Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
  • H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
  • H01G9/20—Light-sensitive devices
  • H01G9/2027—Light-sensitive devices comprising an oxide semiconductor electrode
  • H01G9/2031—Light-sensitive devices comprising an oxide semiconductor electrode comprising titanium oxide, e.g. TiO2
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/02—Hydrogen or oxygen
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/02—Hydrogen or oxygen
  • C25B1/04—Hydrogen or oxygen by electrolysis of water
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/50—Processes
  • C25B1/55—Photoelectrolysis
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
  • H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
  • H01G9/20—Light-sensitive devices
  • H01G9/2068—Panels or arrays of photoelectrochemical cells, e.g. photovoltaic modules based on photoelectrochemical cells
  • H01G9/2072—Panels or arrays of photoelectrochemical cells, e.g. photovoltaic modules based on photoelectrochemical cells comprising two or more photoelectrodes sensible to different parts of the solar spectrum, e.g. tandem cells
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M14/00—Electrochemical current or voltage generators not provided for in groups H01M6/00 - H01M12/00; Manufacture thereof
  • H01M14/005—Photoelectrochemical storage cells
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F10/00—Individual photovoltaic cells, e.g. solar cells
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
  • H10F19/00—Integrated devices, or assemblies of multiple devices, comprising at least one photovoltaic cell covered by group H10F10/00, e.g. photovoltaic modules
  • H10F19/40—Integrated devices, or assemblies of multiple devices, comprising at least one photovoltaic cell covered by group H10F10/00, e.g. photovoltaic modules comprising photovoltaic cells in a mechanically stacked configuration
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy
  • Y02E10/542—Dye sensitized solar cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/36—Hydrogen production from non-carbon containing sources, e.g. by water electrolysis
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/10—Process efficiency
  • Y02P20/133—Renewable energy sources, e.g. sunlight

Definitions

• This invention relates to so-called photoelectrochemical systems.
• light for example solar illumination
• one or more photoactive semiconductors i.e. semiconductors that generate electrons or holes on illumination by light
• an electrolytic medium for example an aqueous electrolytic medium
• One previously proposed system consists of two photosystems connected in series.
• the first of these systems comprises a tungsten trioxide or iron (iii) oxide semiconducting film that absorbs the blue and green parts of the electromagnetic spectrum and transmits the red and yellow parts to a second system arranged behind the first.
• the second system acts as an electric bias to increase the electrochemical potential of electrons generated by the first to beyond the point where cleavage of the electrolyte to liberate hydrogen becomes possible.
• This previously proposed system provides an overall solar light to chemical conversion efficiency of something in the order of 5 to 10 percent, and whilst this represents a significant improvement over previously proposed systems the commercial viability of a large scale system could be improved if the efficiency of the system were to be increased.
• one presently preferred embodiment of the present invention provides a photoelectrochemical system for the generation of hydrogen by cleavage of an aqueous electrolyte, the system comprising: an electrolyser for housing an electrolyte, said electrolyser comprising first and second electrodes and being configured for collection of hydrogen liberated at one of said electrodes on operation of the system; a photoactive element separated from said electrolyte and consisting of first and second photoactive systems electrically connected to one another in series, said first and second photoactive systems being electrically coupled to respective ones of said first and second electrodes; wherein said first photoactive system is configured to absorb and be responsive to illumination of a first range of wavelengths and at least substantially transparent to the remainder of said illumination, and said second photoactive system is configured to be responsive to illumination of a second range of wavelengths comprising at least part of the remainder transmitted by said first system.
• This arrangement is advantageous, for example as. compared with that described in the abovementioned European Patent, for a number of reasons.
• the use of a discrete electrolyser means that only those semiconductors which absolutely must be in contact with electrolyte need be so. Indeed, in the preferred arrangement, none of the photoactive semiconductors are in contact with the electrolyte in the electrolyser. This helps avoid problems associated with the electrolyte acting to degrade the semiconductor, as might happen with the previously proposed arrangement (for example seawater electrolyte may, over time, act to degrade the photoactive film in the front cell of the system). It is even possible, in a highly preferred embodiment of the present invention, to avoid using electrolytic solutions in the electrolyser by utilising an electrolyser with a solid electrolytic membrane (such as a so-called PEM electrolyser for example). This arrangement is particularly advantageous as it then allows water, for example pure water, to be used in place of an electrolyte.
• the photoactive element includes, in said first photoactive system, a dye-sensitised solar cell (DSC) that is traversed by incident illumination.
• DSC dye-sensitised solar cell
• Another presently preferred embodiment of the present invention provides, in or for a photoelectrochemical system for the generation of hydrogen, a photoactive element substantially as herein described. Investigations have shown that this element is significantly more efficient than the photoactive element described in the aforementioned European patent. In general, the efficiencies achieved are comparable to those achieved with the aforementioned single crystal devices, and as DSCs are relatively inexpensive the cost of the presently proposed system is significantly less than the cost of a corresponding single crystal system.
• Fig. 1 is a schematic representation of a previously proposed so-called tandem cell
• Fig. 2 is a schematic representation of a photoelectrochemical system in accordance with a preferred embodiment of the invention, that is to say a system comprising a photoactive element and an electrolyser;
• Fig. 3 is a schematic representation of one embodiment of the photoactive element depicted in Fig. 2;
• Fig. 3a is a schematic representation of another embodiment of the photoactive element depicted in Fig. 2;
• Fig. 4 is a schematic representation of the electrolyser depicted in Fig. 2;
• Fig. 5 is a graph illustrating the light transmittance of an illustrative photoactive system incorporated within the element of Fig. 2;
• Fig. 6 is a graph illustrating the spectral response of a second illustrative photoactive system incorporated within the element of Fig. 2;
• Fig. 7 is a graph illustrating the overall performance of the photoactive element as a whole.
• FIG. 1 for illustrative background purposes, there is shown a photoelectrochemical system of the type described in the aforementioned European Patent.
• the system 20 consists of two photoactive systems connected in series.
• the cell on the left contains the aqueous electrolyte that is subjected to water photolysis.
• the electrolyte is composed of water as a solvent to which an electrolyte has been added for ionic conduction. Saline seawater can also be used as a water source, in which case the addition of electrolyte becomes superfluous.
• Light enters from the left side of the cell through a glass window 1. After traversing the electrolyte 2 the light impinges on a back wall of the cell that comprises a mesoporous semiconductor film 3 composed of an oxide such as tungsten trioxide (WO 3 ) or iron (iii) oxide (Fe 2 O 3 ).
• This film is deposited onto a transparent conducting oxide film 4 made from a material such as fluorine-doped tin dioxide that serves as current collector and which is deposited on the glass sheet 1.
• the oxide absorbs the blue and green part of the solar spectrum whilst yellow and red light is transmitted through it.
• the transmitted yellow and red part of the solar spectrum is captured by a second cell mounted behind the back wall of the first cell.
• This second cell contains a dye- sensitized mesoporous TiO 2 film, and functions as a light-driven electric bias increasing the electrochemical potential of the electrons that emerge from the WO3 film under illumination to render the reduction of water to hydrogen possible.
• the second cell further comprises a transparent conducting oxide film 4 deposited on the back side of the glass sheet 1 that constitutes the back wall of the first cell.
• the conducting oxide film is covered by the dye-derivative nanocrystalline titania film 6, and this film is in contact with the organic redox electrolyte 7 and a counter electrode 8 that comprises glass that is rendered conductive on the side of the organic electrolyte by the deposition of a transparent conductive oxide layer.
• a second compartment 9 which contains an aqueous electrolyte of the same composition as in the front compartment 2. Hydrogen is evolved at the cathode 10 that is immersed in this second electrolyte compartment.
• the two electrolyte compartments 2 and 10 have the same composition and are separated by an ion-conducting membrane or a glass frit 11.
• the thin film of nanocrystalline tungsten trioxide absorbs the blue part of the solar spectrum.
• the valence band holes h + created by band gap excitation of the oxide serve to oxidize water forming oxygen and protons:
• the conduction band electrons are collected on the conducting glass support forming the back wall of the first photocell. From there the conduction band electrons are fed into the second photocell that is mounted directly behind the WO3 film to capture the part of the solar spectrum that is transmitted through the top cell.
• the second photocell acts as a photo-driven bias to increase the electrochemical potential of the electrons so that they can reduce the protons produced during water oxidation to hydrogen.
• the overall reaction corresponds to the splitting of water by visible light.
• the photoelectrochemical system of a preferred embodiment of the present invention comprises a photoactive element 22 electrically coupled to a discrete electrolyser 24, such as a so-called PEM electrolyser or a liquid electrolyte electrolyser.
• the electrolyser 24 is depicted schematically as comprising two regions 26, 28 separated by a barrier 30 that may comprise (in the case of a liquid electrolyte electrolyser) a simple gas impermeable membrane or (in the case of a PEM electrolyser) a proton exchange membrane.
• Separation of the electrolyser from the photoactive element may prove to be advantageous as it allows the electrolyser to be remotely located from the photoactive element, and this could prove advantageous in circumstances where the electrolyte of the electrolyser might react adversely to cold ambient temperatures (as might be experienced if the device were to be installed on a rooftop).
• the photoactive element comprises, as aforementioned, a first photoactive system 32 coupled electrically in series to a second photoactive system 34.
• the first photoactive system 32 is configured to absorb and be responsive to a first wavelength range, and be at least substantially transparent to the remaining wavelength range of the incident illumination.
• the second photoactive system 34 is configured to absorb and be responsive to at least a second wavelength range falling within the remaining wavelength range transmitted by the first system 32.
• first photoactive system 32 forms the negative electrode of the photoactive element and is coupled to the cathode of the electrolyser 24.
• the second photoactive system 34 forms the positive electrode of the photoactive element and is coupled to the anode of the electrolyser 24.
• the photoactive element is arranged such that incident illumination (e.g. solar illumination) falls upon the first photoactive system so that a predetermined first wavelength range of the illumination is absorbed by the first system.
• incident illumination e.g. solar illumination
• the remainder of that incident illumination traverses the first system and falls upon the second system, and the second system absorbs and is responsive to a second range of wavelengths comprising at least part of the range of wavelengths transmitted by the first system.
• the first photoactive system comprises a dye sensitised solar cell (DSC).
• DSC dye sensitised solar cell
• Such cells comprises a porous photoactive semiconductor structure, preferably a mesoporous structure (mesoporous structures being those structures which are porous and include pores of in the region of approximately 2 to 50 nm), that has been sensitised by the incorporation of dye particles.
• the mesoporous dye-sensitised photoactive semiconductor is immersed in an organic redox electrolyte, preferably a non-volatile organic redox electrolyte or an ionic liquid.
• the photoactive semiconductor is of titanium dioxide, TiO 2 , and consists of roughly 20nm sized anatase particles.
• Ruthenium dye preferably Ru(4,4'-dicarboxylic acid-2,2'-bipyridine) (4,4'-bisnonly )- 2,2'-bipyridine)(NCS) 2 ] coded as Z907 is preferred as a sensitiser, although it will be apparent to persons skilled in the art that alternative sensitisers such as Ru(4,4'-dicarboxylic acid-2,2'-bipyridine) (4,4'-bis(p- hexyloxystyryl)- 2,2'-bipyridine)(NCS) 2 , coded as K-19 or complexes with a related structure could instead be employed.
• the mesoporous titanium dioxide structure is immersed, in the preferred embodiment, in a non-volatile organic electrolyte based on the ionic liquid, N- propyl-N-methyl imidazolium iodide or in another embodiment the mesopores of the titanium dioxide may be impregnated with a solid organic hole conductor.
• the DSC of the first photoactive system is constructed as a self-contained solar cell, that is to say that the photoactive material, immersed in electrolyte, is produced as a sealed unit.
• Previously DSCs have suffered from degradation of the electrolyte over time, but tests with the DSC of the presently preferred embodiment have indicated that with this proposed DSC design, degradation over time is less of a concern.
• DSCs absorb photons in particular spectral regions, and are at least substantially transparent in others. This makes DSCs a particularly suitable choice for multi-component photoactive elements, as it allows the individual components of the element to be "tuned” to particular spectral regions, to thereby utilise an increased proportion of the incident illumination. It is also the case that the short-circuit photocurrent of DSCs can readily be varied by changing the thickness of the film, the effective pore size or the density, and furthermore that DSCs can readily (and relatively inexpensively) be formed by simple techniques such as screen printing or doctor blading.
• the second photoactive system 34 comprises a thin-film photovoltaic semiconductor, or semiconductors.
• the second photoactive system includes a so-called CIGS cell (Cu(In 1 Ga)Se 2 ) responsive to at least part of the range of wavelengths of illumination that the DSC is transparent to.
• the CIGS cell is electrically coupled in series with the DSC of the first photoactive system by means of an appropriate conductor that is transparent at least to those wavelengths to which the CIGS cell is intended to be responsive.
• a CIGS cell is preferred, other suitable types of cell such as a thin film silicon cell or a TiO 2 film sensitized by a quantum dot or by a near IR wavelength absorbing dye, may instead be used.
• the dye of the DSC On illumination, light of the first range of wavelengths is absorbed by the dye of the DSC thereby exciting the dye molecules and freeing electrons which are injected (in this case) into the mesoporous titanium dioxide that supports the sensitiser.
• the holes generated on excitation of these electrons are transported by the electrolyte out of the first photoactive system to a conducting transparent interconnect, formed in this embodiment by the transparent conductor 39, that separates the top cell 32 from the bottom cell 34 of the photoactive element.
• the holes flow via the transparent conductor 39 to the series- connected second photoactive system, and the second photoactive system acts to increase the electrochemical potential of the holes to the point where those holes can oxidize water at the anode of the electrolyser to oxygen and protons.
• the electrons injected into the semiconductor (which in this case is mesoporous TiO 2 ) are used to reduce the protons to hydrogen at the cathode of the electrolyser.
• FIG. 3 of the accompanying drawings there is depicted a more detailed view of a photoactive element 36 in accordance with a preferred embodiment of the present invention.
• the element 36 comprises a first photoactive system 38 (typically this will be the front system, i.e. the system that is first illuminated), and a second photoactive system 40 closely coupled behind the first in a sandwiched arrangement.
• the second system is coupled to the first in series by means of a transparent interconnect 39 that comprises a transparent conductor such a conducting glass or transparent conducting organic material.
• Behind the second cell there is provided a reflective layer 41 so that illumination traversing through both systems is reflected back through the first and second systems to thereby increase the proportion of illumination absorbed.
• Fig. 3a The arrangement depicted in Fig. 3a is similar to that of Fig. 3, except that in this embodiment the second system 40 comprises two photovoltaic cells 42, 44 coupled together in series. In this instance the second of these photovoltaic cells is separated from the first photoactive system 38 by means of a transparent insulator 46 such as a relatively thin glass or plastic sheet so as to maintain the in-series electrical connection between the first and second photoactive systems 38, 40.
• a transparent insulator 46 such as a relatively thin glass or plastic sheet
• the photoactive element may be encapsulated in a suitably transparent and impervious material so that the first and second photoactive systems are protected from damage.
• Fig. 4 is a schematic representation of a previously proposed PEM electrolyser, the like of which is commonly available, e.g. from H-TEC AG (Germany), and to which the photoactive element of the preferred embodiment can conveniently be connected.
• the electrolyser 24 comprises a housing 48 in which respective electrolyte inlets 50 are formed.
• the electrolyte is shown as being an aqueous electrolyte - specifically water - but it will be appreciated by those persons skilled in the art that substances may be added to water to improve its conductivity (although it will equally be appreciated that in a PEM electrolyser this is not necessary).
• the electrolyte need not necessarily be aqueous, in which case whilst hydrogen would be liberated at one electrode, another gas would be liberated at the other.
• the electrolyte is aqueous (and thus that hydrogen and oxygen will be evolved), but it should be noted and remembered that this need not be the case. To enjoy the fruits of the present invention it is only essential that hydrogen be evolved at one electrode, and some other gas be evolved at the other.
• the housing 48 is divided into an oxygen collection region 52 and a hydrogen collection region 54 by means of a so-called proton exchange membrane or PEM 56.
• PEM proton exchange membrane
• Such membranes are well known in the art, and typically comprise complex polymer bodies which are porous to liquid, but gas impermeable (Nafion ® , manufactured by Dupont, is one example of a suitable PEM material).
• Each side of the PEM 56 is coated with a catalyst 58 (typically of platinum or ruthenium/platinum), and it is these catalysed regions of the PEM that form the electrodes, specifically an anode 60 and a cathode 62, of the electrolyser.
• the anode 60 is coupled, as mentioned above, to the first photoactive system of the photoactive element, and the cathode is coupled to the second photoactive system of the photoactive element.
• One or both of the aforementioned collection regions of the electrolyser can be configured so that generated gasses, in particular hydrogen, are output from the electrolyser at a desired positive pressure.
• Such an arrangement can be advantageous, for example if it should be desired to store the hydrogen in a hydrogen storage device such as a metal halide canister as it obviates the need for a separate device for pressurising generated gasses.
• a DC voltage between the anode and cathode and generated on illumination of the photoactive element causes a DC electric current to flow through the PEM.
• Protons are drawn to the cathode and are discharged as H atoms by combination with electrons (e " ) at the metal cathode surface (M). Pairs of adsorbed H atoms then combine to make molecules of H 2 gas which escape, freeing the electrode surface for more proton discharge:
• a particularly preferred DSC comprises a mesoporous " I ⁇ O2 film of 5 to 10 micron thickness that consists of roughly 20 nm sized anatase particles, ruthenium dye Z907 as a sensitizer and a nonvolatile electrolyte based on the ionic liquid N- propyl-N-methyl imidazolium iodide.
• the thickness of the top cell is controlled to provide a short circuit photocurrent (J sc ) output of roughly 10-11 mA/cm 2 and an open circuit voltage (V oc ) between roughly 800 and 900 mV under AM 1.5 solar radiation.
• the aforementioned DSC has good transparency in the wavelength region above about 700 nm.
• the second photoactive system it is particularly preferred to provide a pair of (for example) CIGS or silicon cells coupled together, and to the first photoactive system, as depicted schematically in Fig. 3a.
• each of these cells When illuminated in isolation (i.e. when decoupled from the first photoactive system) each of these cells provide a photocurrent of roughly 35- 40 mA/cm 2 and an open circuit photovoltage of roughly 650 mV in full sunshine.
• a typical spectral response of the photocurrent of a CIGS cell is shown in Figure 6, and it is clearly apparent that these cells have a good response to light of those wavelengths transmitted by the first photoactive system (namely, light of a wavelength above about 700 nm). Silicon cells exhibit a similar action spectrum of their photocurrent.
• the photocurrent reduces to about 23 mA/cm2 and the V 00 to 630 mV as a consequence of the absorption of a proportion of the incident illumination by the first photoactive system.
• the two in series connected CIGS or silicon cells are capable of supporting the photocurrent of roughly 10-11 mA/cm 2 generated by the top cell.
• the total open circuit photovoltage of the photoactive element is over 2 V.
• the photocurrent of the photoactive element, at the maximum power point is close to 9-10 mA/cm, and the voltage is roughly 1.6 V, which is enough to sustain electrolysis of an electrolyte in a PEM electrolyser.
• the overall efficiency of hydrogen generation by sunlight is roughly 14- 15 %, significantly more than previously achievable with the so-called tandem cell depicted schematically in Fig. 1 , and more than that achievable with more expensive single-crystal type photoelectrochemical systems of the like mentioned above.

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