, the InP substrate 26 and
the first lattice constant transition layer 20 are transparent to
infrared radiation of longer wavelengths than can be absorbed by
their respective bandgap characteristics. Therefore, in the
examples of FIGS. 6 and 8, the lowest bandgap of the
InAs.sub.yP.sub.1-y lattice constant transition layer 20 is 0.90 eV
(see line 62 in FIG. 7), which, of course, is also lower than the
1.35 eV bandgap of the InP substrate. Therefore, infrared radiation
of wavelengths longer than 1.38 .mu.m pass through both the InP
substrate 26 and the InAs.sub.yP.sub.1-y lattice constant
transition layer 20. The first subcell 22, with its 0.70 eV
bandgap, absorbs and converts infrared radiation R wavelengths of
1.77 .mu.m and shorter to electricity, and it transmits infrared
radiation R wavelengths longer than 1.77 .mu.m to the second
subcell 24. The 0.58 eV bandgap of the Ga.sub.xIn.sub.1-xAs second
subcell 24 enables it to absorb and convert infrared radiation
wavelengths of 2.14 .mu.m and shorter to electricity, while
infrared radiation R wavelengths greater than 2.14 .mu.m pass
through the second subcell 24.
[0091] The energy in the infrared radiation R wavelengths longer
than 2.14 .mu.m, which are not absorbed in the second subcell 24
would be wasted in the PV converter 140 embodiment of FIG. 6, but
adding one or more additional subcells, such as subcell 72 in the
three-bandgap PV converter 150 in FIG. 8, can capture and convert
significant amounts of that energy to electricity. However, as
shown by line 60 in FIG. 7, there is no ternary
Ga.sub.xIn.sub.1-xAs or quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y with a bandgap below 0.58 eV
(line 66) that has the same lattice constant (5.94 .ANG.) as the
first and second subcells 22, 24 in the example. Therefore, a
second lattice constant transition layer 70 comprising
InAs.sub.yP.sub.1-y with gradually increasing proportions of
arsenic (As) is positioned between the second subcell 24 and the
third subcell 72. The initial InAs.sub.yP.sub.1-y in the second
lattice constant transition layer 70 is formulated to have a
bandgap of 0.90 eV, so that it has the same lattice constant (5.94
.ANG.) as the second subcell 24. Then, the subsequent
InAs.sub.yP.sub.1-y grown for the second lattice constant
transition layer 70 decreases in bandgap in incremental steps or
gradually toward, and preferably to, the same bandgap 66 as the
second subcell 24, which is 0.58 eV in the example described above.
At that bandgap level, the InAs.sub.yP.sub.1-y is still transparent
to all of the infrared radiation R wavelengths that pass through
the second subcell 24. Therefore, the InAs.sub.yP.sub.1-y second
lattice constant transition layer 70 does not absorb or interfere
with the infrared radiation R that has to reach the third subcell
72, yet it provides a transition from the lattice constant of the
second subcell 24 (line 60 in FIG. 7) to a new, larger lattice
constant (line 74 in FIG. 7) for the third subcell 72. In this
example, the new lattice constant of 6.02 .ANG. will match a
ternary Ga.sub.xIn.sub.1-xAs with a bandgap of 0.45 eV or
quaternary Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y with a bandgap
anywhere along line 94 between 0.58 eV and 0.45 eV, as shown in
FIG. 7. Therefore, if the third subcell 74 in FIG. 8 comprises, for
example, Ga.sub.xIn.sub.1-xAs formulated to have a bandgap of 0.45
eV, it will absorb and convert infrared radiation in wavelengths of
2.76 .mu.m and shorter to electricity.
[0092] Again, as explained above for the first lattice constant
transition layer 20, the gradual change of lattice constant in the
second lattice constant transition layer 70 can be graded gradually
or in discrete stepped increments, the latter of which is
preferred. Also, while not shown in detail in FIG. 8, the
Ga.sub.xIn.sub.1-xAs n/p junction in the third subcell 72 is
preferably clad front and back with cladding layers of
InAs.sub.yP.sub.1-y, which have the same lattice constant as the
Ga.sub.xIn.sub.1-xAs of the third subcell 74, to form the third
subcell 72 as a lattice-matched (LM), double-heterostructure (DH)
subcell. Other auxiliary layers mentioned above can also be
provided.
[0093] All of the PV converter embodiments 10, 110, 140, 150
described above have been monofacial, i.e., grown on only one face
of the substrate. A very significant feature of this invention is
that it can also be implemented in bifacial or buried substrate
structures, as illustrated diagrammatically by the example low
bandgap, monolithic, multi-bandgap, PV converter 80 in FIG. 9.
Essentially, in the PV converter 80 of FIG. 9, a lattice-matched
(LM) first subcell 82 is grown epitaxially on a front surface 83 of
a InP substrate 84, and a lattice-mismatched (LMM) second subcell
86 with an intervening lattice constant transition layer 90 is
grown epitaxially on a back surface 85 of the substrate 84. An
antireflective coating (ARC) 88 on the front surface 81 of the
first subcell 82 and a back surface reflector (BSR) 89 on the back
surface 87 of the second subcell 86 are shown, but they can be
other optical control layer materials, as described above for PV
converter 10. Again, other auxiliary features, layers, and
components that may be used to implement an actual device, such as
buffers, contacts, deep acceptor doping of the InP substrate,
tunnel junctions or isolation layers, and the like are not shown
separately in FIG. 9 in order to avoid unnecessary clutter and
repetition, but persons skilled in the art can use the information
herein to understand, design, and fabricate such components in PV
converter devices according to this invention. Cladding layers (not
shown separately in FIG. 9) can be used as part of the subcells 82,
86 for lattice-matched (LM), double-heterostructure (DH)
implementations of the subcells, as described above in relation to
the PV converter 10. Also, while only one lattice constant
transition layer 90 and two subcells 82, 86 with specific example
bandgaps and lattice constants are illustrated in the example PV
converter 80 in FIG. 9, other numbers of subcells, lattice constant
transition layers, bandgaps, and/or lattice constants can also be
used according to this invention, as explained above.
[0094] In the example bifacial PV converter 80 in FIG. 9, the first
subcell 82 is lattice-matched to the InP substrate 84 (line 12 in
FIG. 10). In this example, subcell 82 is formulated to have the
lowest possible bandgap that can be lattice-matched to the InP
substrate 84, which is the ternary Ga.sub.0.47In.sub.0.53As with a
bandgap of 0.74 eV (line 14 in FIG. 8), although many other
formulations could be illustrated, as explained above. If it is
desired to use a lattice-matched, double-heterostructure for the
first subcell 82, the Ga.sub.0.47In.sub.0.53As n/p junction
material can be clad on both sides with lattice-matched,
epitaxially grown, InP cladding layers, as described above.
[0095] In this example, any incident radiation R of wavelengths
shorter than 1.67 .mu.m will be absorbed by the 0.74 eV bandgap
Ga.sub.0.47In.sub.0.53As in the first subcell 82, and longer
wavelength infrared radiation R will pass through the first subcell
82. The InP substrate 84, which has a much higher bandgap of 1.35
eV (line 13 in FIG. 10) is also transparent to any of such longer
wavelength infrared radiation that passes through the first subcell
82. Therefore, in order for the second subcell 86 to absorb and
convert any of such longer wavelength infrared radiation R to
electricity, it has to have a bandgap E.sub.g2 that is less than
the bandgap E.sub.g1 of the first subcell 82, i.e., less than 0.74
eV in this example. There are many considerations for selecting the
lower bandgap for the second subcell 86, such as targeting the
concentrations of the infrared radiation in various wavelength or
frequency bands, conversion efficiencies, and any additional
subcells (not shown in FIG. 9). However, any bandgap less than 0.74
eV requires a Ga.sub.xIn.sub.1-yAs or
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y that has a larger lattice
constant than the InP substrate 84, so it would not be possible for
it to be lattice-matched to the InP substrate 84. Therefore, a
lattice constant transition layer 90 is needed, and it is preferred
that the second subcell 86 should have a lattice constant that
allows the InAs.sub.yP.sub.1-y lattice constant transition layer 90
to be transparent to the longer infrared radiation wavelengths,
which are not absorbed by, and pass through, the first subcell 82.
In this example, a bandgap E.sub.g2 for the second subcell 86 is
selected to be 0.55 eV, which can be provided with ternary
Ga.sub.xIn.sub.1-xAs having a lattice constant of 5.972 .ANG., as
illustrated by lines 92 and 93 in FIG. 10. However, a quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y with a slightly larger lattice
constant could also be used for the same bandgap and still be able
to accommodate a transparent lattice constant transition layer
90.
[0096] As shown by lines 12, 93 in FIG. 10, the ternary
Ga.sub.xIn.sub.1-xAs with its lattice constant of 5.972 .ANG.
requires the lattice constant transition layer 90 to make the
transition between the lattice constant of the InP substrate (line
12) to the 5.972 .ANG. lattice constant (line 93). As also
illustrated in FIG. 10, increasing the proportion of As in an
InAs.sub.yP.sub.1-y lattice constant transition layer 90 reaches
this 5.972 .ANG. constant without its bandgap ever decreasing below
about 0.82 eV (line 94), which is still higher than the 0.74 eV
bandgap (line 14) of the first subcell 82. Therefore, any infrared
radiation R that is not absorbed by, thus passes through, the first
subcell 82 will also not be absorbed by the InAs.sub.yP.sub.1-y in
the lattice constant transition layer 90. Consequently, such
infrared radiation will reach the second subcell 86, where at least
some of it can be absorbed and converted to electricity.
[0097] If the substrate 84 is doped with a deep acceptor element,
such as Fe or Cr, to be an insulator or semi-insulator, as
explained above, then the first subcell 82 and the second subcell
86 are electrically isolated from each other. Therefore,
electricity has to be extracted independently from each subcell 82,
86, as described above for the electrically isolated subcells 22,
24 of the PV converter device 110 in FIG. 4. This feature has
advantages, such as in voltage-matching of multiple, series and/or
parallel interconnected PV converter subcell circuits, especially
in monolithic, integrated module (MIM) devices, as will be
described in more detail below. In situations where the substrate
86 cannot be made as an insulator or semi-insulator, a separate
isolation layer (not shown in FIG. 9) can be positioned anyplace
between the two subcells 82, 86. For example, an isolation layer
can be grown on either the front surface 83 or back surface 85 of
the substrate 84 or between the lattice constant transition layer
90 and the second subcell 86. Such an isolation layer can be made
as desired above in relation to the isolation layer 39 in FIG. 4,
i.e., a lattice-matched material that is transparent to wavelengths
of radiation not absorbed by the first subcell 82 and doped to make
the material highly-resistive or to create a diode barrier to the
flow of electric current. Of course, there could also be
applications that involve a series connection of subcell 82 on the
front side of the substrate 84 with the subcell 86 on the back side
of the substrate 84, in which case the substrate 84 should be doped
to conduct current, and appropriate tunnel junction layers may be
added to allow current flow as explained above in relation to the
PV converter device 10 of FIG. 2.
[0098] As explained above, any of a wide range of ternary or
quaternary GaInAs(P) alloys with any combinations of bandgaps and
lattice constants can be used in subcells of tandem (more than one
subcell) stacks of low-bandgap, monolithic, multi-bandgap,
optoelectronic devices according to this invention. Another example
of such combinations is illustrated in the alternate example
bifacial PV converter device 160 in FIG. 11, where two lattice
mismatched (LMM) subcells 162, 164 are grown on the front side 165
of an InP substrate 166 and another, even lower bandgap, subcell
168 is grown on the back side 167 of the substrate 166. For
purposes of this illustration, but not for limitation, the first
subcell 162 is shown as LM, DH, quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y with a bandgap of 1.1 eV, but
lattice-matched to a LM, DH, ternary Ga.sub.xIn.sub.1-xAs second
subcell 164 instead of to the InP substrate 166, as shown by lines
170, 172, 174 in FIG. 10. Therefore, a lattice constant transition
layer 176, such as graded InAs.sub.yP.sub.1-y is needed between the
InP substrate 166 and the second subcell 164, as shown in FIG. 11,
to make the transition between the 5.869 .ANG. lattice constant of
the InP substrate 166 (line 12 in FIG. 10) and the 5.905 .ANG.
lattice constant of the ternary Ga.sub.xIn.sub.1-xAs of the second
subcell 164 (line 174 in FIG. 10). Either an isolation layer 178,
as shown in FIG. 11, or a tunnel junction, can be positioned
between the first subcell 162 and the second subcell 164, depending
on whether it is desired to connect the subcells 162, 164
independently or in series, as explained above.
[0099] The third subcell 168 in the PV converter 160 in FIG. 11 has
an even lower bandgap, for example 0.55 eV, to absorb and convert
longer wavelength radiation transmitted through the first and
second subcells 162, 164 to electricity, according to the
principles explained above. Such a third subcell 168 can be, for
example, a LM, DH, ternary Ga.sub.xIn.sub.1-xAs with a 0.55 eV
bandgap, which is not lattice-matched to the InP substrate 166, as
shown by lines 12, 93 in FIG. 10. Therefore, another lattice
constant transition layer 180, such as graded InAs.sub.yP.sub.1-y
is needed between the InP substrate 166 and the third subcell 168
to make the transition between the 5.869 .ANG. lattice constant of
InP (line 12 in FIG. 10) and the 5.952 .ANG. lattice constant of
the ternary Ga.sub.xIn.sub.1-xAs (ine 93 in FIG. 10) of the third
subcell 168.
[0100] Again, if the InP substrate 166 is deep acceptor doped to be
an insulator or semi-insulator, the third subcell 168 will be
electrically isolated from the first and second subcells 162, 164
and can be connected independently to other PV converters or
subcells, such as in a MIM structure (described below). Otherwise,
a separate isolation layer (not shown in FIG. 11) may be needed
somewhere between the substrate 166 and the third subcell 168, as
explained above. Again, contacts, conductive layers, buffers,
optical control layers, and the like, are not shown in FIG. 11, but
can be provided as explained above for other embodiments.
[0101] Another interesting variation of the bifacial embodiment PV
converter 80 in FIG. 9 is the use of epitaxially grown InP for the
higher bandgap first subcell 82 instead of a ternary or quaternary
GaInAs(P). This variation, with an appropriate lower bandgap (lower
than the 1.35 eV bandgap of InP) second subcell 86, can operate as
a highly efficient, stand-alone, tandem solar cell. This bifacial
or buried substrate configuration of the PV converter 80 is
particularly advantageous for use as a solar cell, because the
buried InP substrate 84 is not in a position to block or absorb
shorter wavelength solar radiation before it reaches the first
subcell 82.
[0102] An illustration of this principle in a slightly more complex
bifacial, monolithic, multi-bandgap, solar photovoltaic (SPV)
converter device 190, multiples of which can also be incorporated
into a MIM structure, is shown in FIG. 12. In this example SPV
device 190, there are three lattice-matched subcells 192, 194, 196
grown on the front side 197 of a InP substrate 198 and two, lower
bandgap, lattice-mismatched (LMM) subcells 200, 202 grown on the
back side 199 of the substrate 198. Again, isolation and/or tunnel
junction layers 204, 206 can be included between front-side
subcells 192, 194 and/or between subcells 194, 196, respectively,
for either independent electrical connection or series electrical
connection, respectively, within the SPV device 190, as explained
above. Similarly, either an isolation layer 208 or a tunnel
junction can be provided between the back-side subcells 200, 202,
depending on whether it is desired to electrically connect them
independently or in series within the SPV device 190.
[0103] In the example SPV device 190, the first subcell 192 is
shown as a LM, DH, InP subcell with a bandgap of 1.35 eV, while the
bandgaps of the second and third subcells 194, 196 have lower
bandgaps, e.g., 1.0 eV, LM, DH, quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y for the second subcell 194 and
0.74 eV, LM, DH, ternary Ga.sub.0.47In.sub.0.53As for the third
subcell 196, which is the lowest bandgap GaInAs that can be
lattice-matched to the InP substrate 198 (see lines 13, 14, 210 in
FIG. 13). Since all the front-side subcells 192, 194, 196 are
lattice-matched (line 12 in FIG. 13) to the InP substrate 198, no
lattice constant transition layer is needed on the front side of
the substrate 198.
[0104] Again, the variations of conductive or highly-resistive
substrate 198, isolation and/or tunnel junction layers, electrical
contacts, buffer layers, optical control layers, more or fewer
subcells, different bandgaps, and the like, as described above for
other embodiments, are also applicable to the SPV device 190
described above.
[0105] Also, AlInAs in slightly higher bandgaps than the 1.35 eV
bandgap of the InP can also be lattice-matched to InP, so a first
subcell of such AlInAs lattice-matched to the InP substrate could
also be used as part of a bifacial, monolithic, multi-bandgap, PV
converter of this invention. Of course, Ga could be added to
produce AlGaInAs, if a slightly lower bandgap than AlInAs is
desired for either the first subcell or a subsequent subcell of a
bifacial PV converter of this invention.
[0106] The PV converters described above can be used alone or in
combinations with myriad other devices. For example, any of the PV
converters, especially the SPV device 190, but also, PV converters
10, 110, 112, 140, 150, 80, 160, can be used for the bottom cell
device in a mechanical stack of higher bandgap (higher than the
1.35 eV bandgap of InP) PV converters, such as GaAs based PV
converters, in solar cell and other applications. Such other,
higher bandgap, PV converters (not shown) can selectively absorb
and convert shorter wavelength solar energy to electricity, while
the lower bandgap PV converters, e.g., PV converters 10, 110, 112,
150, 80, 160, 190, absorb and convert longer wavelength solar
radiation to electricity.
[0107] As mentioned above, the bifacial, monolithic, multi-bandgap,
optoelectronic devices of this invention, for example, the bifacial
PV converters 80, 160, 190 shown in FIGS. 9, 11, and 12 and
described above, as well as myriad variations of such bifacial
configurations, are particularly adaptable to use in monolithic,
integrated modules (MIMs). An example MIM PV converter device 230
with a plurality of bifacial, monolithic, multi-bandgap,
photovoltaic converters 190 of FIG. 12 grown on a single substrate
198 is shown in FIG. 14. Essentially, all of the PV converter
subcell stacks 190 are grown in unison on the common substrate 198,
and then they are separated into a plurality of individual subcell
stacks 190' by etching away or otherwise removing material to form
isolation trenches 232 between the front-side subcell stacks 190'
and to form isolation trenches 234 between the back-side subcell
stacks 190''. Then, various conductors 236, 238, 240, 242, 244,
246, 248, 250, 252, 254, 256, 258, 260, and others are added in
various electrical connection patterns to interconnect the subcells
together, as will be described in more detail below. The spaces
between the conductors are filled with insulator material, such as
silicon nitride or any of a variety of other suitable insulator
materials.
[0108] In the bifacial MIM PV converter device 230 illustrated in
FIG. 14, there are twice as many, albeit smaller, back-side subcell
stacks 190'' as front-face subcell stacks 190'. Further, there can
be any desired ratio of back-side subcell stacks 190'' to
front-side subcell stacks 190'. The ratio of two back-side subcell
stacks 190'' to one front-side subcell stack 190' shown in FIG. 14
is only an example.
[0109] One advantage of being able to have different numbers of
subcell stacks 190', 190'' on front and back of the substrate 198
is more flexibility to design voltage-matched subcell circuits. A
schematic diagram of an equivalent electrical circuit corresponding
to the example voltage-matched subcell circuits 262, 264, 266, 270,
272 of the MIM 230 of FIG. 14 is shown in FIG. 15. zAccording to
fundamental electrical principles, a circuit comprising a plurality
of subcells connected in parallel will have an output voltage equal
to the subcell output voltage, but current from the parallel
connected subcells add. Conversely, a circuit comprising a
plurality of subcells connected in series will have a current
output equal to the subcell output current, but the output voltages
of the series connected subcells add. Therefore, connecting a
plurality of higher voltage subcells together in parallel can build
current as output voltage remains constant, while connecting a
plurality of lower voltage subcells together in series can boost
the voltage output of the subcell circuit to the level of the
higher voltage subcell circuit. Also, higher bandgap subcells
produce higher voltage than lower bandgap subcells. Therefore, if,
for example, the voltage output of each of the lower bandgap
subcell stacks 190'' in FIG. 14 is half as much as the voltage
output of each of the higher bandgap subcell stacks 190', and if
all the higher bandgap subcell stacks 190' are connected in series
with each other while all the lower bandgap subcell stacks 190''
are connected in series with each other, then the total voltage
output of the back-side subcell stacks 190'' circuits 270, 272
would equal the total voltage output of the front-side subcell
stacks 190' circuits 262, 264, 266, because there are twice as many
low voltage subcell stacks 190'' as there are higher voltage
subcell stacks 190'.
[0110] However, the front-side subcells 192, 194, 196 of the
front-side stacks 190' can be connected in myriad combinations of
series and/or parallel electrical connections, as illustrated in
FIGS. 14 and 15 to create voltage-matched subcell circuits 262,
264, 266. The same goes for the back-side subcells 200, 202 of the
back-side stacks 190'' to create voltage-matched subcell circuits
270, 272. Such electrical connection options are facilitated by the
isolation layers 204, 206, 208 and highly-resistive substrate 198,
as described above. Additional options can be provided by tunnel
junctions instead of isolation layers or even making the substrate
198 conductive rather than resistive for intra-subcell stack series
connections, as explained above.
[0111] To illustrate several series and parallel connection
options, the bifacial MIM PV converter device 230 in FIGS. 14 and
16, is shown, for example, with all of its highest bandgap, thus
highest voltage, subcells 192 connected together in parallel to
form the subcell circuit 262. The next highest bandgap, thus next
highest voltage, subcells 194 are connected together in a
combination of parallel 244, 246 and series 247 connections to form
a subcell circuit 264 that is voltage-matched to the subcell
circuit 262. The subcells 196, which are the lowest bandgap, thus
lowest voltage, of the subcells on the front side of the substrate
198, are shown in this example illustration of FIGS. 14 and 15, as
all being connected in series in subcell circuit 266 by conductors
242 to add their voltages in order to match the output voltage of
subcell circuit 266 to the output voltages of the subcell circuits
262, 264. These parallel and series-connected subcell circuits 262,
264, 266 are connected in parallel to each other at conductors 236,
238, 240 and at 236', 238', 240' to add their respective current
outputs.
[0112] The back-side subcells 200, 202 are even lower voltage than
the front-side subcells 196, but there are more of them than the
front-side subcells 192, 194, 196, so the back-side voltage can be
matched to the front-side voltage. In the example of FIGS. 14 and
15, the lowest voltage subcells 202 are connected together in
series in the subcell circuit 272 by conductors 260 to add their
voltages in order to match the output voltage of subcell circuit
272 to the output voltage of the parallel connected 256, 258,
higher voltage subcells 200 in the subcell circuit 270. Then, the
subcell circuit 272 is connected in parallel to the subcell circuit
270 by conductors 252, 254 and 252', 254' to add their respective
current outputs.
[0113] Finally, the front-side subcell circuits 262, 264, 266 are
connected in parallel to the back-side subcell circuits 270, 272 at
terminal contacts 258, 258' to add their respective current
outputs. Therefore, the bifacial MIM PV converter 230 can be
connected electrically to other devices or loads via the two
terminal contacts 256, 258, which is a very desirable feature of
this invention. Other MIM structures, circuit connections, and
advantages can be made according to these principles within this
scope of this invention. For example, but not for limitation, the
monofacial, monolithic, LM, DH, multi-bandgap, PV converters
described above can also be incorporated into MIM structures (not
shown), although the bifacial embodiments described above have the
advantage of using the substrate 198 as a built-in isolation
structure between subcells on the front side and subcells on the
back side, as explained above.
[0114] Any of the PV converter embodiments 10, 110, 150, described
above and shown in FIGS. 1, 2, 4, and 6 can be modified to provide
an ultra-thin, monolithic, multi-bandgap, PV converter by
fabricating it in such a way as to enable removal of the InP
substrate 26. For example, as shown in FIG. 16, a monolithic,
multi-bandgap (tandem), PV converter 100 is fabricated much the
same as the PV converter 10 in FIG. 2 on an InP substrate 26,
except that a stop-etch layer 98 is added between the buffer layer
32 and the front cladding layer 40 of the first subcell 22. The
stop-etch layer 98 can be, for example, n-Ga.sub.0.47In.sub.0.53As
with the same lattice constant as the InP substrate 26, so that the
subsequent layers of the first and second subcells 22, 24, tunnel
junction 44, 46, (or isolation layer for independently connected
subcells) and lattice constant transition layer 20 can be grown
epitaxially, as described above.
[0115] The purpose of the stop-etch layer 98 is to enable the InP
substrate 26 and buffer layer 32 to be removed by etching or other
selective chemical removal to create an ultra-thin, monolithic,
multi-bandgap (tandem) PV converter 100 without etching or damaging
any of the first subcell 22. After the several layers of the
structure in FIG. 16 are grown epitaxially on the InP substrate 26,
the structure 100 is top-mounted on another object 102, such as a
solar panel, heat sink, printed circuit board, or other useful
platform. Then the substrate 26 and buffer layer 32 are removed by
etching or other selective chemical removal, leaving the
ultra-thin, monolithic, multi-bandgap, PV converter 100 mounted on
the object 102, as shown in FIG. 17. The stop-etch layer 98 can be
an electically conductive material, so it can also serve as a
contact layer. If desired, part of such a conductive stop-etch
layer 98 can be removed by etching or other selective chemical
removal with a different chemical in which it is soluble to leave a
grid pattern, which would be useful if the material of layer 98 is
not transparent to the incident radiation R
[0116] Mounting the PV converter 100 on the object 102 can be
accomplished with a suitable adhesive or by any other suitable
mounting mechanism. An anti-reflective coating 97 can be added to
reduce reflection of incident radiation, or layer 97 can be any
other optical control material for purposes described above for the
PV converter 10.
[0117] This ultra-thin, monolithic, multi-bandgap, PV converter 100
enables this device to be used as a solar cell, because elimination
of the InP substrate 26 allows all of the incident solar radiation
SR to reach the subcells 22, 24, which can convert it to
electricity. Otherwise, the InP substrate 26, which has a bandgap
of 1.35 eV, would absorb large amounts of solar radiation SR in
wavelengths shorter than 0.93 .mu.m, before such solar radiation SR
could reach the first subcell 22. There is no n/p junction in the
substrate 22, and it cannot convert radiant energy to electricity,
so any solar energy absorbed by the substrate 26 would be
thermalized and wasted as heat.
[0118] Even without the InP substrate, however, there could be
significant production of heat in the PV converter 100, when it is
used as a solar cell, because there is a substantial amount of
energy in higher frequencies (shorter wavelengths) of the solar
spectrum, where wavelengths are substantially shorter than the
longest wavelength that can be absorbed by the first subcell 22.
Therefore, there is significant thermalization of excess energy
that is not needed for carriers to transcend the bandgap E.sub.g1
of the first subcell 22, thus a significant production of heat that
must be dissipated from the PV converter 100. However, the PV
converter 100 is ultra-thin and has no thick substrate, so heat can
flow through the PV converter 100 is substantially one-dimensional,
and it can flow quickly and easily to the back surface 104. If the
object 102 on which the PV converter 100 is mounted is a good heat
sink, i.e., good thermal conductivity and sufficient mass and/or
surface area to conduct heat away from the PV converter 100, the
combination provides very good thermal management and minimizes
heat build-up in the PV converter 100.
[0119] The ultra-thin, monolithic, multi-bandgap, PV converter 100
can also be grown in a polycrystalline form on less expensive
substrates, such as graphite, which is amorphous and does not
impose a lattice constant on the first subcell 22, or in
single-crystal form on compliant substrate or bonded substrate
systems, which provide a lattice constant match to accommodate
epitaxial growth. A typical compliant substrate may be made, for
example, with an inexpensive substrate material, such as silicon,
and with an amorphous oxide of the substrate material followed by a
layer of perovskite oxide. Therefore, a first subcell 22 of InP,
GaInAs, or GaInAsP will grow with its natural lattice constant.
Such first subcell 22 can then be followed by a InAs.sub.yP.sub.1-y
lattice constant transition layer 20 and another, lower bandgap,
second subcell 24, as described above. Then, the resulting
ultra-thin PV converter 100 is mounted on another object 102 and
the compliant substrate is removed.
[0120] Compliant substrates can also be used on any of the
monofacial PV converter embodiments 10, 110, 112, 140, 150
described above, and they can possibly be used for the bifacial
embodiments 80, 160, 190, 230. Possible uses of compliant
substrates in the embodiments of this invention depend on the
transparency and other properties of the compliant substrate
materials and systems being considered and/or applied.
[0121] While the description of this invention has focused
primarily on photovoltaic converters, persons skilled in the art
know that other optoelectronic devices, such as photodetectors and
light-emitting diodes (LEDs) are very similar in structure,
physics, and materials to PV converters with some minor variations
in doping and the like. For example, photodetectors can be the same
materials and structures as the PV converters described above, but
perhaps more lightly-doped for sensitivity rather than power
production. On the other hand LED's can also be much the same
structures and materials, but perhaps more heavily-doped to shorten
recombination time, thus radiative lifetime to produce light
instead of power. Therefore, this invention also applies to
photodetectors and LEDs with structures, apparatus, compositions of
matter, articles of manufacture, and improvements as described
above for PV converters.
[0122] Since these and numerous other modifications and
combinations of the above-described method and embodiments will
readily occur to those skilled in the art, it is not desired to
limit the invention to the exact construction and process shown and
described above. For example, accordingly, resort may be made to
all suitable modifications and equivalents that fall within the
scope of the invention as defined by the claims which follow. The
words "comprise," "comprises," "comprising," "include,"
"including," and "includes" when used in this specification and in
the following claims are intended to specify the presence of stated
features or steps, but they do not preclude the presence or
addition of one or more other features, steps, or groups thereof.
Also, GaInAs(P) is used as a shorthand, generic term that includes
any ternary Ga.sub.xIn.sub.1-xAs and/or quaternary
Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y, and similar notation
conventions apply to AlGaInAs(P).
* * * * *