U.S. patent application number 16/580179 was filed with the patent office on 2020-04-16 for devices and methods for photoelectrochemical water splitting.
The applicant listed for this patent is Alliance for Sustainable Energy, LLC. Invention is credited to Isabel Barraza Alvarez, Todd Gregory Deutsch, Daniel Joseph Friedman, John David Simon, Myles Aaron Steiner, James Luke Young.
Application Number | 20200115810 16/580179 |
Document ID | / |
Family ID | 70161205 |
Filed Date | 2020-04-16 |
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United States Patent
Application |
20200115810 |
Kind Code |
A1 |
Steiner; Myles Aaron ; et
al. |
April 16, 2020 |
DEVICES AND METHODS FOR PHOTOELECTROCHEMICAL WATER SPLITTING
Abstract
The present disclosure relates to a photoelectrochemical
electrode that includes an absorber layer having a quantum well,
where the photoelectrochemical electrode is configured to perform a
first reaction defined as, 4H.sup.-+4e.sup.-2H.sub.2, or a second
reaction defined as, 2H.sub.2OO.sub.2+4H.sup.++4e.sup.-, when the
photoelectrochemical electrode is configured to be in contact with
water.
Inventors: |
Steiner; Myles Aaron;
(Denver, CO) ; Friedman; Daniel Joseph; (Lakewood,
CO) ; Simon; John David; (Austin, TX) ;
Deutsch; Todd Gregory; (Arvada, CO) ; Young; James
Luke; (Golden, CO) ; Alvarez; Isabel Barraza;
(Santa Barbara, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Alliance for Sustainable Energy, LLC |
Golden |
CO |
US |
|
|
Family ID: |
70161205 |
Appl. No.: |
16/580179 |
Filed: |
September 24, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62743977 |
Oct 10, 2018 |
|
|
|
62754689 |
Nov 2, 2018 |
|
|
|
62836138 |
Apr 19, 2019 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 11/0478 20130101;
C25B 1/003 20130101; C25B 11/0405 20130101; C25B 1/04 20130101 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C25B 1/00 20060101 C25B001/00; C25B 1/04 20060101
C25B001/04 |
Goverment Interests
CONTRACTUAL ORIGIN
[0002] The United States Government has rights in this disclosure
under Contract No. DE-AC36-08G028308 between the United States
Department of Energy and the Alliance for Sustainable Energy, LLC,
the Manager and Operator of the National Renewable Energy
Laboratory.
Claims
1. A photoelectrochemical electrode comprising: an absorber layer
comprising a quantum well, wherein: the photoelectrochemical
electrode is configured to perform a first reaction defined as,
4H.sup.++4e.sup.-2H.sub.2, or a second reaction defined as,
2H.sub.2OO.sub.2+4H.sup.++4e.sup.-, when the photoelectrochemical
electrode is configured to be in contact with water.
2. The photoelectrochemical electrode of claim 1, wherein: the
quantum well comprises, in order: a first barrier layer having a
first bandgap; a well layer having a second bandgap; and a second
barrier having a third bandgap, wherein; the second bandgap is less
than the first bandgap and the third bandgap.
3. The photoelectrochemical electrode of claim 2, wherein: the
first bandgap and the third bandgap are between about 1.4 eV and
about 1.66 eV, inclusively, and the second bandgap is between about
1.13 and about 1.4 eV, inclusively.
4. The photoelectrochemical electrode of claim 3, wherein: the
first barrier layer and the second barrier both comprise
GaAs.sub.1-yP.sub.y, and y is between about 0.0 and about 0.2,
inclusively.
5. The photoelectrochemical electrode of claim 3, wherein: the well
layer comprises Ga.sub.1-xIn.sub.xP, and x is between about 00.0
and about 0.2, inclusively.
6. The photoelectrochemical electrode of claim 2, wherein: the
first barrier layer and the second barrier layer have a thickness
between about 1 nm and about 50 nm, and the well layer has a
thickness between about 1 nm and about 20 nm.
7. The photoelectrochemical electrode of claim 1, wherein the
absorber layer comprises between 1 and 100 quantum wells.
8. The photoelectrochemical electrode of claim 1, further
comprising: a substrate layer; and a reflecting layer, wherein: the
reflecting layer is positioned between the substrate and the
absorber layer, and the reflecting layer is a Bragg reflector.
9. The photoelectrochemical electrode of claim 8, wherein the
substrate is lattice-matched to the quantum well.
10. The photoelectrochemical electrode of claim 8, wherein the
substrate is lattice-mismatched to the quantum well.
11. The photoelectrochemical electrode of claim 8, wherein: the
Bragg reflector comprises: a first layer having a first index of
refraction; and a second layer having a second index of refraction,
wherein: the first index of refraction is different than the second
index of refraction.
12. The photoelectrochemical electrode of claim 1, wherein both the
first index of refraction and the second index of refraction are
between 2.5 and 4.0, inclusively.
13. The photoelectrochemical electrode of claim 12, wherein: the
first index of refraction is about 3.5, and the second index of
refraction is about 3.0.
14. The photoelectrochemical electrode of claim 13, wherein: the
first layer comprises GaAs, and the second layer comprises
AlAs.
15. The photoelectrochemical electrode of claim 8, wherein: the
absorber layer further comprises: a first doped layer, and a second
doped layer, wherein: the quantum well is positioned between the
first doped layer and the second doped layer, the second doped
layer is positioned between the substrate and the second doped
layer, and the second doped layer is the opposite doping of the
first doped layer.
16. The photoelectrochemical electrode of claim 15, wherein: the
first doped layer is p-type or n-type, the second doped layer is
p-type or n-type, and the doping type of the second doped layer is
the opposite of the doping type of the first doped layer.
17. The photoelectrochemical electrode of claim 16, wherein: the
first doped layer and the second doped layer each have a bandgap
between about 1.0 eV and about 1.5 eV, inclusively.
18. The photoelectrochemical electrode of claim 17, wherein: both
the first doped layer and the second doped layer comprise
Al.sub.xGa.sub.yIn.sub.zAs.sub.vP.sub.wN.sub.tSb.sub.u, x is
between 0.0 and 1.0, y is between 0.0 and 1.0, z is between 0.0 and
1.0, v is between 0.0 and 1.0, w is between 0.0 and 1.0, t is
between 0.0 and 1.0, u is between 0.0 and 1.0, and
x+y+z=u+t+v+w=1.0.
19. The photoelectrochemical electrode of claim 18, wherein at
least one of the first doped layer or the second doped layer
comprises GaAs.
20. The photoelectrochemical electrode of claim 15, further
comprising: a second absorber layer, wherein: the absorber layer
having the quantum well is positioned between the reflecting layer
and the second absorber layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S Provisional
Patent Application Nos. 62/743,977, 62/754,689, and 62/836,138
filed Oct. 10, 2018, Nov. 2, 2018, and Apr. 19, 2019, respectively,
the contents of which are incorporated herein by reference in their
entirety.
BACKGROUND
[0003] Water splitting refers to the chemical reaction of water to
its constituent elements in the form of diatomic hydrogen and
diatomic oxygen. Photoelectrochemical (PEC) water splitting is a
promising pathway to the economical production of solar hydrogen.
Using a semiconductor to directly split water eliminates the high
capital costs of an electrolyzer. The key parameters that dictate
hydrogen costs from the PEC approach are semiconductor efficiency,
stability, and cost. Technoeconomic analysis for PEC hydrogen
production reveals that high solar-to-hydrogen (STH) efficiency is
the most critical figure of merit when designing such a system.
[0004] STH efficiency is proportional to the photocurrent generated
by the device, as long as the voltage generated is sufficient to
overcome the thermodynamic potential and any surface kinetic
effects. The photocurrent in a PEC device depends on the number of
absorbed photons per unit time. The absorption, and therefore the
STH efficiency, can be increased by extending the wavelength range
of photon absorption.
[0005] The baseline standard for a PEC device is a GaInP/GaAs
tandem cell, grown lattice-matched on a GaAs substrate. This cell
absorbs light with wavelengths less than approximately 870 nm. One
technique for extending the absorption range is to use a
semiconductor "dilute-nitride-antimonide" alloy GaInAsNSb
lattice-matched to GaAs and containing small amounts of nitrogen
(N) and/or antimony (Sb) for the bottom junction absorber layer.
This approach has the drawback that these alloys are very difficult
to grow with the required optoelectronic quality. Another technique
for extending the absorption range is to change the lattice
constant to enable growth of a second absorber layer of GaInAs,
with a lower bandgap but larger lattice constant than the
substrate. The absorption range has been extended out to 1033 nm by
devices incorporating this design feature. This metamorphic
technique has demonstrated high efficiencies, but the cell is grown
inverted and is more complicated to grow and process; requires
removal of the substrate; and may not be compatible with some
advanced processing techniques. Thus, there remains a need for
improved PEC designs and systems that provide higher STH
efficiencies to enable more affordable hydrogen production.
SUMMARY
[0006] An aspect of the present disclosure is a
photoelectrochemical electrode that includes an absorber layer
having a quantum well, where the photoelectrochemical electrode is
configured to perform a first reaction defined as,
4H.sup.-+4e.sup.-2H.sub.2,
and/or a second reaction defined as,
2H.sub.2OO.sub.2+4H.sup.++4e.sup.-,
when the photoelectrochemical electrode is configured to be in
contact with water.
[0007] In some embodiments of the present disclosure, the quantum
well may include, in order, a first barrier layer having a first
bandgap, a well layer having a second bandgap, and a second barrier
having a third bandgap, where the second bandgap is less than the
first bandgap and the third bandgap. In some embodiments of the
present disclosure, the first bandgap and the third bandgap may be
between 1.4 eV and 1.66 eV, inclusively, and the second bandgap may
be between 1.13 and 1.4 eV, inclusively. In some embodiments of the
present disclosure, the first barrier layer and the second barrier
may both include GaAs.sub.1-yP.sub.y, where y may be between about
0 and about 0.2, inclusively. In some embodiments of the present
disclosure, the well layer may include Ga.sub.1-xIn.sub.xP, where x
may be between about 0 and about 0.2, inclusively. In some
embodiments of the present disclosure, the first barrier layer and
the second barrier layer may have a thickness between about 1 nm
and about 50 nm, and the well layer may have a thickness between
about 1 nm and about 20 nm. In some embodiments of the present
disclosure, the absorber layer may have between 1 and 100 quantum
wells.
[0008] In some embodiments of the present disclosure, the
photoelectrochemical electrode may further include a substrate
layer, and a reflecting layer, where the reflecting layer is
positioned between the substrate and the absorber layer, and the
reflecting layer is a Bragg reflector. In some embodiments of the
present disclosure, the substrate may be lattice-matched to the
quantum well. In some embodiments of the present disclosure, the
substrate may be lattice-mismatched to the quantum well. In some
embodiments of the present disclosure, the Bragg reflector may
include a first layer having a first index of refraction, and a
second layer having a second index of refraction, where the first
index of refraction is different than the second index of
refraction. In some embodiments of the present disclosure, both the
first index of refraction and the second index of refraction may be
between 2.5 and 4.0, inclusively. In some embodiments of the
present disclosure, the first index of refraction may be about 3.5,
and the second index of refraction may be about 3.0. In some
embodiments of the present disclosure, the first layer may include
GaAs, and the second layer may include AlAs.
[0009] In some embodiments of the present disclosure, the absorber
layer may further include a first doped layer, and a second doped
layer, where the quantum well is positioned between the first doped
layer and the second doped layer, the second doped layer is
positioned between the substrate and the second doped layer, and
the second doped layer is the opposite doping of the first doped
layer. In some embodiments of the present disclosure, the first
doped layer may be p-type or n-type, the second doped layer may be
p-type or n-type, the doping type of the second doped layer is the
opposite of the doping type of the first doped layer. In some
embodiments of the present disclosure, the first doped layer and
the second doped layer may each have a bandgap between about 1.0 eV
and about 1.5 eV, inclusively. In some embodiments of the present
disclosure, both the first doped layer and the second doped layer
may include Al.sub.xGa.sub.yIn.sub.zAs.sub.vP.sub.wN.sub.tSb.sub.u,
where x is between 0.0 and 1.0, y is between 0.0 and 1.0, z is
between 0.0 and 1.0, v is between 0.0 and 1.0, w is between 0.0 and
1.0, t is between 0.0 and 1.0, u is between 0.0 and 1.0, and
x+y+z=u+t+v+w=1.0 In some embodiments of the present disclosure, at
least one of the first doped layer or the second doped layer may
include GaAs.
[0010] In some embodiments of the present disclosure,
photoelectrochemical electrode may further include a second
absorber layer, where the absorber layer having the quantum well is
positioned between the reflecting layer and the second absorber
layer. In some embodiments of the present disclosure, the second
absorber layer may have a bandgap that is greater than the bandgap
of the first doped layer and the second doped layer. In some
embodiments of the present disclosure, the bandgap of the second
absorber layer may be between about 1.5 eV and about 1.9 eV,
inclusively. In some embodiments of the present disclosure, the
second absorber layer may include an alloy of GaInP. In some
embodiments of the present disclosure, the second absorber layer
may further include an n-doped layer and a p-doped layer.
DRAWINGS
[0011] Exemplary embodiments are illustrated in referenced figures
of the drawings. It is intended that the embodiments and figures
disclosed herein are to be considered illustrative rather than
limiting.
[0012] FIG. 1 illustrates a photoelectrochemical device for
capturing solar energy and utilizing the energy captured toward
water electrolysis to produce H.sub.2 and O.sub.2, according to
some embodiments of the disclosure.
[0013] FIG. 2 illustrates a first electrode of the
photoelectrochemical device of FIG. 1, according to some
embodiments of the present disclosure.
[0014] FIG. 3 illustrates an absorber layer of the first electrode
of FIG. 2, according to some embodiments of the present
disclosure.
[0015] FIG. 4 illustrates a first electrode of the
photoelectrochemical device of FIG. 1, according to some
embodiments of the present disclosure.
[0016] FIG. 5A illustrates a schematic of the tandem solar cell
device, including the QW region and a 20-layer Bragg reflector,
according to some embodiments of the present disclosure. Some
devices in this disclosure exclude one or both of those layer
sets.
[0017] FIG. 5B illustrates an isolated PEC electrode, with SU-8
isolating the mesa, epoxy protecting the cleaved sidewalls, and
copper foil electrical contact to the back, according to some
embodiments of the present disclosure.
[0018] FIG. 5C illustrates photographs of PV (top) and PEC (bottom)
devices, according to some embodiments of the present disclosure.
The PV device shows 4 cells and a Hall bar; the PEC device shows a
single device.
[0019] FIG. 6A illustrates the external quantum efficiency (EQE) of
PV devices, according to some embodiments of the present
disclosure. The legend indicates the device name and some details
of the device architecture.
[0020] FIG. 6B illustrates the incident photon to current
efficiency (IPCE) of PEC, according to some embodiments of the
present disclosure. The IPCE data appear slightly higher than the
EQE of FIG. 6A because the aqueous electrolyte acts as an
anti-reflection layer.
[0021] FIG. 7A illustrates the current-voltage curve for a PV
device and FIG. 7B illustrates the current-voltage curve for a PEC
device, according to some embodiments of the present disclosure.
The PEC devices were measured in 0.5M sulfuric acid, with respect
to an iridium oxide counter electrode. For both data sets, the
intensity was set to 1000 W/m.sup.2 to simulate the AM1.5 reference
spectrum. Some light is collimated in the PEC measurement,
effectively increasing the concentration.
[0022] FIG. 8 illustrates short-circuit photocurrent of the four
devices, at 0 V (red) and -0.5 V (blue) relative to an IrO.sub.x
counter electrode, according to some embodiments of the present
disclosure. Only curves that exhibited the light-limited
photocurrent at 0 V were included in the analysis. The error bars
indicate the standard deviation.
[0023] FIG. 9 illustrates a full set of JV curves for the PEC
electrodes, according to some embodiments of the present
disclosure. The line types are the same as in FIGS. 5 and 6.
REFERENCE NUMBERS
[0024] 100 . . . device
[0025] 110 . . . first electrode
[0026] 120 . . . second electrode
[0027] 130 . . . connector
[0028] 140 . . . membrane
[0029] 150 . . . electrolyte
[0030] 210 . . . Ohmic contact
[0031] 215 . . . tunnel junction
[0032] 220 . . . substrate layer
[0033] 230 . . . reflecting layer
[0034] 240 . . . passivation layer
[0035] 250 . . . first absorber layer
[0036] 260 . . . second absorber layer
[0037] 270 . . . capping layer
[0038] 280 . . . catalyst
[0039] 300 . . . quantum well
[0040] 310 . . . well layer
[0041] 320 . . . barrier layer
[0042] 330 . . . first doped layer
[0043] 340 . . . second doped layer
DETAILED DESCRIPTION
[0044] The present disclosure may address one or more of the
problems and deficiencies of the prior art discussed above.
However, it is contemplated that some embodiments as disclosed
herein may prove useful in addressing other problems and
deficiencies in a number of technical areas. Therefore, the
embodiments described herein should not necessarily be construed as
limited to addressing any of the particular problems or
deficiencies discussed herein.
[0045] References in the specification to "one embodiment", "an
embodiment", "an example embodiment", "some embodiments", etc.,
indicate that the embodiment described may include a particular
feature, structure, or characteristic, but every embodiment may not
necessarily include the particular feature, structure, or
characteristic. Moreover, such phrases are not necessarily
referring to the same embodiment. Further, when a particular
feature, structure, or characteristic is described in connection
with an embodiment, it is submitted that it is within the knowledge
of one skilled in the art to affect such feature, structure, or
characteristic in connection with other embodiments whether or not
explicitly described.
[0046] As used herein the term "substantially" is used to indicate
that exact values are not necessarily attainable. By way of
example, one of ordinary skill in the art will understand that in
some chemical reactions 100% conversion of a reactant is possible,
yet unlikely. Most of a reactant may be converted to a product and
conversion of the reactant may asymptotically approach 100%
conversion. So, although from a practical perspective 100% of the
reactant is converted, from a technical perspective, a small and
sometimes difficult to define amount remains. For this example of a
chemical reactant, that amount may be relatively easily defined by
the detection limits of the instrument used to test for it.
However, in many cases, this amount may not be easily defined,
hence the use of the term "substantially". In some embodiments of
the present invention, the term "substantially" is defined as
approaching a specific numeric value or target to within 20%, 15%,
10%, 5%, or within 1% of the value or target. In further
embodiments of the present invention, the term "substantially" is
defined as approaching a specific numeric value or target to within
1%, 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, or 0.1% of the
value or target.
[0047] As used herein, the term "about" is used to indicate that
exact values are not necessarily attainable. Therefore, the term
"about" is used to indicate this uncertainty limit. In some
embodiments of the present invention, the term "about" is used to
indicate an uncertainty limit of less than or equal to .+-.20%,
.+-.15%, .+-.10%, .+-.5%, or .+-.1% of a specific numeric value or
target. In some embodiments of the present invention, the term
"about" is used to indicate an uncertainty limit of less than or
equal to .+-.1%, .+-.0.9%, .+-.0.8%, .+-.0.7%, .+-.0.6%, .+-.0.5%,
.+-.0.4%, .+-.0.3%, .+-.0.2%, or .+-.0.1% of a specific numeric
value or target.
[0048] For the reasons discussed in the previous paragraph, there
are advantages to an upright, lattice-matched device compared to an
inverted metamorphic device or a device employing
dilute-nitride-antimonides as absorber layer(s). Thus, the present
disclosure relates to the innovation of extending the absorption
range of the bottom cell in the tandem PEC device out to longer
wavelengths by including multiple quantum wells (MQWs) in the
upright lattice-matched structure, thus increasing the efficiency
while retaining the advantages of the upright lattice-matched
structure. The MQWs can be lattice-matched or strain-balanced, but
in either case the net strain is zero and the device remains
lattice-matched. Quantum wells include a lower bandgap material
sandwiched between higher bandgap barriers. The absorption edge of
the well depends on the bandgaps of the well and barrier materials,
and the thickness of the well layer.
[0049] Some embodiments of the disclosure described herein provide
tandem absorber photoelectrochemical devices with very high
water-splitting efficiencies. For example, FIG. 1 illustrates a
photoelectrochemical device 100 for capturing solar energy and
utilizing the captured energy for electrolysis to produce diatomic
hydrogen and diatomic oxygen from water, according to the following
reactions.
At the cathode (first electrode): 4H.sup.++4e.sup.-2H.sub.2 (1)
At the anode (second electrode): 2H.sub.2OO.sub.2+4H.sup.++4e.sup.-
(2)
Total reaction: 2H2O2H.sub.2+O.sub.2 (3)
[0050] The photoelectrochemical device 100 may include a first
electrode 110 and a second electrode 120 electrically linked
together by a connector 130, for example, an electrically
conductive wire such as copper, stainless steel, titanium, silver,
gold, platinum, palladium, aluminum and/or any other suitable
electrically conducting material. The first electrode 110 and the
second electrode 120 may be immersed in a liquid electrolyte 150
contained in an appropriate vessel. In some embodiments, the
container may include a membrane 140, which physically separates
the first electrode 110 from the second electrode 120 and divides
the electrolyte into two portions, 150A and 150B, with electrolyte
150A associated with the first electrode 110, and electrolyte 150B
associated with the second electrode 120. In some embodiments, the
container may be filled with an aqueous electrolyte that may be
acidic, basic, or neutral. An aqueous electrolyte may be
characterized by a pH ranging from greater than about 0 to about
14. In other embodiments, an aqueous electrolyte may be
characterized by a pH ranging from greater than about -0.5 to about
7. An aqueous electrolyte may be an acid solution containing, for
example, a strong acid such as HI, HBr, HClO.sub.4, HClO.sub.3,
H.sub.2SO.sub.4, and/or HNO.sub.3. In some embodiments, the
electrolyte may include a weak acid such as CH.sub.3COOH (acetic
acid), HCOOH (formic acid), HF, HCN (hydrocyanic acid), and/or
HNO.sub.2 (nitrous acid). Aqueous electrolytes may also include
basic solutions that include, for example, KOH and/or NaOH
solutions. Any other suitable acidic or basic solutions may be
used.
[0051] A membrane 140 may be configured to only allow ions to pass
through it, for example protons (W). In some embodiments, a
membrane 140 may be porous. A membrane 140 may be configured to
separate the electrolyte into two portions (150A and 150B), such
that ions may be transported from one side of the membrane 140 and
one portion of the electrolyte to the other side of the membrane
and other portion of the electrolyte, while simultaneously
preventing the reaction products (H.sub.2 and O.sub.2) from
contacting each other. Examples of porous membranes include
fluorinated polymers (e.g. Nafion.RTM., Flemion.RTM., Teflon.RTM.,
and Aciplex.RTM.), polyether-ether ketone (e.g. SPEEK.RTM.),
poly(phenylquinoxalene) polymers, and/or copolymers thereof. Anion
exchange membranes may also be used, for example, with basic
electrolytes. Methods of making one or more of the films and/or
layers of the devices described herein may include metal organic
chemical vapor deposition (MOCVD), metal organic vapor phase
epitaxy (MOVPE), molecular beam epitaxy (MBE), hydride vapor phase
epitaxy (HVPE), chemical vapor deposition methods (CVD), liquid
phase epitaxy (LPE), and/or other suitable methods.
[0052] In the example of FIG. 1, the first electrode 110, the
cathode, absorbs light resulting in the formation of electrons (not
shown) near the surface of the first electrode 110, which combine
with the protons to form diatomic hydrogen per reaction (1). These
electrons are replaced by the formation of electrons occurring at
the anode according to reaction (2). These electrons flow through
the connector 130 from the second electrode 120 to the first
electrode 110 to maintain the electron balance, resulting in the
total reaction (3). Referring again to FIG. 1, this example
illustrates the case where the first electrode 110, receives and
absorbs the light and operates as a photocathode. However, elements
described herein apply generally to photoelectrochemical devices
for both devices that utilize a photocathode and cases where the
light-receiving electrode is the anode (photoanodes). The switch
from a photocathode to a photoanode can be achieved by choosing the
opposite polarity. This can be achieved by inverting the doping of
the light absorbing electrode, while perhaps also selecting a
different dark electrode with more suitable catalytic activity
toward the cathodic reaction.
[0053] FIG. 2 illustrates a first electrode 110, according to some
embodiments of the present disclosure. A first electrode 110 may
include a number of stacked elements or layers. For example, a
first electrode 110 may begin with a catalyst 280 positioned on the
outer surface of the first electrode 100 that is in contact with
the electrolyte 150 (not shown in FIG. 2). In the case of a
photocathode, the catalyst 280 may promote reaction (1), the
reduction of protons to produce diatomic hydrogen. For the case
where the first electrode 110 is a photocathode, the catalyst may
promote the conversion of protons to diatomic hydrogen. Similarly,
the second electrode 120 may contain in its bulk and/or on its
surface a catalyst (not shown) that may catalyze the conversion of
water to diatomic oxygen. Catalysts for one or both of the
electrodes of the present disclosure include at least one of
molybdenum disulfide, nickel/nickel-oxides, NaTaO.sub.3:La,
K.sub.3Ta.sub.3B.sub.2O.sub.12, GaZnNO alloys, Pt, TiO.sub.2,
cobalt, CoP, NiP, bismuth, ruthenium, RuO.sub.2, and/or IrO.sub.x.
Some embodiments may also include surface attached homogeneous
catalysts, interface modifiers, or surface texture for
anti-reflection or other purposes.
[0054] The catalyst 280 may be positioned on an underlying capping
layer 270, such that the catalyst 280 completely covers the outer
surface of the capping layer 270 or partially covers the capping
layer 270 such that at least a portion of the capping layer 270 is
in physical contact with the electrolyte 150. The capping layer 270
protects the underlying passivation layer 240D (if present) from
decomposing in the electrolyte. Suitable materials for the capping
layer 270 include GaInP, GaInPN, GaInAsP, and possibly others,
where the indicated compositions are nominal. The capping layer 270
may be positioned between the catalyst 280 and an underlying
passivation layer 240D, where the passivation layer 240D is
positioned on a second absorber layer 260. The purpose of the
passivation layer 240D like all of the other passivation layers
shown in FIG. 2 (240A, 240B, and 240C), is to minimize carrier
recombination, thus improving the photoelectrode's conversion of
light to an electrical current. Suitable materials for passivation
include AlInP and AlGaInP for layer 240D, and AlGaInP or AlGaAs for
layer 240C. Thus, in some embodiments of the present disclosure,
the second absorber layer 260 may be sandwiched between two
passivation layers 240C and 240D.
[0055] In general, the second absorber layer 260 is constructed of
a semiconductor material, which may be an AlGaInAsPNSb alloy,
having a larger bandgap than the bandgap of the underlying first
absorber layer 250. In some embodiments, the second absorber layer
260 may include at least one of a p-type absorber layer and/or an
n-type layer (not shown). The second absorber layer 260 may have a
relatively high bandgap (relative to the first absorber layer 250)
between about 1.5 eV and about 1.9 eV. In some embodiments, the
second absorber layer 260 may have a bandgap between about 1.6 eV
and about 1.8 eV. In some embodiments of the present disclosure,
the second absorber layer 260 may have a p-type conductivity due to
doping of the AlGaInAsPNSb alloy with one or more dopants such as
zinc, beryllium, magnesium, and/or carbon.
[0056] The AlGaInAsPNSb alloy used for the second absorber layer
260 may be described as having a first mixture of Group III
elements combined with a second mixture of Group V elements. Group
III elements include boron, aluminum, gallium, indium, and
thallium. Group V elements include nitrogen, phosphorus, arsenic,
antimony, and bismuth. Thus, the elemental stoichiometry of the
AlGaInAsPNsb alloy used for the second absorber layer 260 may be
described as,
Al.sub.xGa.sub.yIn.sub.zAs.sub.vP.sub.wN.sub.tSb.sub.u,
where x is between 0.0 and 1.0, y is between 0.0 and 1.0, z is
between 0.0 and 1.0, v is between 0.0 and 1.0, w is between 0.0 and
1.0, t is between 0.0 and 1.0, u is between 0.0 and 1.0, and
x+y+z=u+t+v+w=1.0. In some embodiments of the present disclosure,
x=v=u=w=t=about 0.0, resulting in an alloy for the second absorber
layer 260 that may be described as,
Ga.sub.yIn.sub.zP.
[0057] In some embodiments of the present disclosure, the second
absorber layer 260 may be constructed from an alloy defined as
Ga.sub.yIn.sub.zP.sub.w where y is between about 0.4 and about 0.6,
z is between about 0.4 and about 0.6, y+z=1.0, and w=1.0. In some
embodiments of the present invention, y may equal about 0.5. For
example, a first electrode may have a second absorber layer with a
composition of about Ga.sub.0.5In.sub.0.5P with a bandgap of about
1.8 eV. In some embodiments of the present invention, the first
electrode may have a second absorber layer having a composition of
Ga.sub.xIn.sub.1-xAs.sub.tP.sub.1-t, where x is about 0.68, t is
about 0.34, and a bandgap of about 1.7 eV.
[0058] Referring again to FIG. 2, a first electrode 110 may also
include a tunnel junction 215 positioned between the second
absorber layer 260 and the first absorber layer 250. In some
embodiments, a tunnel junction 215 may be positioned between a
first passivation layer 240C in contact with the second absorber
layer 260 and a second passivation layer 240B in contact with the
first absorber layer 250. As used herein, a tunnel junction 215 is
electrical interconnection between the top and bottom absorber
structures. In some embodiments of the present disclosure, a tunnel
junction 215 may be selected from a semiconductor alloy that is
heavily p++/n++ doped and otherwise is identical to or very similar
to the semiconductor alloy selected for the second absorber layer
to provide a transparent monolithic electrical interconnection that
is lattice-matched to the second absorber layer. Examples of
semiconductor layers that may be used as a tunnel junction 215
between the second absorber layer and the first absorber layer
include AlAs, GaAs, AlGaAs, GaInP, GaInAsP, AlGaInP.
[0059] To improve device efficiencies (the amount of hydrogen gas
produced per unit of incident solar energy), a first electrode 110
may include a first absorber layer 250. Details regarding a first
absorber layer 250 follow below with reference to FIG. 3. Referring
again to FIG. 2, a first absorber layer 250 may be positioned
between a first passivation layer 240B and a second passivation
layer 240A. Suitable materials for passivating layer 240B include
Al.sub.0.47In.sub.0.53P, and
(Al.sub.xGa.sub.1-x).sub.0.5In.sub.0.49P. Suitable materials for
passivating layer 240A include GaInP and Al.sub.xGa.sub.1-xAs. In
addition, a first electrode 110 may include a reflecting layer 230.
As shown in FIG. 2, in some embodiments a passivation layer 240A
may be positioned between the first absorber layer 250 and the
reflecting layer 230. The purpose of the reflecting layer 230 is to
reflect any light that passes through both the second absorber
layer 260 and the first absorber layer 250 to give that light a
second pass through at least the first absorber layer 250 and a
second chance to be absorbed by the first absorber layer 250. Any
such recovered/absorbed light provides the potential to increase
the conversion efficiency of the device. As discussed in more
detail below, a reflecting layer 230 may include a Bragg
reflector.
[0060] A Bragg reflector is an alternating sequence of layers with
alternating index of refraction. The thicknesses of the layers and
the total number of layers are tuned to create a reflector with a
desired peak reflectance at a specified wavelength. An example
includes an alternating sequence of 57 nm of GaAs and 82 nm of
AlAs, repeated 20 times. Other materials or alloys can be used
instead of GaAs or AlAs. In some embodiments of the present
disclosure, a Bragg reflector may be designed such that its peak
reflectance occurs in the wavelength region at which the MQW
increases the first absorber's absorbance. In some embodiments of
the present disclosure, a Bragg reflector constructed using an
alternating sequence of GaAs and AlAs will have an alternating
index of refraction of 3.5 and 3.0 respectively (at a wavelength of
903 nm). In some embodiments of the present disclosure, the
alternating layers of a Bragg reflector may have indices of
refraction between 2.5 and 4.0.
[0061] Finally, a first electrode may include a substrate layer 220
positioned between the reflecting layer 230 and an Ohmic contact
210 (current collector) that is electrically connected to the
connector 130, which in turn, electrically connects the first
electrode 110 to the second electrode 120. The substrate serves as
a template for the epitaxial growth so that highly crystalline
semiconductor layers can be sequentially deposited. The substrate
also provides mechanical rigidity to the epitaxial semiconductor
electrode, during growth, post-growth processing, and for the final
device operation. Note that although the exemplary first electrode
110 of FIG. 2 utilizes a total of twelve individual layers, other
embodiments of a first electrode that use more or fewer layers fall
within the scope of the present disclosure. For example, in some
embodiments of the present disclosure, at least one of the
passivation layers (240A, 240B, 240C, and/or 240D), the capping
layer 270, and/or the substrate layer 220 may be omitted from the
first electrode 110 architecture or stack. In some embodiments
where the reflecting layer 230 is omitted and the substrate layer
220 may be polished on both sides by the manufacturer, the Ohmic
contact 210 can also serve as a reflector to enable a second pass
of light through at least the first absorber layer 250.
[0062] Referring to FIG. 3, the first absorber layer 250 of a first
electrode 110 may include a number of individual layers or
elements. For example, the first absorber layer 250 may include a
first doped layer 340 and a second doped layer 330, where the
doping type, n-type or p-type, of the first doped layer 340 is the
opposite of the doping type of the second doped layer 330. In some
embodiments, at least one of the first doped layer 340 and/or the
second doped layer 330 may be constructed of a semiconductor alloy
including at least one of GaAs, AlGaAsN, InGaAsN, GaAsSbN,
GaInNAsSb, or any other suitable alloys that are lattice-matched to
GaAs and provide the desired bandgap; e.g. a bandgap that is less
than the bandgap of the second absorber layer 260. In general, at
least one of the first doped layer 340 and/or the second doped
layer 330 may be constructed of an AlGaInAsPNSb alloy having a
first mixture of Group III elements (Al, Ga, In) combined with a
second mixture of Group V elements (N, As, P, Sb). Thus, the
elemental stoichiometry of the AlGaInAsPNSb alloy used for at least
one of the first doped layer 340 and/or the second doped layer 330
may be defined as,
Al.sub.xGa.sub.yIn.sub.zAs.sub.vP.sub.wN.sub.tSb.sub.u,
where x is between 0.0 and 1.0, y is between 0.0 and 1.0, z is
between 0.0 and 1.0, v is between 0.0 and 1.0, w is between 0.0 and
1.0, t is between 0.0 and 1.0, u is between 0.0 and 1.0, and
x+y+z=u+t+v+w=1.0. In some embodiments, x=z=w=t=u=0, such that at
least one of the first doped layer 340 and/or the second doped
layer 330 is a two-component system of GaAs, having a bandgap of
about 1.4 eV. In some embodiments of the present disclosure, at
least one of the first doped layer 340 and/or the second doped
layer 330 may have a bandgap between about 1.0 eV to about 1.5 eV.
In some embodiments of the present disclosure, at least one of the
first doped layer 340 and/or the second doped layer 330 may have a
bandgap between about 1.0 eV to about 1.2 eV.
[0063] FIG. 3 illustrates a first absorber layer 250 having a
quantum well (QW) 300, according to some embodiments of the present
disclosure. As defined herein, a QW 300 is a combination of two
barrier layers (320A and 320B) sandwiching a well layer 310. The
first absorber layer 250 shown in FIG. 3 illustrates only one QW
300, however, some embodiments of the present disclosure may have
one or more QWs 300. The barrier layers have a higher bandgap than
the well layer. The two barrier layers (320A and 320B) are
typically constructed of the same material, though they could be
different. The combination of the two barrier layers (320A and
320B) and the well layer 310 thickness determines the absorption
properties of the quantum well structure. In some embodiments of
the present disclosure, barrier layers (320A and 320B) may be
constructed using an alloy of GaAs.sub.0.9P.sub.0.1 and having a
bandgap of about 1.53 eV, with a well layer 310 positioned between
the two barrier layers, constructed using an alloy of
Ga0.89In0.11As having a bandgap of about 1.27 eV. The bandgaps of
the barrier layers (320A and 320B) and the well layer 310 combine
to form an effective bandgap for the quantum well 300 containing
these three layers. This QW bandgap is commonly referred to as the
absorption edge and also depends on the thickness of the well layer
310; e.g. a thinner well layer 310 results in more confinement and
a higher bandgap. Thus, in some embodiments of the present
disclosure, a QW 300 may be constructed using well layers 310
having a thickness of about 8.5 nm and barrier layers (320A and
320B) having a thickness of about 17 nm, resulting in an adsorption
edge for the QW 300 of about 1.34 eV. In some embodiments of the
present disclosure, a well layer 310 of a QW 300 may be constructed
using an alloy of Ga.sub.1-xIn.sub.xP, where x is from 0 to 0.2
inclusively with a corresponding bandgap between 1.13 and 1.4 eV.
In some embodiments of the present disclosure, a barrier layer
(320A and 320B) of a QW 300 may be constructed using an alloy of
GaAs.sub.1-yP.sub.y, where y is from 0 to 0.2 inclusively with a
corresponding bandgap between 1.4 and 1.66 eV.
[0064] A quantum well 300 may be designed to have a net strain of
zero so that, on average, it is lattice-matched to the rest of
layers making up the device. A QW structure (barrier layers and
well layers) may be constructed of materials that have the same
lattice constant as the substrate layer 220, or QWs may be formed
of lattice-mismatched materials. An example of a lattice-matched
materials is GaInP used to construct both of the barrier layers
(320A and 320B), and GaInAsN for the well layer 310. An example of
lattice-mismatched materials is the use of GaAsP for the barrier
layers (320A and 320B), and GaInAs for the well layer 310. For the
case of utilizing lattice-mismatched materials, the thicknesses and
compositions of the three layers (barrier layers and well layers)
are not strictly independent and should be chosen so that the net
strain is zero (the structure is strain-balanced). The three-layer
quantum well 300 structure may be grown in the depletion region
between the second doped layer 330 and first doped layer 340 and
repeated approximately 20-100 times. It is important for carrier
transport that the three layers be depleted, and therefore the
layers may be grown without any dopant. The total number of
repeating quantum well 300 layers that can be grown may be
determined by dividing the total width of the semiconductor
depletion region (as determined by the carrier concentrations in
the first and second doped layers 330 and 340) by the thickness of
the quantum well structure 300. A Bragg reflector, as described
above, may be grown beneath the second doped layer 330 and serves
to increase the absorption in the first absorber 250, including the
region containing the quantum well(s) 300.
[0065] The tandem cell having the quantum well(s) 300 extends the
range of absorption of the device, relative to a device without the
quantum well(s)300. If the second absorber layer 260 is designed
with an appropriate bandgap and/or thickness so as to evenly
distribute the total photocurrent, the first absorber having
quantum well(s) 300 will lead to a higher STH efficiency. Other
techniques for extending the absorption range may include changing
the lattice constant to enable growth of a first absorber layer of
lower bandgap but larger lattice constant than the substrate. This
metamorphic technique has demonstrated high efficiencies, but is
more complicated to grow and process, requires removal of the
substrate, and may not be amenable to some processing techniques
such as high-temperature deposition of a catalyst layer 280.
[0066] Referring again to FIG. 2, the materials chosen for
fabricating the second absorber layer 260 and the first absorber
layer 250 are selected to provide a combination of bandgaps that
maximize the photoelectrochemical device's STH efficiency and to
provide the voltage necessary split water into hydrogen and oxygen.
In some cases, the second absorber layer 260 may have a bandgap
ranging from about 1.5 eV to 1.9 eV. In some further cases, the
second absorber layer 260 may have a bandgap ranging from about 1.6
eV to 1.8 eV. In some cases, the first absorber layer 250 may have
a bandgap ranging from about 1.0 eV to 1.5 eV. In some further
cases, the first absorber layer 250 may have a bandgap ranging from
about 1.0 eV to 1.2 eV. Still further examples include second
absorber layer/first absorber layer bandgap combinations of about
1.8 eV to about 1.2 eV, and/or 1.7 eV to about 0.95 eV.
[0067] Referring again to FIG. 1, in some embodiments of the
present disclosure, the photoelectrochemical device 100 for water
splitting may provide a voltage from about 0.5 V to about 5 V. In
some embodiments, the photoelectrochemical device 100 for water
splitting may provide from about 1.23 V to about 2.5 V. In some
embodiments, the voltage provided by the photoelectrochemical
device 100 may be supplemented by a voltage provide by some other
additional device. Layer and/or film thicknesses for some
embodiments of the present disclosure, may range from about 0.01
microns to about 10 microns. Alternatively, layers and/or films may
range from about 1 micron to about 10 microns in thickness. For
example, an absorber layer may have a thickness between about 0.5
microns and about 3 microns, or between about 1 micron and about 3
microns.
[0068] Although only a few embodiments of this disclosure have been
described in detail above, those skilled in the art will readily
appreciate that many modifications are possible in to the examples
provided herein without materially departing from the novel
teachings and advantages of this disclosure. The disclosure now
being generally described will be more readily understood by
reference to the following examples, which are included merely for
the purposes of illustration of certain aspects of the embodiments
of the present disclosure. The examples are not intended to limit
the disclosure, as one of skill in the art would recognize from the
above teachings and the following examples that other techniques
and methods can satisfy the claims and can be employed without
departing from the scope of the claimed disclosure.
EXAMPLES
[0069] FIG. 4 illustrates an example of a first electrode 110 of a
water-splitting device, where the first electrode 110 is a
photocathode, according to some embodiments of the present
disclosure. The first electrode 110 has a stack architecture as
follows, with each layer/element described in order, starting with
the side in contact with electrolyte (not shown) and finishing with
the back contact, or Ohmic contact 210.
[0070] The first electrode 110 includes a catalyst 280 that is in
contact with the electrolyte (not shown). This may be placed on a
capping layer 270 constructed of GaInP. The capping layer 270 is
placed on a first passivation layer 240D constructed of AlInP with
a nominal composition of 53% In and 47% Al. The first passivation
layer 240D is the first of two, with the second being 240C, which
together sandwich the second absorber layer 260 between them. In
the example of FIG. 4, the second passivation layer 240C is
constructed of Al.sub.0.27Ga.sub.0.23In.sub.0.5P (nominal
composition). The second absorber layer 260 contains a first
n-doped layer (not shown) of GaInP and a second p-doped layer (not
shown), also of nominally Ga.sub.0.5In.sub.0.49P. The second
passivation layer 240C is positioned on a tunnel junction 215
constructed of GaAs and Al.sub.0.5Ga.sub.0.5As. The tunnel junction
215 is followed by a third passivation layer 240B constructed of
Ga.sub.0.51In.sub.0.49P or Al.sub.0.47In.sub.0.53P. The third
passivation layer 240B is one of two passivation layers, with the
second being passivation layer 240A, that sandwich the first
absorber layer 250 between them. The bottom of the two passivation
layers 240A may be constructed of nominally
Ga.sub.0.51In.sub.0.49P.
[0071] The first absorber layer 250, as described above, contains
several elements, starting with a "top" n-doped layer 340 and a
"bottom" p-doped layer 330. The intermediate multiple quantum well
absorber layer (not numbered) is positioned between a "top" n-doped
layer 340 and a "bottom" p-doped layer 330. As described above, the
intermediate layer (not numbered) includes at least one triplet of
layers, a well layer 310 positioned between a first barrier layer
320A and a second barrier layer 320B. In this example, an arbitrary
number of intermediate absorber layers is illustrated, but first
absorber layers 250 having any number of intermediate absorber
layers fall within the scope of the present disclosure. In this
example, the well layer 310 is constructed of
Ga.sub.0.89In.sub.0.11As and each barrier layer (320A and 320B) are
constructed of GaAs0.9P.sub.0.1. The fourth passivation layer 240A
(described above) is sandwiched between the first absorber layer
250 and the reflecting layer 230, in this case a Bragg reflector
having 20 repeating layers of 57 nm of GaAs and 82 nm of AlAs. The
reflecting layer 230 is positioned on a substrate layer 220 made of
GaAs, which is positioned on the Ohmic contact 210 (back contact).
In this example, the absorption edge of the bottom cell absorber
250 is extended to 930 nm.
Additional Examples
[0072] According to some embodiments of the present disclosure,
exemplary devices described herein were grown by atmospheric
pressure metalorganic vapor phase epitaxy (MOVPE) on a custom-built
reactor. Metalorganic sources included triethylgallium,
trimethylgallium, trimethylindium and trimethylaluminum for
group-III elements, arsine and phosphine for group-V elements,
diethylzinc and carbon tetrachloride for the p-type dopants zinc
and carbon, and disilane and hydrogen selenide for the n-type
dopants silicon and selenium. Semiconductor material was deposited
on an (001) GaAs substrate layer, miscut 4.degree. toward the
<111>B to maximize the kinetic CuPt ordering in the GaInP
alloy. The bandgap of GaInP can vary from 1.8 eV (ordered) to 1.9
eV (disordered), so the lower bandgap ordered alloy leads to
increased photocurrent generation in the top junction.
[0073] The devices were grown upright, starting from the GaAs
bottom absorber layer at 650.degree. C., followed by an AlGaAs/GaAs
tunnel junction at 600.degree. C. and finally the GaInP top
absorber layer at 700.degree. C. In the devices with QWs, a Bragg
reflecting layer was grown below the bottom absorber layer. The top
absorber layer included a thin AlInP passivation layer to passivate
the emitter, and a thin GaInP capping layer to protect the AlInP
passivatin layer from etching in the electrolyte. For devices that
included quantum wells, the GaInP top absorber layer was thickened
to .about.3 .mu.m to current-match the two absorber layers. The
growth also included a heavily doped front contact layer for aid in
making test structures, that was later removed to make PEC devices.
A schematic of the full device is shown in FIGS. 5A-5C, including
additional cladding layers to improve minority carrier confinement
and transport. FIG. 5A illustrates a schematic of the tandem solar
cell device, including the QW region and a 20-layer Bragg
reflector, according to some embodiments of the present disclosure.
Some devices in this disclosure exclude one or both of those layer
sets. FIG. 5B illustrates an isolated PEC electrode, with SU-8
isolating the mesa, epoxy protecting the cleaved sidewalls, and
copper foil electrical contact to the back, according to some
embodiments of the present disclosure. FIG. 5C illustrates
photographs of PV (top) and PEC (bottom) devices, according to some
embodiments of the present disclosure. The PV device shows 4 cells
and a Hall bar; the PEC device shows a single device.
[0074] After growth, a portion of the material was cleaved off for
photovoltaic (PV) test devices, while the remainder was used for
PEC electrodes. All processing was carried out in a class 1000
cleanroom using standard photolithographic and wet-chemical etching
techniques. For both sets of devices, gold was electroplated to the
substrate to form a back electrical contact. For the PV devices,
gold front contacts were electroplated through a positive
photoresist mask and individual devices were isolated. The devices
had mesa areas of 11.6 mm.sup.2 and illuminated areas of 10.0
mm.sup.2.
[0075] For the PEC electrodes, the front contact layer was etched
away. Individual devices were isolated and the sidewalls were
protected with SU-8 2002 transparent dielectric epoxy. The exposed
surface of the isolated mesas were "flash" sputtered with a PtRu
catalyst for 3 seconds at 20 watts in a 10 mTorr UHP argon
background. This deposition yielded 2-5 nm PtRu particles covering
a fraction of the semiconductor surface and total PtRu loading of
approximately 500 ng/cm.sup.2. The PEC devices had a nominal front
area of 11.6 mm.sup.2. Further details about the electrode
fabrication are provided below.
[0076] For the PV measurements, external quantum efficiency was
measured on custom-built and calibrated instrumentation based on a
tungsten-halogen lamp and a 270m monochromator, and SR830
dual-channel lock-in amplifiers. The data resolution was 5 nm. High
brightness Mightex LEDs peaking at 470 nm and 850 nm were used to
light-bias the devices, to observe the characteristics of the
individual junctions. Current-voltage measurements were taken on a
solar simulator with a xenon lamp and high brightness LEDs to shape
the spectrum. The intensity was set using calibrated GaInP and GaAs
reference cells and the spectrum was measured with a Spectral
Evolution high speed spectrophotometer.
[0077] For PEC measurements, the incident photon-to-current
efficiency (IPCE) was measured with 10 nm FWHM resolution in a
three-electrode configuration using the solar cell as the working
electrode, a mercury/mercurous sulfate reference electrode (MSE)
with 0.5 M sulfuric acid filling solution, and a platinum flag
counter electrode. The electrodes were submerged in a glass cell
containing 0.5 M sulfuric acid with 1 mM Triton X-100 surfactant.
Light from a Xe arc lamp was passed through an Acton Research SP-50
monochromator and focused onto the middle of the device in an
"underfill" configuration to mitigate any uncertainty in active
surface area. Before obtaining measurements, the monochromated
output was measured using a calibrated photodiode. Light biases of
808 nm or 532 nm were applied when measuring the top or bottom cell
response, respectively.
[0078] Two-electrode PEC current-voltage (JV) measurements were
performed with an IrO.sub.x-coated Ti mesh (Water Star Inc.)
counter electrode and 0.5 M sulfuric acid electrolyte with 1 mM
Triton X-100 surfactant added. The illumination from an ABET
Technologies Sun 3000 solar simulator was set using calibrated
GaInP and GaAs reference cells for top-limited and bottom-limited
devices, respectively. Because of refraction through the glass
window of the sample holder, the light was estimated to be
concentrated to .about.1.1.times., so that the measured
photocurrents are systematically inflated relative to the reference
spectrum. The absolute STH efficiency from the IPCE data was
estimated, as described below.
[0079] Several devices were grown and characterized, as indicated
in Table 1. EQE and IPCE data are shown in FIG. 6A and 6B,
respectively, for one sample of each device. JV and PEC data are
shown in FIGS. 7A and 7B, for the one sample of each device,
respectively, with additional data provided below.
TABLE-US-00001 TABLE 1 Device Data PV PEC Jsc Voc Jsc V(0.99
.times. Device Device Top cell Bottom cell Color (mA/cm.sup.2) (V)
(mA/cm.sup.2) Jsc) ID Baseline 1-.mu.m GaInP GaAs Black 9.34 2.354
12.1 +0.075 MR555 Cntrl1 1-.mu.m GaInP GaAs + QW + Bragg Light blue
9.68 2.305 12.4 +0.065 MR635 Cntrl2 3-.mu.m GaInP GaAs Dark blue
8.83 2.325 11.1 >0 MR657 QW1 3-.mu.m GaInP GaAs + QW + Bragg Red
10.48 2.327 13.1 +0.088 MR706
[0080] The baseline device, shown as solid lines, was a nominally
current-matched GaInP/GaAs tandem with a .about.0.8 .mu.m top
absorber layer and a 3 .mu.m bottom absorber layer. This tandem
device did not include a Bragg reflector because the 3-.mu.m GaAs
was already optically thick. Integrating the IPCE curves over the
AM1.5 direct solar spectrum indicates that this exemplary tandem
device was, in fact, slightly bottom-limited.
[0081] Adding quantum wells and a Bragg reflector to the bottom
absorber layer (long-dashed lines) shifted the absorption edge to
930 nm which in turn increased the photogenerated current in the
bottom absorber layer. The response of the top absorber layer
remained the same, as shown in the EQE and IPCE curves, but the
increased current in the bottom absorber layer made the device
top-limited. Because the baseline was slightly bottom-limited the
overall short-circuit current increased slightly, as shown in both
sets of JV curves. FIG. 7A shows that the open-circuit voltage
(V.sub.oc) has dropped by .about.70 mV, corresponding to the shift
in absorption edge due to the incorporation of QWs. The device
still exhibits the light-limiting photocurrent at zero volts.
[0082] The control device (short-dashed lines) had a 3-.mu.m top
cell but only a GaAs bottom absorber layer. FIG. 6A shows an
increase in the long wavelength EQE and FIG. 6B shows the IPCE
response of the top absorber layer, because of the increased
thickness. In this configuration the tandem was bottom-limited, and
the short-circuit current dropped compared to the baseline device.
The V.sub.oc of this GaInP/GaAs device is largely unchanged from
the baseline because the bandgaps have not changed; the changes in
generated photocurrent in the two cells have only logarithmic
effects on the V.sub.oc.
[0083] Finally, a device MR706 with a 3-.mu.m top cell, 80 QWs in
the bottom cell, and a Bragg reflector was constructed and tested
(short-long-dashed lines). This device showed the increased
absorption range due to the QWs, as well as the increased response
in the top absorber layer. With increased photoresponse in both
junctions, the short-circuit current of device MR706 increased by
1.0 mA/cm.sup.2 compared to the baseline. This increase corresponds
to a relative increase in STH efficiency of >8%.
[0084] FIG. 8 shows the photocurrents extracted from the full set
of electrodes; data are described below. The first three sample
sets showed good catalytic effects and a light-limiting
photocurrent at 0V, whereas the last sample set showed marginal
behavior at 0V. Thus, the photocurrent at -0.5 V was also included,
to represent the potential performance with a slightly better
co-catalyst. Focusing on the solid data points, the QW-containing
electrode exhibited a 9.8% improvement in photocurrent compared to
the baseline electrode.
[0085] For completeness, the absolute efficiency of the best device
was estimated. The light-limited photocurrent can be estimated by
integrating the IPCE over the global spectrum. For the
hollow-circle data in FIG. 8, the estimated photocurrents are 12.38
and 11.03 mA/cm.sup.2 for the top and bottom cells. Assuming 100%
faradaic efficiency and 1.23 V potential, the limiting current
leads to an STH efficiency of 13.6%. With slightly improved
collection efficiency in the QW region and optimized current
matching, the STH could increase to 11.94.times.1.23=14.7%.
[0086] For PV electricity generation applications, the absorption
edge is ideally extended out to 930 nm which corresponds to the
edge of a broad water-absorption dip in the global spectrum.
Extending to slightly longer wavelengths comes at the expense of a
further drop in voltage but without a significant increase in
photocurrent, and since the PV efficiency depends on both current
and voltage, it is not beneficial to extend the absorption beyond
930 nm. For PEC applications, however, only the photocurrent is
important, provided that sufficient voltage still exists to split
water. Thus, in principle, the QW architecture can be used to
extend the absorption range to even longer wavelengths beyond 1000
nm where the water absorption band ends.
[0087] In summary, GaInP/GaAs tandem electrodes have been
demonstrated for photoelectrochemical water splitting, that
incorporate strain-balanced quantum wells to extend the absorption
edge out to 930 nm. The solar-to-hydrogen conversion efficiency
increases by .about.8% relative to the baseline, with a pathway to
additional increases by further extending the absorption edge and
lowering the top cell bandgap.
Experimental:
[0088] Growth of quantum wells: GaAsP is a mixed group-V alloy that
exhibits temperature-dependent Langmuir adsorption of adatoms on
the group-V sites, resulting in a strongly non-linear distribution
coefficient. That is, the ratio of As:P in the solid phase is not
the same as the ratio of AsH.sub.3:PH3 in the vapor phase. The
growth conditions were calibrated by growing layers of GaAsP with
fixed AsH.sub.3 flow and varying PH.sub.3 flow and measuring the
solid composition by means of a high-resolution x-ray diffraction
(224) reciprocal space map.
[0089] To grow QWs with as sharp interfaces as possible, the flows
in the GaAsP and GaInAs portions were carefully adjusted to
minimize transitions. The AsH.sub.3 flow was fixed at 20 sccm, and
the trimethylgallium (TMGa) was fixed at 3.75 sccm. The
trimethylindium (TMIn) flow was adjusted to give a solid
composition of Ga.sub.0.894In.sub.0.106As, based on a calibrated
distribution coefficient of approximately unity at 650.degree. C.
At the interfaces, the AsH3 and the PH3 and TMIn flows were cycled
between the run and vent lines. Since the growth rate is dominated
by the group-III flow, this strategy effectively sets the growth
rate of the GaInAs at .about.1.1.times. the rate of the GaAsP.
[0090] GaInAs is typically grown at 620.degree. C. by MOVPE, but
here we grew the QWs at 650.degree. C. to help keep the background
doping as low as possible, which is important for efficient carrier
collection from the quantum wells.
[0091] Growth of Bragg reflectors: Bragg reflectors are formed from
an alternating sequence of high and low index-of-refraction
materials. At 930 nm, the indices of GaAs and AlAs are .about.3.6
and .about.3.1, respectively. While this is not as much contrast as
is available with ex-situ evaporated dielectrics, a 20 layer
alternative stack of AlAs/GaAs can nonetheless lead to reflectance
>95%. In some laser devices, III-V Bragg reflectors can
demonstrate >99% reflectance.
[0092] Processing Details: Gold was electroplated from a TSG-250
Sulfide Gold (Transene) plating solution. We used Shipley 1818
positive photoresist for patterning of the front contacts (PV
devices) and mesas (PV and PEC devices). During mesa isolation,
phosphide-based layer were etched with concentrated hydrochloric
acid; arsenide layers were etched with
NH.sub.4OH:H.sub.2O.sub.2:H.sub.2O (2:1:10 by volume) or
H.sub.3PO.sub.4:H.sub.2O.sub.2:H.sub.2O (3:4:1 by volume).
[0093] To protect the sidewalls of the PEC devices, we used a
highly transparent SU-8 2002 negative dielectric photoresist epoxy.
Only the central 3 mm.times.3 mm of the 3.4 mm.times.3.4 mm mesa
was left exposed, but due to the high transparency of the SU-8, the
encapsulated edges of the mesa were still able to generate charge
carriers and were therefore included in the photoactive area of the
mesa. The exposed surface of the isolated mesas were "flash"
sputtered with a PtRu catalyst for 3 seconds at 20 Watts in a 10
mTorr UHP Argon background. This deposition yielded 2-5 nm PtRu
particles covering a fraction of the semiconductor surface and very
low total PtRu loading of approximately 500 ng/cm.sup.2.
[0094] Individual PEC devices were fashioned into electrodes. A
diamond scribe was used to cleave the substrate between isolated
mesas. Cleaved mesas were mounted on copper tape adhered to a glass
slide using colloidal silver liquid and semi-transparent Loctite
E-120HP Hysol adhesive epoxy. Kapton tape and the adhesive epoxy
were used to isolate electrical contacts from electrolyte. The
adhesive epoxy was left at room temperature to cure for .about.24
hours prior to submerging the electrodes in electrolyte for PEC
measurement and characterization.
[0095] IPCE and PEC measurement: Photoelectrochemical (PEC)
characterization consisted of current-voltage (JV) and incident
photon to current efficiency (IPCE) measurements. JV measurements
of tandems utilized a 2-electrode setup: a tandem solar device
working electrode and an IrO flag counter electrode. Both
electrodes were submerged in a glass cell containing 0.5 M sulfuric
acid with 1 mM Triton X-100 surfactant. The electrodes were
connected to a Solartron 1287A potentiostat. The cell was
positioned in the center of the light field of an ABET Technologies
Sun 3000 solar simulator and distanced from the light source so
that the illumination intensity to which the device was exposed was
approximately one sun. The one sun position was pinpointed by
moving a calibrated reference cell package through the light field
until the current produced by the reference cell corresponded to
one sun exposure. GaInP and GaAs reference cells were used to
calibrate the one sun position for top-limited and bottom limited
tandem devices respectively. The potentiostat swept from -1 V to
the open circuit voltage (V.sub.oc) of the device recording the
corresponding current. The device was not exposed to the light
source for the first 0.1 V of the measurement in order to obtain a
dark current reading. Devices were measured three consecutive times
to determine initial device performance and changes resulting from
time spent in electrolyte.
[0096] IPCE utilized a 3-electrode setup: a tandem solar device
working electrode, a mercury-sulfide reference electrode, and a
platinum flag counter electrode. The electrodes were submerged in a
glass cell containing 0.5 M sulfuric acid with 1 mM Triton X-100.
Light from the solar simulator was passed through monochromator and
slit and focused onto the middle of the device. Before obtaining
measurements, the light source was calibrated using a photodiode
with a known QE. The slit width was adjusted so that the photodiode
produced approximately 30 .mu.A when exposed to 500 nm light. The
device IPCE was measured by recording the current response produced
from exposing the device for 2 seconds to a single wavelength and
then chopping the light for 2 seconds before exposing it to the
next wavelength. A red and green light bias was applied to the
device to measure first the GaInP top cell and GaAs bottom cell
respectively.
[0097] FIG. 9 illustrates a full set of JV curves for the PEC
electrodes, according to some embodiments of the present
disclosure. The line types are the same as in FIGS. 6A-7B.
Additional Examples
[0098] Example 1. A photoelectrochemical electrode comprising: an
absorber layer comprising a quantum well, wherein: the
photoelectrochemical electrode is configured to perform a first
reaction defined as, 4H.sup.30 +4e.sup.-2H.sub.2, or a second
reaction defined as, 2H.sub.2OO.sub.2+4H.sup.30 +4e.sup.-, when the
photoelectrochemical electrode is configured to be in contact with
water.
[0099] Example 2. The photoelectrochemical electrode of Example 1,
wherein: the quantum well comprises, in order: a first barrier
layer having a first bandgap; a well layer having a second bandgap;
and a second barrier having a third bandgap, wherein; the second
bandgap is less than the first bandgap and the third bandgap.
[0100] Example 3. The photoelectrochemical electrode of Example 2,
wherein: the first bandgap and the third bandgap are between about
1.4 eV and about 1.66 eV, inclusively, and the second bandgap is
between about 1.13 and about 1.4 eV, inclusively.
[0101] Example 4. The photoelectrochemical electrode of Example 3,
wherein: the first barrier layer and the second barrier both
comprise GaAs.sub.1-yP.sub.y, and y is between about 0.0 and about
0.2, inclusively.
[0102] Example 5. The photoelectrochemical electrode of Example 3,
wherein: the well layer comprises Ga.sub.1-xIn.sub.xP, and x is
between about 00.0 and about 0.2, inclusively.
[0103] Example 6. The photoelectrochemical electrode of Example 2,
wherein: the first barrier layer and the second barrier layer have
a thickness between about 1 nm and about 50 nm, and the well layer
has a thickness between about 1 nm and about 20 nm.
[0104] Example 7. The photoelectrochemical electrode of Example 1,
wherein the absorber layer comprises between 1 and 100 quantum
wells.
[0105] Example 8. The photoelectrochemical electrode of Example 1,
further comprising: a substrate layer; and a reflecting layer,
wherein: the reflecting layer is positioned between the substrate
and the absorber layer, and the reflecting layer is a Bragg
reflector.
[0106] Example 9. The photoelectrochemical electrode of Example 8,
wherein the substrate is lattice-matched to the quantum well.
[0107] Example 10. The photoelectrochemical electrode of Example 8,
wherein the substrate is lattice-mismatched to the quantum
well.
[0108] Example 11. The photoelectrochemical electrode of Example 8,
wherein: the Bragg reflector comprises: a first layer having a
first index of refraction; and a second layer having a second index
of refraction, wherein: the first index of refraction is different
than the second index of refraction.
[0109] Example 12. The photoelectrochemical electrode of Example 1,
wherein both the first index of refraction and the second index of
refraction are between 2.5 and 4.0, inclusively.
[0110] Example 13. The photoelectrochemical electrode of Example
12, wherein: the first index of refraction is about 3.5, and the
second index of refraction is about 3.0.
[0111] Example 14. The photoelectrochemical electrode of Example
13, wherein: the first layer comprises GaAs, and the second layer
comprises AlAs.
[0112] Example 15. The photoelectrochemical electrode of Example 8,
wherein: the absorber layer further comprises: a first doped layer,
and a second doped layer, wherein: the quantum well is positioned
between the first doped layer and the second doped layer, the
second doped layer is positioned between the substrate and the
second doped layer, and the second doped layer is the opposite
doping of the first doped layer.
[0113] Example 16. The photoelectrochemical electrode of Example
15, wherein: the first doped layer is p-type or n-type, the second
doped layer is p-type or n-type, and the doping type of the second
doped layer is the opposite of the doping type of the first doped
layer.
[0114] Example 17. The photoelectrochemical electrode of Example
16, wherein: the first doped layer and the second doped layer each
have a bandgap between about 1.0 eV and about 1.5 eV,
inclusively.
[0115] Example 18. The photoelectrochemical electrode of Example
17, wherein: both the first doped layer and the second doped layer
comprise Al.sub.xGa.sub.yIn.sub.zAs.sub.vP.sub.wN.sub.tSb.sub.u, x
is between 0.0 and 1.0, y is between 0.0 and 1.0, z is between 0.0
and 1.0, v is between 0.0 and 1.0, w is between 0.0 and 1.0, t is
between 0.0 and 1.0, u is between 0.0 and 1.0, and
x+y+z=u+t+v+w=1.0.
[0116] Example 19. The photoelectrochemical electrode of Example
18, wherein at least one of the first doped layer or the second
doped layer comprises GaAs.
[0117] Example 20. The photoelectrochemical electrode of Example
15, further comprising: a second absorber layer, wherein: the
absorber layer having the quantum well is positioned between the
reflecting layer and the second absorber layer.
[0118] Example 21. The photoelectrochemical electrode of Example
20, wherein the second absorber layer has a bandgap that is greater
than the bandgap of the first doped layer and the second doped
layer.
[0119] Example 22. The photoelectrochemical electrode of Example
21, wherein the bandgap of the second absorber layer is between 1.5
eV and 1.9 eV, inclusively.
[0120] Example 23. The photoelectrochemical electrode of Example
22, wherein the second absorber layer comprises an alloy of
GaInP.
[0121] Example 24. The photoelectrochemical electrode of Example
23, wherein the second absorber layer further comprises an n-doped
layer and a p-doped layer.
[0122] The foregoing discussion and examples have been presented
for purposes of illustration and description. The foregoing is not
intended to limit the aspects, embodiments, or configurations to
the form or forms disclosed herein. In the foregoing Detailed
Description for example, various features of the aspects,
embodiments, or configurations are grouped together in one or more
embodiments, configurations, or aspects for the purpose of
streamlining the disclosure. The features of the aspects,
embodiments, or configurations, may be combined in alternate
aspects, embodiments, or configurations other than those discussed
above. This method of disclosure is not to be interpreted as
reflecting an intention that the aspects, embodiments, or
configurations require more features than are expressly recited in
each claim. Rather, as the following claims reflect, inventive
aspects lie in less than all features of a single foregoing
disclosed embodiment, configuration, or aspect. While certain
aspects of conventional technology have been discussed to
facilitate disclosure of some embodiments of the present invention,
the Applicants in no way disclaim these technical aspects, and it
is contemplated that the claimed invention may encompass one or
more of the conventional technical aspects discussed herein. Thus,
the following claims are hereby incorporated into this Detailed
Description, with each claim standing on its own as a separate
aspect, embodiment, or configuration.
* * * * *