WO2021038462A1 - Metal foil protective layer for photovoltaic cells - Google Patents

Abstract

Protective films based on metal foils can be used to protect PV cells from corrosion in electrolyte without significantly degrading performance of the PV cells. One example of such a metal foil is Nickel, but can also include Nickel oxide. The metal foil can be deposited on a PV cell. The foil provides improved corrosion protection by reducing or eliminating contact between semiconductor materials (such as Ge) in the PV cells from the surrounding electrolyte in an aqueous composition. Such a foil improves stability of the PV cell under water splitting conditions. The nickel may be used to protect the side of the PV cell where the photo-oxidation (OER) half reaction takes place. The PV cell protected by the Nickel film may be a three-junction structure with at least one III-V subcell based on a InGaP/InGaAs/Ge structure.

Description

DESCRIPTION

METAL FOIL PROTECTIVE LAYER FOR PHOTOVOLTAIC CELLS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U. S. Provisional Patent Application No. 62/891,659 filed August 26, 2019, which is hereby incorporated by reference in its entirety.

FIELD OF INVENTION

[0002] The present invention generally relates to photovoltaic cells (PVs). More specifically, the present invention relates to protective layers for photovoltaic cells (PVs) used for water splitting.

BACKGROUND OF THE INVENTION

[0003] Photovoltaic cells (PVs) can be used to convert light into electricity using semiconducting materials. One example of PVs is solar cells used to power residential or commercial facilities. Materials in these PVs exhibit the photovoltaic effect. In these materials, electrons in the materials are excited by photons of light, which separates the electrons from holes in the material, producing a flow of electrons that creates a direct current (DC) current. In solar cells, this DC current is used to power a light bulb, power a computer, charge a battery, or otherwise consumed. However, the electrons making up this DC current can also be used for other purposes.

[0004] Water splitting is a sustainable-energy technique that generates hydrogen by splitting water molecules, and that hydrogen can be used as fuel in, for example, a car. Energy is harvested when the hydrogen is recombined with oxygen, with the resulting byproduct being only water. Such an energy cycle of splitting water to form hydrogen and recombining hydrogen with oxygen to form water is a green process, resulting in no pollution or waste products. However, energy is required to split the water molecule to generate the hydrogen. Using solar energy to perform the water splitting creates an entirely green process for the generation and consumption of fuel. Photovoltaic cells (PVs) can be used in this process as part of a photoelectrochemical (PEC) water splitting device. The PEC water splitting process uses semiconductor materials to convert solar energy directly to chemical energy in the form of hydrogen. The semiconductor materials used in the PEC process are similar to those used in photovoltaic solar electricity generation, but for PEC applications the semiconductor material is immersed in a water-based electrolyte, where sunlight drives the water-splitting process. [0005] FIG. 1 shows an example PEC apparatus according to the prior art and how hydrogen is generated. A PEC device 110 includes an anode 112, a photoelectrode panel 114, and a cathode 116. The PEC 100 is submerged in an electrolyte 102. Sunlight strikes the anode 112, which through the photovoltaic effect and other chemical processes, causes hydrogen and oxygen in the water in the electrolyte 102 to disassociate, which generates oxygen gas 122 and hydrogen gas 124. The hydrogen gas 124 can be collected and used as fuel. Water 104 can be replenished in the electrolyte such that hydrogen gas 124 production can continue as long as light continues to strike the anode 112.

[0006] A challenge for the PEC water splitting process is the stability of the PEC device 110 when submerged in the electrolyte 102. The materials of the PEC device 110 corrode when submerged, and that corrosion is accelerated under the excitation of the sunlight. This corrosion process is shown in FIG. 2. Photo-generated holes (h⁺) may oxidize the semiconductor first, rather than water. This is dependent on the alignment of F_oxi (oxidation potential) relative to F(02/H20), and Fred (reduction) relative to F(H⁺/H2). Some PEC devices have shown stability for up to tens of hours of use. However, this is inadequate for long-term useful applications of PEC water splitting. One conventional solution is to protect the PV cells with thin metal oxide films, such as titanium dioxide (T1O2) deposited by atomic layer deposition (ALD). However, such solutions are still lacking in providing long-term protection for the PEC water splitting device without negatively affecting the efficiency of the device, because the stability is still only a few hours.

BRIEF SUMMARY OF THE INVENTION

[0007] A PV cell with enhanced corrosion resistance and photovoltaic performance can be made of III-V semiconductor materials and include a protective films based on a metal foils. One example of such a metal foil is Nickel. The metal foil can be deposited on a PV cell. Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysts can be deposited on the foil to form a PEC device, or the Ni foil can act as the catalyst without separate catalysts. In some embodiments, the nickel foil can function as the catalyst. The foil provides improved corrosion protection by reducing or eliminating contact between semiconductor materials (such as Ge) in the PV cells from the surrounding electrolyte in an aqueous composition. Such a foil as described in the embodiments below can be stable under water splitting conditions, impermeable (e.g., no cracks or pinholes in the foil), and provide good charge transport properties such that photo-generated charge carriers reach the catalyst surface. In embodiments having a Nickel film, the Nickel can protect the PV cells in addition to acting as a catalyst for oxygen production. The nickel may be used to protect the side of the PV cell where the photo-oxidation (OER) half reaction takes place. In some embodiments, the PV cell protected by the Nickel film is a three-junction structure with at least one III-V subcell based on a InGaP/InGaAs/Ge structure.

[0008] According to some embodiments, a water-splitting multi -junction (MJ) III-V device includes a first photovoltaic (PV) subcell having a top surface and an opposing bottom surface; a second PV subcell comprising one or more III-V semiconductor materials and having a top surface and an opposing bottom surface, wherein the bottom surface of the second subcell is positioned on the top surface of the first subcell; and/or a continuous electrically-conductive water-impermeable nickel-containing layer having a thickness of 0.05 mm to 1 mm positioned on the bottom surface of the first subcell, wherein the continuous electrically conductive water- impermeable nickel containing layer is configured to produce oxygen (O2) from hydroxide (OH ) anions.

[0009] In some embodiments, the first subcell may be comprised of a p-type base layer and/or p-type back surface field layer comprising Germanium; the bottom surface of the first subcell may be comprised of the p-type base layer and/or the p-type back surface field layer; the first subcell further may be comprised a layer having one or more III-V semiconductor materials; and/or the continuous electrically-conductive water-impermeable nickel containing layer may be adhered to the bottom surface of the first subcell, preferably with a composition comprising indium (In) and gallium (Ga). In one embodiment, the adhesive may be a silver paste and can be cured at low temperatures, such as 60-70 degrees Celsius, which reduces damage to the subcell. Other adhesives may also be used, although a high thermal conductivity may be preferred to provide cooling to the subcell when under concentrated light.

[0010] The device may also include a third subcell comprising one or more III-V semiconductor materials and having a top surface and an opposing bottom surface, wherein the bottom surface of the third subcell is positioned on the top surface of the second subcell. The third subcell may have a band gap greater than the second subcell, and the second subcell has band gap greater than the first subcell, such as wherein the third subcell has a band gap of 1.70 eV to 1.90 eV, preferably 1.86 eV, the second subcell has a band gap of 1.1 eV to less than 1.7 eV, preferably 1.4 eV, and the first subcell has a band gap of 0.4 to less than 1.1 eV, preferably 0.65eV. [0011] The water-splitting device according to embodiments of this disclosure may be include a container comprising an aqueous composition and a multi -junction (MJ) III-V PV cell for the production of hydrogen (Th) and/or oxygen (O2) from water. The water may have a pH of at least 13, preferably 13 to 15. In some embodiments, the water splitting may be performed without bias applied to the MJ III-V cell. A method for water splitting using the device may include contacting the MJ III-V PV cell with an aqueous composition and a light source to produce H2 and/or O2 from the aqueous composition, and in some embodiments without an external bias is not used to produce the H2 and/or O2. The cell may be coupled to a counter electrode configured to produce H2 and OH anions from the aqueous composition during the water splitting.

[0012] Foils used in some embodiments described herein are made in a top-down approach involving rolling metal slabs cast from molten metal in a rolling mill to the desired thickness by applying an appropriate pressure on the slab. Using such a foil has an advantage of allowing attachment of the foil to the MJ PV structure in ambient conditions, such as without high vacuum, chemical bath, or high temperatures.

[0013] The terms “about” or “approximately” are defined as being close to as understood by one of ordinary skill in the art. In one non-limiting embodiment the terms are defined to be within 10%, preferably, within 5%, more preferably, within 1%, and most preferably, within 0.5%.

[0014] The terms “wt.%”, “vol.%” or “mol.%” refers to a weight, volume, or molar percentage of a component, respectively, based on the total weight, the total volume, or the total moles of material that includes the component. In a non-limiting example, 10 moles of component in 100 moles of the material is 10 mol.% of component.

[0015] The term “substantially” and its variations are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%.

[0016] The terms “inhibiting” or “reducing” or “preventing” or “avoiding” or any variation of these terms, when used in the claims and/or the specification, includes any measurable decrease or complete inhibition to achieve a desired result.

[0017] The term “effective,” as that term is used in the specification and/or claims, means adequate to accomplish a desired, expected, or intended result.

[0018] The use of the words “a” or “an” when used in conjunction with the term “comprising,” “including,” “containing,” or “having” in the claims or the specification may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.”

[0019] The words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “includes” and “include”) or “containing” (and any form of containing, such as “contains” and “contain”) are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0020] The process of the present invention can “comprise,” “consist essentially of,” or “consist of’ particular ingredients, components, compositions, etc., disclosed throughout the specification.

[0021] Other objects, features and advantages of the present invention will become apparent from the following figures, detailed description, and examples. It should be understood, however, that the figures, detailed description, and examples, while indicating specific embodiments of the invention, are given by way of illustration only and are not meant to be limiting. Additionally, it is contemplated that changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. In further embodiments, features from specific embodiments may be combined with features from other embodiments. For example, features from one embodiment may be combined with features from any of the other embodiments. In further embodiments, additional features may be added to the specific embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] For a more complete understanding, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0023] FIG. 1 shows an example PEC apparatus according to the prior art and how hydrogen is generated.

[0024] FIG. 2 is a graph showing the chemical reaction occurring during corrosion of the PEC apparatus during electrolyte exposure.

[0025] FIG. 3 shows a modified surface of a PV cell having a protective layer and a catalyst according to some embodiments of the disclosure.

[0026] FIG. 4 shows an example three-junction photovoltaic (PV) cell based on a InGaP/InGaAs/Ge structure according to some embodiments of the disclosure. [0027] FIG. 5 is a block diagram showing operation of a protected PV cell according to embodiments of the disclosure operated in a water splitting apparatus.

[0028] FIG. 6 is a graph showing dry cell photovoltaic measurements of a three-junction PV cell based on InGaP/InGaAs/Ge under one sun according to embodiments of the disclosure.

[0029] FIG. 7 is a graph illustrating cyclic voltammetry of the protected 3J cell (InGaP/InGaAs/Ge/Ni) photoanode according to embodiments of the disclosure measured using a three-electrode system in 0.5 M KOH (pH ~ 14) under 1 sun illumination and with Pt as a counter electrode for HER.

[0030] FIG. 8 is a graph illustrating chronoamperometry (CA) measurement of the Nickel foil protected 3J cell under zero bias conditions and in pH 14 solution according to embodiments of the disclosure with a left axis showing measured photocurrent and a right axis showing H2/O2 ratio.

[0031] FIG. 9 is a graph illustrating x-ray diffraction results of a protective Ni foil according to embodiments of this disclosure before and after 400 hours of stability testing.

[0032] FIG. 10 is a graph illustrating a SIMS depth profile of a protective Ni foil according to embodiments of this disclosure before and after 400 hours of stability testing.

DETAILED DESCRIPTION OF THE INVENTION

[0033] FIG. 3 shows a modified surface of a PV cell having a protective layer and a catalyst according to some embodiments of the disclosure. A semiconductor material 310 may be a semiconductor material of a PV cell or any other electronic device that could benefit from protection from corrosive environments, such as electrolyte solutions. A thin protective film 320 is formed on a side of the semiconductor material 310 that will be exposed to the corrosive environment. In some embodiments, a catalyst 330 may be formed on the protective film 320 A catalyst may be used with PV cells used, for example, in water splitting or other electrochemical reactions. The protective film 320 may be stable under water splitting conditions, may be impermeable (e.g., no cracks or pinholes in the film), and/or may have good charge transport properties to allow photo generated charge carriers from the semiconductor material 310 to reach the catalyst 330 The Nickel used for a continuous electrically-conductive water-impermeable nickel-containing layer may be a nickel oxide and/or nickel metal. A continuous layer is non-permeable, and no coating is required to make the foil non-permeable. Foils may be tempered annealed making them dense, and thus diffusion through the foil is very slow. Such a foil may be particularly useful on III-V multi -junction PV cells. In one embodiment, the protective film 320 may be a continuous electrically-conductive water- impermeable nickel-containing layer having a thickness of approximately 0.05 mm to 1 mm, or preferably 0.05-0.5 mm, positioned on a bottom surface of a subcell of a N-cell PV device. The Nickel foil may be used to protect the cells, as well as to act as catalyst for O2 production. The Nickel foil may be used to protect the side where the photo-oxidation (OER) half reaction takes place. Continuity of the Nickel foil may be provided by forming the Nickel foil separate from the manufacturing of the PV subcells, and the continuous Nickel foil adhered with conductive paste onto a PV subcell. In embodiments in which the protective film 320 is Nickel, the Nickel protective film 320 may function as a catalyst in the water splitting process, such that no separate catalyst 330 is used.

[0034] The protective film of FIG. 3 may be used on PV cell structures such as the three- junction PV cell shown in FIG. 4, however different PV cell structures, such as 2-cell, 4-cell, 5-cell, and N-cell structures may also benefit from the protective film shown in FIG. 3. FIG. 4 shows an example three-junction photovoltaic (PV) cell based on an InGaP/InGaAs/Ge structure according to some embodiments of the disclosure. The three-junction structure 400 includes a first photovoltaic (PV) subcell 410, a second photovoltaic (PV) subcell 420, and a third photovoltaic (PV) subcell 430. The first PV subcell 410 may be based on InGaP materials; the second PV subcell 420 may be based on InGaAs materials; and the third PV subcell 430 may be based on Ge materials. However, other embodiments may include different materials for the one or more subcells in the structure 400. For example, another embodiment includes InGaP materials in the first PV subcell 410, GaAs materials in the second PV subcell 420, and InGaAs materials in the third PV subcell 430. In specific embodiments, the three subcells may be based on InGaP(1.88eV)/GaAs(1.43eV)/Ge(0.67eV), respectively, or based on InGaP(1.88eV)/GaAs(1.43eV)/InGaAs(0.98eV), respectively. In other embodiments, the third subcell may have a band gap greater than the second subcell, and the second subcell may have a band gap greater than the first subcell, such as when the third subcell has a band gap of 1.70 eV to 1.90 eV, preferably 1.86 eV, the second subcell has a band gap of 1.1 eV to less than 1.7 eV, preferably 1.4 eV, and the first subcell has a band gap of 0.4 to less than 1.1 eV, preferably 0.65eV. In these embodiments, hydrogen gas is formed on the n-type side of the structure 400 and oxygen gas is formed on the p-type side of the structure 400.

[0035] Each of the PV subcells 410, 420, and 430 may include an emitter layer along with other supporting layers. In some embodiments, the subcell 410 may include a p-Ge base layer 412 of approximately 150 micrometers, a n-Ge emitter layer 414 of approximately 300 nanometers, and a n-Ga(In)As layer 416 of approximately 1000 nm. In some embodiments, the subcell 420 may include a back surface layer 422 of approximately 30 nanometers, a p- Ga(In)As base layer 424 of approximately 2500 nm, a n-Ga(In)As emitter layer 426 of approximately 100 nm, and a n-AlGaAs window layer of approximately 100 nm. In some embodiments, the subcell 430 may include a back surface layer 432 of approximately 50 nanometers, a p-GalnP base layer 434 of 680 nm, a n-GalnP emitter layer 436 of approximately 30 nm, and a n-AlInP window layer 438 of approximately 30 nm. Each of the two or more subcells 410, 420, and 430 may be separated by a bi-layer 442 of a n++-tunnel junction and a p++-tunnel junction, each of approximately 10 nanometers.

[0036] In an N-cell structure, the photo-excited p-n junctions generate electrons that are collected on one side of the cell and holes that are collected on the other side of the cell. In the structure 400 shown in FIG. 4, electrons are collected on a top surface of the third PV cell 430 and holes are collected at a bottom surface of the first PV cell 410. A protective film, such as that of FIG. 3, is applied to the structure 400 of FIG. 4 such that there is no contact between a semiconductor surface and the electrolyte to prevent corrosion of the semiconductors in electrolyte. That protective film may be a continuous electrically-conductive water- impermeable nickel-containing layer 440 having a thickness of 0.05 mm to 1 mm positioned on the bottom surface of the bottom subcell 410, wherein the continuous electrically- conductive water-impermeable nickel containing layer is configured to produce oxygen (O2) from hydroxide (OH-) anions. OER and HER catalysts may be deposited on the protective surface.

[0037] In this work, the cells were tested in a PEC setup in which the three-junction (3 J) cell acts as photo anode against a Pt counter-electrode for HER as shown in FIG. 5. Stability testing was performed in 0.5 M KOH (pH ~ 14) under 1 sun illumination while monitoring the photocurrent and H2/O2 production. FIG. 5 is a block diagram showing operation of a protected PV cell according to embodiments of the disclosure operated in a water splitting apparatus. An environment 500 shows the experimental setup in which a 3J cell (InGaP/InGaAs/Ge) 510 protected by a Ni foil 512 of 250 micrometer thickness was used as a photo anode with a Pt wire 520 as counter-electrode in an electrolyte 530.

[0038] FIG. 6 is a graph showing dry cell photovoltaic measurements of a three-junction PV cell based on InGaP/InGaAs/Ge under one sun according to embodiments of the disclosure. FIG. 6 shows that these cells exhibit high efficiency of - 28.2% and high Voc 2.54 V. Water splitting can be performed with > 1.7 V from the cell based on a sum of the potential difference needed to split water (1.23 V) plus over potential of best known catalysts plus resistance of the electrolyte. The high V_oc of the 3J cell (2.54 V) allows efficient water splitting. The Jsc from the cell is ~ 13.5 mA/cm², which indicates that possible STH for the cell is ~ 15.7% assuming 95% faradaic efficiency, with STH calculated from the following equation:

STH =

[0039] FIG. 7 is a graph illustrating cyclic voltammetry of the protected 3J cell (InGaP/InGaAs/Ge/Ni) photoanode according to embodiments of the disclosure measured using a three-electrode system in 0.5 M KOH (pH ~ 14) under 1 sun illumination and with Pt as a counter electrode for HER. The graph of FIG. 7 shows linear sweep voltammetry (LS V) of the Ni-protected 3J photoanode in 0.5 M KOH (pH ~ 14) under 1 sun illumination in three electrode setup with Pt as a counter electrode for HER. At ~ -0.3 V vs. RHE the photocurrent saturates to ~10 mA/cm². This is the light-limiting current value from the cell, irrespective of the catalyst used for OER, and thus water splitting can be performed with the 3 J cell without external bias.

[0040] FIG. 8 is a graph illustrating chronoamperometry (CA) measurement of the Nickel foil protected 3J cell under zero bias conditions and in pH 14 solution according to embodiments of the disclosure with a left axis showing measured photocurrent and a right axis showing H2/O2 ratio. The graph of FIG. 8 shows the stability of the Ni-protected 3 J photoanode in 0.5 M KOH (pH ~ 14) based on testing using chronoamperometry and photo-catalytic measurements under zero bias. Both the photocurrent and H2/O2 were measured under a flow setup using Argon as a carrier gas. The reactor was purged with 25 seem Ar flow before starting the reaction to remove all oxygen from reactor. During the reaction, the Argon flow was 10 seem. FIG. 8 shows that the photocurrent was stable as function of time, up to 400 hours. Further, the H2/O2 ratio remains at 2:1 indicating a stable cell and real water splitting. This indicates high stability for III-V cells for PEC water splitting, which is made possible with the Ni foil protection layer. The STH calculated from current is ~ 11.7% and from H2 is ~ 11.2%.

[0041] FIG. 9 is a graph illustrating x-ray diffraction (XRD) results of a protective Ni foil according to embodiments of this disclosure before and after 400 hours of stability testing. The nickel foil was characterized before and after 400 hours of exposure to electrolyte. A line 902 shows XRD results for Ni foil before exposure to electrolyte, and line 404 shows XRD results for Ni foil after 400 hours of electrolyte exposure. The five characteristic peaks for nickel 2Q = 44.45°, 51.71°, 76.41°, 92.96°, and 98.46°, corresponding to Miller indices (111), (200), (220), (311), and (222), respectively, were observed in line 902 and line 904. This observation indicates that the foil is comprised of the face centered cubic (fee) phase of metallic nickel. The nickel peaks are identical before and after reaction indicating no change in the bulk of the protection layer.

[0042] FIG. 10 is a graph illustrating a SIMS depth profile of a protective Ni foil according to embodiments of this disclosure before and after 400 hours of stability testing. The extent of oxidation of the Ni surface before and after electrolyte exposure were analyzed with secondary ion mass spectrometry (SIMS) with results shown in FIG. 10. The signal of NiO ions as shown in FIG. 10 for the sample before electrolyte exposure as line 1002 and the sample after electrolyte exposure as line 1004. The graphs indicate that the reference sample has a native oxide layer which is ~ 100 nm thick. The sample after 400 hours of electrolyte exposure and operation shows oxidation up to about 400 nm. Based on the experimental rate of oxidation shown in this measurement of approximately 1 nm per hour during water splitting, an apparatus protected by a 1 mm thick nickel foil can provide 100 years or more of operational time before failure resulting from loss of protection.

[0043] Although embodiments of the present application and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the above disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

Claims

1. A water-splitting multi -junction (MJ) III-V device comprising: a first photovoltaic (PV) subcell having a top surface and an opposing bottom surface; a second PV subcell comprising one or more III-V semiconductor materials and having a top surface and an opposing bottom surface, wherein the bottom surface of the second subcell is positioned on the top surface of the first subcell; and a continuous electrically-conductive water-impermeable nickel-containing layer having a thickness of 0.05 mm to 1 mm positioned on the bottom surface of the first subcell, wherein the continuous electrically conductive water-impermeable nickel containing layer is configured to produce oxygen (O2) from hydroxide (OH) anions.

2. The MJ III-V device of claim 1, wherein the first subcell comprises a p-type base layer and/or p-type back surface field layer comprising Germanium, and wherein the bottom surface of the first subcell is comprised of the p-type base layer and/or the p-type back surface field layer.

3. The MJ III-V PV device of any one of claims 1 to 2, wherein the thickness of the continuous electrically conductive water-impermeable nickel containing layer is 0.05 mm to 0.5 mm.

4. The MJ III-V PV device of any one of claims 1 to 2, wherein the nickel in the continuous electrically conductive water-impermeable nickel containing layer is nickel oxide and/or nickel metal.

5. The MJ III-V PV device of any one of claims 1 to 2, wherein the continuous electrically conductive water-impermeable nickel containing layer is nickel foil.

6. The MJ III-V PV device of any one of claims 1 to 2, wherein the first subcell further comprises a layer having one or more III-V semiconductor materials.

7. The MJ III-V PV device of any one of claims 1 to 2, wherein the continuous electrically- conductive water-impermeable nickel containing layer is adhered to the bottom surface of the first subcell, preferably with a composition comprising indium (In) and gallium (Ga).

8 The MJ III-V PV device of any one of claims 1 to 2, further comprising a third subcell comprising one or more III-V semiconductor materials and having a top surface and an opposing bottom surface, wherein the bottom surface of the third subcell is positioned on the top surface of the second subcell.

9. The MJ III-V PV device of any one of claims 1 to 2, wherein the third subcell has a band gap greater than the second subcell, and the second subcell has band gap greater than the first subcell.

10. The MJ III-V PV device of claim 9, wherein the third subcell has a band gap of 1.70 eV to 1.90 eV, preferably 1.86 eV, the second subcell has a band gap of 1.1 eV to less than 1.7 eV, preferably 1.4 eV, and the first subcell has a band gap of 0.4 to less than 1.1 eV, preferably 0.65eV.

11. A water-splitting system for generating hydrogen (Th) and/or oxygen (O2) from water, the system comprising a container comprising an aqueous composition and the MJ III- V PV cell of any one of claims 1 to 2.

12. The water-splitting system of claim 11, wherein the water has a pH of at least 13, preferably 13 to 15.

13. The water-splitting system of claim 11, wherein the water-splitting system is a non- biased water-splitting system.

14. The water-splitting system of any claim 11, further comprising a counter electrode connected to the MJ III-V PV cell, wherein the counter electrode is configured to produce H2 and OH anions from the aqueous composition.

15. A method for producing hydrogen (H2) and/or oxygen (O2) from water, the method comprising contacting the MJ III-V PV cell of any one of claims 1 to 2with an aqueous composition and a light source to produce H2 and/or O2 from the aqueous composition.

16. The method of claim 15, wherein an external bias is not used to produce the H2 and/or O2.

17. The method of claim 15, wherein the MJ III-V PV cell is connected to a counter electrode, and wherein the counter electrode is configured to produce Th and OH anions from the aqueous composition.

18. The method of claim 15, wherein the aqueous composition has a pH of at least 13, preferably 13 to 15.

19. The method of claim 17, wherein the light source is sunlight, preferably concentrated sunlight.

20. The method of claim 17, wherein the first subcell comprising Ge does not come into direct contact with the aqueous composition.