TiO2-Protected Photoelectrochemical Tandem Cu ... - ACS Publications

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TiO2-Protected Photoelectrochemical Tandem Cu(In,Ga)Se2 Thin Film Membrane for Light-Induced Water Splitting and Hydrogen Evolution B. Neumann,*,† P. Bogdanoff, and H. Tributsch Department Solare Energetik 5, Helmholtz-Zentrum Berlin fu¨r Materialien und Energie, Glienickerstrasse 100, 14109 Berlin, Germany ReceiVed: April 20, 2009; ReVised Manuscript ReceiVed: August 29, 2009

A flexible, thin film Cu(In,Ga)Se2 solar cell deposited on a titanium foil was combined with a TiO2 photocatalyst layer and modified by a niobium-doped titanium oxide front electrode to function as a photoelectrochemical tandem cell/membrane for a direct light-driven hydrogen evolution from aqueous solution. The P680/P700 tandem system in the plant photosynthetic unit is successfully imitated by the concerted and constructive interaction of both semiconductors. Under illumination with UV/vis light, the monolithic TiO2/Ti/Cu(In,Ga)Se2/ CdS/Nb0.03Ti0.97O1.84 tandem membranes produced up to 0.052 µL of hydrogen s-1 cm-2. The hydrogen formation rate is about 7250 µmol h-1 g-1, relative to the amount of TiO2 used. In the long term a significant cost reduction of solar hydrogen evolution will be possible within this system, due to the reduction of solar cell encapsulation costs, the absence of a grid connection between several solar cell modules, and the omission of typical solar cell module equipment, such as a converter and a bypass diode. In addition, no expensive rare-metal-based electrolyzer stack is necessary for hydrogen evolution. The layered structure of the membrane allows an easy substitution and variation of single components, an important criterion for further research and tests with photooxidatively active materials that were not solely limited to the solar UV light fraction, like TiO2 is. Typical electronic and electrochemical requirements of such substitutions are discussed within this paper. Furthermore, the proposed incorporation of the TiO2/Ti/Cu(In,Ga)Se2/CdS/Nb0.03Ti0.97O1.84 tandem membrane into an optical Winston collector system shows that a combined light and thermal solar energy conversion into hydrogen and heat is in principle possible. 1. Introduction Research in photoelectrochemical cells (PECs) often stands for a direct conversion of light energy into chemical energy in the form of storable fuels, such as hydrogen, methane, or methanol.1-4 The basis of all classic PEC-based fuel-generating routes is chemical reactions of photoinduced charge carriers with potential electron donators and acceptors at the surface of at least one semiconductor electrode. While the reduction of protons to hydrogen is unproblematic for most PECs, the absence of a cheap and abundant electron donor that can be photooxidized with high yield and efficiency by a single catalyst under AM1.5 conditions has limited the success of photoelectrochemical fuel production so far. Water is one example of a cheap and abundantly available electron donor for solar fuel generation, but with the exception of plant photosynthesis,5,6 most of the technical photocatalysts, such as titanium dioxide (TiO2), tungsten oxide (WO3), iron oxide (Fe2O3), or several niobium- and tantalum-based oxides, reach only low conversion efficiencies between 0.1% and 1% yield (η).4,7-14 Photovoltage-supported photoelectrodes15-19 and systems that are based on standard solar cells in combination with an (water) electrolysis stack20-24 show higher yields in solar H2 formation. Examples of the first group mentioned are the two in series connected Cu(In,Ga)Se2 solar cells combined with a ruthenium disulfide (RuS2) photoelectrode reported by N. G. Dhere et al. (η ) 2.90%)20 and the two ruthenium dye solar * To whom correspondence should be addressed. Phone: +049-33817975-169. Fax: +49-3381-7975-269. E-mail: [email protected]. † Present address: Johanna-Solar-Technology GmbH, Mu¨nstersche Str. 24, 14772-Brandenburg a.d. Havel, Germany.

cells in use with a WO3 photoelectrode reported by the group of M. Gra¨tzel et al. (η ) 8.0%).21 Representatives of the second group mentioned above, where the photovoltage provided by two or more in series connected solar cells exceeds the level of 1.3 V, so that finally a (dark) electrolysis of water becomes possible, show up with efficiencies near 20%. Examples are the ruthenium oxide (RuO2) and platinum (Pt) supported GaAs/ GaInP2/p-GaInP2 system reported by X. Gao et al. (η ) 6.0%),22 the triple a-Si solar cell system with a RuO2 anode and a Co/ Mo cathode reported by R. Rocheleau (η ) 7.80%),23 and, the record-holder so far, a triple-junction p-AlGaAs/n-GaAs//p-Si/ n-Si system with a RuO2 and a Pt-black layer reported by S. Licht and H. Tributsch et al. (η ) 18.30%).24 In this paper, the development of a new type of monolithic, hydrogen-producing PEC membrane is described. Opposite most so far mentioned systems, the TiO2/Ti/Cu(In,Ga)Se2/CdS/ Nb0.03Ti0.97O1.84 membrane works completely immersed in the aqueous electrolyte. This strongly increases the demands in functionality and corrosion stability, especially when sulfideand selenide-containing semiconductors are used. It is for this reason that the first membrane prototype was developed to be lined on both surfaces by TiOx-based electronic catalytic materials that have been shown to be potentially stable under the desired reaction conditions (light, aqueous electrolyte). This new example of a monolithical combination between a metal oxide-based photocatalyst layer and a flexible, surface-modified Cu(In,Ga)Se2 solar cell has the potential to reduce the costs of PV-based hydrogen production significantly, due to the absence of encapsulation and grid connection costs as well due to the

10.1021/jp903630k CCC: $40.75  2009 American Chemical Society Published on Web 11/13/2009

Photoelectrochemical Membrane for H2 Production

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Figure 1. (A) Simplified energy scheme of the photoelectrochemical tandem membrane with semiconductor 1 described in the text as a “metal oxide photoelectrode” (band gap energy Eg1) and semiconductor 2 described in the text as a “solar cell” (Eg2). The combined use of both semiconductors allows a favorable fit of the valence band (VB) and conduction band (CB) relative to the standard redox potential for the OH radical-based water splitting reaction and the reduction of protons to hydrogen, within the amount of energy provided by the solar AM1.5 spectrum. (B) Model of the Cu(In,Ga)Se2-solar-cell-based tandem PEC membrane for direct hydrogen evolution from aqueous solutions. The photocatalyst and the TCO layer have to fulfill several tasks according to optical, electronic-, and corrosion-related requirements.

fact that typical solar module equipment, such as a converter and a bypass diode, can be omitted. General schemes of the composition and the operating mode of the photoelectrochemical TiO2/Ti/Cu(In,Ga)Se2/CdS/ Nb0.03Ti0.97O1.84 tandem membrane are given in parts A and B, respectively, of Figure 1. The model for the membrane is the P680/P700 tandem system of the plant photosynthetic unit,5,6 where each absorber unit differs in its band gap energy and in its energetic position of the reactive HOMO and LUMO orbital to achieve maximum work. In the tandem PEC system described in Figure 1A in the form of an energy scheme and in Figure 1B in the form of a schematic cross-section, two kinds of light-absorbing semiconductors, one of which is active in photoinduced oxidation reactions due to a quite positive energetic position of its valence band (HOMO) and the second of which is potent in electrochemical reduction reactions with a conduction band quite negative of the standard potential of hydrogen (E < -4.5 eV/Evac or 0.0 V/normal hydrogen electrode (NHE)), are combined similarly to those in the plant photosynthetic system.5 The concept of a monolithic connection between a metal oxide photoelectrode and a Cu(In,Ga)Se2 thin film solar cell on top of a flexible titanium foil brings together the advantages of several classic PEC systems. Depending on its preparation method, the more or less porous photocatalyst layer contributes a high surface to volume ratio and low distances between the charge carrier photogenerated in the bulk and the photoelectrode surface.1,2,8 The structuring as a thin film photoelectrode results in a spatial separation of oxidation and reduction reactions and a controlled expansion of the space charge region by the internal supplied (photo)voltage,1,2,8,9 both of which can help to reduce the charge carrier recombination significantly.1,2 Last but not least, the combination of the system with a Cu(In,Ga)Se2 solar cell provides a selfsustained photovoltage source together with a top-layered front electrode, that conduction band is strongly negatively positioned.25

2. Experimental Section The reactively sputtered niobium-doped and niobium-free TiO2 films were prepared in an argon/oxygen atmosphere from a pure titanium target and an alloyed metallic niobium/titanium target containing 6 wt % niobium (both sputter targets from FHR-Dresden, Ø ) 76 mm). The sputtering power was kept constant at 200 W (dc, 25 kHz), the distance between the target and substrate in a ∼60 L recipient was around 6.5 cm, and the substrates were heated from the backside via a halogen lamp to temperatures between 400 and 420 K (150 W lamp, Ni/Cr thermocouple). Unless indicated, the sputtering pressure was 0.6 Pa in all experiments. The oxygen content in the main argon flow was varied between 0 and 10 vol %. In a typical deposition experiment the target was first intensively conditioned in a pure argon atmosphere, before the oxygen flow was successively increased until the desired working point/discharge voltage was reached. During the relatively short deposition times of 5-10 min, the discharge voltage was kept constant by small manually operated variations of the oxygen flow rate. From pure “oxidic” sputter conditions to pure “metallic” sputter conditions the sputter rate increased from 0.33 nm s-1 (1.66 nm s-1 kW-1) to values around 1 nm s-1 (5 nm s-1 kW-1). For resistivity, Hall, and spectroscopic analyses, thin films were prepared on bare 22 × 22 mm float glass substrates. The stoichiometrical composition of these films was qualitatively and quantitatively determined by elastic recoil detection analysis (ERDA; Helmholtz-Zentrum Berlin fu¨r Materialien und Energie (HZB), 350 MeV accelerated gold ions, angle of incidence 15°; for more information please see ref 26). For these experiments, hydrofluoric acid etched silicon wafers (100 orientation, 4 vol % HF, immediately before film deposition) were used as the substrate. The preparation of the flexible Cu(In,Ga)Se2 solar cell on a thin titanium foil is based on a coevaporation process that has been developed in the HZB (for a detailed description see ref 27).

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The titanium dioxide used for the photoelectrodes was purchased from Degussa (P25, mixture of 30% rutile and 70% anatase). They were prepared by the doctor-blade process onto conductive glass substrates (F-SnO2, 18 Ω-1 cm-1) or onto the titanium foil of the photoelectrochemical tandem membrane. A typical suspension consisted of 50 mg of TiO2-P25, 300 µL of ethanol (Merck, puriss.), 10 µL of acetylacetone (Fluka, puriss.), and 8 µL of Triton X (Merck, surfactant, puriss.). After the samples were annealed in an electric furnace at 720 K for 60 min in air, around 1.0-2.0 µm thick photoactive layers could be obtained. The spray pyrolytic TiO2 films were prepared by spraying 400 mL of a 2-propanol solution containing 1 vol % titanium isopropyl onto a preheated substrate. Final annealing at 720 K for 1 h finished the spray-TiO2 photoelectrode preparation. The mesoporous TiO2 films were prepared by the EISA method from titanium tetrachloride, ethanol, and water by using a prestructureforming polymer.28 After the dip-coating procedure, the films were transferred into an oven and treated successively at 373, 573, and 720 K for 12 h to solidify and dehydrate the mesostructured TiO2 network carefully. For more details, please see ref 28. The bismuth vanadium(V) oxide (BiVO4) photoelectrodes were prepared in a three-step procedure oriented on a synthesis route established by A. Kudo et al.29 First, powderlike potassium vanadium(V) oxide (K3V5O14) was synthesized by a solid-state reaction between potassium carbonate (KCO3; 14 mmol, Fluka, p.a.) and vanadium(V) oxide (V2O5; 21 mmol, Merck) at 723 K for 5 h in air. Thereafter the compound BiVO4 was obtained as a precipitate by the reaction between K3V5O14 (0.38 mmol) and bismuth nitrate (Bi(NO3)3; 1.9 mmol, Merck) in an aqueous solution. After several drying steps, thin BiVO4 layers were deposited by the doctor-blade method already described for the preparation of TiO2-P25 photoelectrodes. The complete TiO2/Ti/Cu(In,Ga)Se2/CdS/Nb0.03Ti0.97O1.84 tandem membrane was prepared in five steps: (a) deposition and annealing of the metal oxide photoelectrode layer onto a thin titanium foil already coated on one side by molybdenum (see above), (b) deposition of the Cu(In,Ga)Se2 absorber by coevaporation of the elements and thermal annealing,27 (c) wet deposition of the CdS buffer layer,27 (d) sputtering of the Nb0.03Ti0.97O1.84 front contact (see above), and (e) platinization with a 5 vol % water/ethanol solution containing hexachloroplatinic acid (H2PtCl6) with a subsequent short UV light illumination. The active area of the membranes is about 1.5 cm2. The electrochemical measurements of single photoelectrodes were done in a classical three-electrode compartment, where the electrochemical cell was additionally connected to a quadrupole mass spectrometer (Balzers, QMI 420) via a gas porous polymer membrane (Scimat 200/40/60). A platinum wire was used as the counter electrode together with a mercury sulfate electrode as the reference (0.630 V vs NHE, 298 K, pH 1). The electrical contact was made by gluing a copper wire with silver epoxy resin onto the FTO-glass substrate surface, followed by a subsequent insulation with standard epoxy resin. The control of the electrode potential was provided by a HEKA potentiostate (typical scanning rate 20 mV s-1). Prior to and during all measurements, the electrolyte (0.5 M H2SO4, Merck) was purged with nitrogen. All photoelectrodes investigated in section 3.1.1 showed a similar film thickness (0.8-1.0 µm). The presented I-V curves were chosen from cyclovoltammograms that were cycled until a stable behavior was observed. The potentials are given with reference to the NHE. In addition, standard conditions

Neumann et al. regarding temperature and pressure were present during the experiments. The photoelectrodes were irradiated from the backside with a 150 W xenon arc lamp in combination with a convex lens and an overall light intensity of ∼1.3 W cm-2. The light-induced hydrogen production of the TiO2/Ti/ Cu(In,Ga)Se2/CdS/Nb0.03Ti0.97O1.84 tandem membranes was qualitatively and quantitatively analyzed with the help of a mass spectrometer setup (Balzers, EI, QMA-125, QMI-420, QME125-2, pressure-measuring units PKR-251 and IKR-020) that was equipped only for this kind of experiment with a 1 m long, about 373 K heated, quartz glass capillary to allow gas measurements under normal air pressure (Figure 5). To increase the sensitivity of the mass spectrometer for hydrogen and to exclude interference H2 signals from the room humidity and hydrocarbons released by the vacuum pumps, deuterated chemicals were used. Before an experiment was started, the fully prepared photoelectrochemical membrane was sealed into a 20.0 × 46.7 mm sized, rubber septum crown capped boerdel glass inside a nitrogen-filled glovebox. After injection of the electrolyte, typically 3.0 mL of 0.62 M deuterated sulfuric acid (D2SO4) plus 0.88 M methanol (MeOD) or 0.88 M formic acid (DCOOD), with the help of a fine nozzle, the membrane in the cell was illuminated for 15 min with visible (HLX-64655 tungsten white light lamp, 25 mW cm-2) and with UV/vis light (150 Xe arc lamp combined with a H2O IR filter and a KG-3 wavelength filter, ∼110 mW cm-2). To estimate the amount of hydrogen accumulated during the illumination time, the crown glass cell was then transferred into a special, continuously argon flushed “measurement chamber” and connected with the tip of the quartz glass capillary of the mass spectrometer (Figure 5). Simultaneously, the injection of a second capillary secured an all-time pressure balance to the argon atmosphere in the measurement chamber (see Figure 5). The fractions of the gas components were calculated by eq 1, where the peak currents of each estimated gas i (IMS i ) were multiplied by gas-specific correction factors (qMS i , e.g., due to different ionization probabilities) and finally correlated to the overall sum of MS signals.

Aigas (%) )

IiMSqiMS

∑ (IiMSqiMS)

× 100

(1)

A reference unit (Y-0, Table 2) investigated without any photoelectrochemically active component showed the following gas fractions: N2, 98.29 vol %; O2, 1.0 vol %; Ar, 0.62 vol %; CO2, 0.028 vol %; D2, 0.008 vol %. These values represent the composition of the inner glovebox atmosphere (N2-based) and the typical “air” contamination during the electrolyte injection. The thickness of the TCO and the photoelectrochemically active layers was measured with a profilometer (DEKTAK-8). Resistance measurements were performed either in a two- or in a four-contact mode geometry with a Keithley 616 electrometer using a pre-evaporated lateral titanium contact grid (d ) 200 nm, Rsheet ) 48 Ω cm-2). The Hall measurement setup consists of a Keithley DMM19/705 electrometer, a 0.85 T magnet, and a liquid N2 cooled cryometer. The conductivity, Hall mobility of the charge carrier, and electron density itself were estimated with the help of the van der Pauw contact and analyzing procedure. To investigate the morphology of the tandem membrane and several photoelectrodes, a scanning electron microscope (Leo44) was used. The optical characterization was performed with the help of a Cary-2000 UV/vis



spectrometer including an Ulbricht sphere for reflection measurements. The performance of the Cu(In,Ga)Se2 solar cells was measured with an Oriel sun simulator (1000 W Xe lamp, AM0 + AM1.5 filter, 100 mW cm-2, Si reference cell) at room temperature (293 K, water-active cooling unit) including a fourpoint contact system. 3. Results 3.1. Combination of an n-Type Photooxidation Electrode with a Cu(In,Ga)Se2 Thin Film Solar Cell. 3.1.1. Selection of Potential n-Type Photoelectrodes. Typical requirements for an oxidative working photocatalyst are n-semiconducting properties, a high oxidation potential, a strong absorption coefficient, a band gap that fits well with the solar AM1.5 spectrum, a stable but nevertheless highly active surface, and last but not least a certain abundance of its precursor elements in nature. While several reaction mechanisms of water and formic acid photooxidation are discussed in the literature,4,9,30-36 significant conversions in oxygen and carbon dioxide evolution were observed with semiconductors only, whose valence band edge is much more positive than 1.23 V/NHE. Typical examples were titanium dioxide (TiO2),12 lanthanum-based titanium(IV) oxides (La2Ti2O7),11 tungsten trioxide (WO3),18,19 metal-based niobium(V) oxides (BiNbO4 and Ca2Nb3O10/RuO2),14,37 or natrium tantalum(V) oxide (NaTaO3).13 A second precondition is that the electrons, photogenerated from the semiconductor valence band into its conduction band, have to react instantly with a suitable redox couple in the electrolyte (preferred for powderlike photocatalysts) or they have to migrate to the back contact with the help of an external potential (photoelectrode) to minimize recombination losses of charge carriers and to maximize the (photo)chemical conversion reaction of the remaining defect electrons at the valence band edge. Well-known examples of oxidative working photoelectrodes are titanium-based di- and trioxides (TiO2, SrTiO3), tungsten trioxide (WO3), and iron(III) oxide (R-Fe2O3) electrodes.16-19 These representatives typically show photocurrents around 0.41 mA cm-2 (TiO2),15 0.6 mA cm-2 (R-Fe2O3),16 1.24 mA cm-2 (WO3),19 and 1.48 mA cm-2 (SrTiO3)38 with a strong dependence on the film thickness, porosity, surface activity, pH value of the solution, external applied potential (typically 0.5-1.0 V/NHE), and the light spectrum as well as the light intensity used in the experiments. Electrodes that can electrolyze water in the dark, such as ruthenium oxide- and iridium oxide-based electrodes, reach much higher current densities (∼20 mA cm-2, 1.6 V/NHE39), but the potentials needed are much higher than 1.0 V/NHE, and the reaction mechanism works without using the solar energy directly. In its first setup a single and not in series with other cells connected Cu(In,Ga)Se2 solar cell was used as the external potential source in the photoelectrochemical membrane. Therefore, the maximum voltage output usable for the photoelectrode layer was limited to the Cu(In,Ga)Se2 solar cell open circuit voltage (Voc) of about 550 mV. In Figure 2 a typical I-V curve of a 0.50 cm2 sized, flexible Cu(In,Ga)Se2 solar cell is plotted together with the photocurrent curves of several metal oxide photoelectrodes investigated in the photooxidation of water. This comparison shows that due to the small photocurrents provided by the photoelectrodes (please note the log scale of the y axis) the solar cell in the membrane will work near its Voc point of 0.56 V. The precondition for achieving a high voltage for the membrane photoelectrode is fulfilled as long as the photocurrent provided by the photoelectrode is significantly lower than the

Figure 2. Typical I-V curve of a flexible Cu(In,Ga)Se2 solar cell with a ZnO window layer (AM 1.5, sun simulator, MPP ) maximum power point) together with the photocurrent curves of potential photoelectrodes, such as a TiO2-P25, a mesoporous m-TiO2, a spray pyrolytic produced spray-TiO2, a mesoporous m-WO3, and a BiVO4 photoelectrode. The photoelectrodes were investigated in diluted sulfuric acid solution under illumination with the UV/vis light of a 150 W Xe arc lamp. (Note: V/NHE in the case of photoelectrodes.)

solar cell current at its maximum power point (IMPP). The intersection between the solar cell and the photoelectrode I-V curves characterizes the solar cell behavior under load and is further denoted as the working point of the tandem membrane. The photocurrent onset potential of the TiO2-P25 photoelectrode is the lowest of all photocatalysts presented in Figure 2. The current density rose already above 0.08 V/NHE and started to saturate for potentials exceeding 0.3 V/NHE. Near the membrane working point (0.56 V/NHE), the titanium oxidebased photoelectrodes showed very similar photocurrent densities that were in addition much higher than for the WO3 and BiVO4 electrodes, but for potentials above 1.0 V/NHE, the mesoporous m-WO3 electrode would win the competition (Figure 2). One advantage of using WO3 photoelectrodes in the membrane would be the ∼0.50 eV lower band gap energy relative to that of TiO2 (Eg(anatase) ) 3.20 eV)15,19 and the therefore extended solar light absorption. The photocatalytical properties of powderlike bismuth vanadium oxide (BiVO4) were investigated by K. Hirota and A. Kudo in detail,29 but resynthesized material that was used as a thin film photoelectrode and not as a dispersed powder showed no clear activity in the photooxidation of water (Figure 2). The photocurrent present between 0.20 and 0.38 and between 0.55 and 0.80 V/NHE could not be correlated to an O2 mass signal in the electrochemical mass spectrometer measurements, contrary to the other investigated photoelectrodes in Figure 2. Only in the presence of formic acid, an electron donor with lower oxidation potential, was a photocurrent density up to 0.40 mA cm-2 observed for the BiVO4 photoelectrodes (0.55 V/NHE). This time, the onset of the photocurrent correlated well with the onset of the carbon dioxide mass signal (m/z 44), the final reaction product of the photooxidation of formic acid. As a consequence, the first membrane prototypes were built with TiO2-P25 photoelectrode layers due to their favorable activity at low potentials. It should also be noted here that experiments done with carbon-doped titanium oxide (Cx-TiO2-x)15 showed in the case of a combined UV + vis light illumination no significantly increased photocurrent relative to that of undoped TiO2 reference materials. 3.1.2. Substitution of the Aqueous Solution CorrosionSusceptible Al:ZnO Front Electrode. In several thin film solar cells, such as a-Si, µ-Si, CdTe, and CIS/CIGSe solar cells,

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TABLE 1: Specific Resistivity (R), Hall Mobility (µHall), and Electron Density (ne) of TiO1.87 and Nb0.03Ti0.97O1.84 Films as Alternative TCO Layers Relative to the Common Standard in Thin Film Solar Cell Production: Al:ZnOa

a

material

R/Ω cm

TiO1.87 Nb0.03Ti0.97O1.84 Al:ZnO

22.0 ( 17.0 3.0 ( 1.6 0.0005-0.00125

µHall/cm2 V-1 s-1 0.05 ( 0.02 0.90 ( 0.70 20-100

ne/cm-3

refs

∼5.0 × 10 ∼2.4 × 1020 5.0 × 1019 to 5.0 × 1020

46 46 50, 51

20

The table shows mean values together with their standard deviations.

commonly aluminum- or gallium-doped zinc oxide films (Al: ZnO; Ga:ZnO) of 0.5-1 µm thickness are used as a transparent conductive front contact (TCO).40 In chalcopyrite-based solar cells, the interface between p- and n-type semiconductors additionally consists of a thin cadmium sulfide (CdS) and a thin intrinsically doped zinc oxide (i-ZnO) film to achieve a stepwise band gap alignment between the p- and n-type semiconductors, to prevent short-circuits between the back and front contacts, and to secure an inversion zone in the CIS layer near the heterojunctions interface.25,27 All these points are beneficial to reduce the charge carrier recombination and to improve the solar cell performance. Aluminum-doped zinc oxide (Al:ZnO) functions as a very favorable optical window (Table 1) because of its high charge carrier mobilities of about 20-100 cm2 V-1 s-1 and its low specific resistivities of about 5 × 10-4 to 1.25 × 10-3 Ω cm. Both quantities are important to minimize the serial resistance in the solar cell and in the solar cell module. In addition, it is highly transparent and maximizes therefore the transmissions of photons. In the Al:ZnO system a good compromise between the number of doped aluminum atoms and oxygen vacancies relative to the film transmission and conductivity properties was found. However, this material has a major drawback for its use in outside-placed solar cell modules and especially for its use in the photoelectrochemical membrane described in this paper. It corrodes in aqueous solution, especially at acidic or alkaline pH values.41 The corrosion is additionally enhanced during UV light irradiation. Even in a complete glass- and polymer-foilencapsulated solar module, a long-term influence of humidity is present. Hence, the module efficiency is reduced mainly due to an increasing serial cell and module resistance, caused by a Al:ZnO layer that becomes less and less conductive.42 For its use in the photoelectrochemical membrane, where a direct contact of the membrane TCO layer to the aqueous electrolyte is necessary to reduce the protons (H+) to hydrogen (H2), the Al:ZnO layer has to be protected against corrosion or replaced by a more stable material with similar optical and electronic properties. Besides experiments with artificial tenside hydrophobicized TiO2 protection layers and the use of nonaqueous ionic solvents, such as ethylammonium nitrate ([N(CH2CH3)H3]NO3), the best results for hydrogen evolution were observed by replacing the Al:ZnO layer of a standard Cu(In,Ga)Se2 solar cell by a niobium-doped NbxTi1-xOy film (x ) 0.03, y ) 1.80-1.85). This material, recently investigated as a TCO layer by Furubayashi, Zhang, Liu, and Tributsch et al.,43-46 is promising due to its properties which are similar to those of ZnO, such as a high band gap energy of about 3.0-3.2 eV and nearly the same energetic position of the conduction band (ZnO, -4.3 eV vs Evac;2 TiO2, -4.4 eV vs Evac2). In addition, a high degree of freedom in niobium atom insertion exists for this material,43,47-49 and it shows a good corrosion stability against water and UV light.4,9 Niobium-doped titanium oxide films with a transparency of more than 90% and a resistivity as low as 3 × 10-4 Ω cm could be produced (NbxTi1-xO2 with x ) 0.02-0.2, ∼40 nm

Figure 3. Correlation of the transmission (T, at 700 nm, with substrate) against the conductivity (σ) of several NbxTi1-xOy films (x ) 0.03; 0 < y < 1.96). Metallic films were apparently not transparent but conductive, and films prepared under “oxidic” sputter conditions showed a poor conductivity but a high transmission. The broad plateau between 20% and 80% T where σ did not change significantly is interesting because of the different influences of the defect sites.

film thickness, 820 K deposition temperature) with pulsed laser deposition from a sintered TiO2/Nb2O5 target,43 but a prerequisite for an application of such NbxTi1-xOy films in chalcopyrite solar cells is a deposition temperature below 480 K to prevent a deteriorating cation diffusion at one of the solar cell interfaces.27 With the help of reactive magnetron sputtering, transparent and conductive films with a similar niobium concentration but a lowered oxygen content relative to those of the films of Y. Furubayashi et al. could be prepared at deposition temperatures below 480 K (Table 1). These films were successfully used as a TCO layer for Cu(In,Ga)Se2 solar cells and the photoelectrochemical membrane. The best results in the synthesis of transparent and conductive titanium oxide-based TCO layers were observed with the niobium-doped titanium target and a niobium atom inclusion of 3 atom % in combination with an oxygen deficiency of about 8 atom % (Table 1). The use of the pure titanium target at similar sputter conditions (6.50 vol % oxygen) led to significantly higher resistive metal oxide films (Table 1). This clearly indicates that both oxygen defect sites and niobium atoms are important to improve the electronic properties. The transparency of the typically 150 nm thick Nb0.03Ti0.97O1.84 films ranges between 55% and 75% in the wavelength region between 350 and 1200 nm, whereas the Hall mobility is about 0.90 cm2 V-1 s-1 and the specific resistivity around 3 Ω cm (Table 1). In comparison to Al:ZnO, the niobium-doped titanium oxide TCO layer has a 1-2 orders of magnitude lower mobility and higher resistivity (Table 1). The positive effect of a niobium atom insertion in the titanium oxide layers observed within the experiments is discussed in more detail in a paper by Neumann et al.46 In Figure 3, the transmission values for several Nb0.03Ti0.97Oy (0 < y < 2) films with varying oxygen content were correlated to the measured conductivity. Naturally, the pure metallic films show the highest conductivity accompanied by the poorest transmission, whereas the highly oxidic sputtered films show a



Figure 4. REM cross-section of a full TiO2/Ti/Cu(In,Ga)Se2/CdS/Nb0.03Ti0.97O1.84 membrane. On the left, the porous structure of the TiO2-P25 photocatalyst layer differs clearly in structure from the right-hand more compact layers of the molybdenum back contact, the Cu(In,Ga)Se2 absorber layer, and the on-top-deposited CdS buffer and Nb0.03Ti0.97O1.84 TCO layer (all polished).

high transmission together with an extremely poor conductivity. Oxygen-saturated Nb0.03Ti0.97Ox films (x ) 1.95-2.00) are more or less isolating like niobium-free TiO2 films with resistivity values in the megaohm region.52 The broad plateau between 20% and 80% transmission where the conductivity is nearly constant between values of 0.30 and 1.50 Ω-1 cm-1 is an interesting but unexpected finding. It indicates a nonlinear creation of defect sites within the semiconductor band gap and shows further that a certain level of defect sites in the film is necessary to significantly improve the electronic film properties. In this plateau region, the film transmission is much more sensitive to the number of oxygen vacancy sites than the film conductivity (Figure 3). This contradictive behavior may be explained by the fact that oxygen defect sites can improve the conductivity by the formation of Ti3+ states, for instance,46,52 but also an increase of the charge transport disturbing crystal defect sites is reasonable. The possible present influence of Magneli phases (TinO2n-1, n ) 2, ...) on the optical and electrical film properties is discussed more in detail in ref 46. In comparison to the transparency and the electronic properties of sputtered Al:ZnO films, the Nb0.03Ti0.97O1.84 films show in their recent stage of optimization markedly lower values, but their functionality as a TCO layer in flexible Cu(In,Ga)Se2 solar cells was proven successfully.46 Solar cells equipped with this new TCO layer typically showed a short-circuit current of about 10 mA cm-2 (Isc), an open circuit voltage of about 550 mV (Voc), a fill factor around 40% (FF), and an overall efficiency of about 2.50% (η). For comparison, the i-ZnO/Al:ZnO reference Cu(In,Ga)Se2 cells provide Isc ) 30.5 mA cm-2, Voc ) 570 mV, FF ) 69.00%, and η ) 13.70% (all presented values are mean values from 4-8 samples).46 For a more detailed discussion, please see ref 46. This reduced solar cell performance is still satisfactory for an application in the photoelectrochemical membrane (Figures 1B and 4), where the used metal oxide photoelectrodes typically provide photocurrent densities between 0.10 and 0.46 mA cm-2 (Figures 2 and 7). 3.1.3. Preparation of the Final TiO2/Ti/Cu(In,Ga)Se2/ Nb0.03Ti0.97O1.84 Tandem Membrane. Finally, the preparation of the complete TiO2/Ti/Cu(In,Ga)Se2/Nb0.03Ti0.97O1.84 tandem membrane consists of four newly developed production steps and six standard Cu(In,Ga)Se2-solar-cell-based production steps.

First, a film of TiO2-P25 nanoparticles was deposited by the doctor-blade technique on top of a polished titanium foil. A subsequent heating process at 720 K in air is necessary for solidification and sintering, which is essential for good charge transport properties in the photoelectrode layer. Thereafter, the backside of the molybdenum-coated titanium foil was deposited with a sodium fluoride (NaF) layer, the Cu(In,Ga)Se2 absorber layer, and a cadmium sulfide (CdS) buffer layer. Details for the Cu(In,Ga)Se2 solar cell preparation are described elsewhere.27 Finally, a 150 nm thick Nb0.03Ti0.97O1.84 front contact layer was reactively sputtered on top of the solar cell in an argon/ oxygen atmosphere. As a last step, small platinum islands were deposited as a hydrogen-evolution cocatalyst by using an ethanolic hexachloroplatinic acid solution (H2PtCl6) in combination with a short UV light illumination process. Figure 4 shows an REM cross-section (polished) of the complete PEC membrane, in which the porous TiO2 photoelectrode layer can be seen at the left and the more dense Mo, Cu(In,Ga)Se2, and Nb0.03Ti 0.97O1.84 layers at the right-hand side of the dominating titanium foil in the center. 3.2. Analysis of the Tandem PEC Cell Hydrogen Production. 3.2.1. Experimental Mass Spectrometer Setup. The hydrogen production of the PEC membranes was qualitatively and quantitatively analyzed with the help of a standard mass spectrometer setup that was additionally equipped with a 1 m long quartz capillary to secure measurements under normal pressure (Figure 5). Before each experiment was started, one of the produced photoelectrochemical membranes was transferred into a glass sample box, filled with the inert gas nitrogen (N2), and sealed with a crown cap rubber septum in a nitrogenfilled glovebox. After the injection of a defined volume of a deuterated electrolyte with a nozzle, the cell was illuminated for 15 min with vis and with UV/vis light. To estimate the hydrogen evolution, the experimental cell was then placed within a continuously argon flushed measurement chamber and connected with the tip of the mass spectrometer quartz capillary. An additionally introduced capillary secured a pressure balance between the sample cell and measurement chamber at all times. In the following, the results in H2 evolution of several TiO2/ Ti/Cu(In,Ga)Se2/CdS/Nb0.03Ti0.97O1.84 membrane experiments with various electrolytes, different pH values, and two different electron donors are compared with those of single TiO2/Ti

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Neumann et al.

Figure 5. Experimental setup of the two-sided optical illumination pathway for a N2 gas sealed sample cell including the PEC membrane on the left and the qualitative and quantitative mass spectrometer setup (QMS, QMH, SEV-217) with a heated quartz capillary, a pressure exchange, and a continuously argon flushed measurement chamber on the right. The inserted “pressure diagram” should show the pressure drop in the analytical setup exemplarily, which is necessary to analyze a gas mixture at standard pressure conditions.

TABLE 2: Observed Volumes of Hydrogen (mL) per Experiment from Which the Other Time-, Area-, and Mass-Dependent Hydrogen Formation Rates Were Calculateda sample

pH e- donor D2 vol/mL D2 formation rate/mL h-1 D2 formation rate/µmol h-1 cm-2 D2 formation rate/µmol h-1 gcat-1

Y0 0 D3COD Y11 12 D3COD Y12 0 D3COD Y12, first repeat 0 D3COD Y12, second repeat 0 D3COD Y22 0 DCOOD

0.0009 0.026 0.048 0.046 0.048 0.065

0.0038 0.106 0.192 0.187 0.196 0.262

0.15 4.3 10.28 10.01 10.47 11.16

2795 6682 6507 6805 7255

a In the right-most column the estimated amount of evolved hydrogen (µmol) per hour was correlated to the mass of TiO2 photocatalyst used on the anodic membrane side.

electrodes and titanium-foil-deposited Cu(In,Ga)Se2 layers without a protective Nb0.03Ti0.97O1.84 window layer. 3.2.2. Hydrogen EWolution as a Function of the Electron Donor, pH Value, and SolWent. The TiO2/Ti/Cu(In,Ga)Se2/CdS/ Nb0.03Ti0.97O1.84 membranes were able to produce hydrogen in acidic and alkaline aqueous solutions (Table 2). The best results, namely, 11.16 µmol of hydrogen h-1 cm-2 (Table 2), were observed with formic acid as the electron donor under acidic conditions because the current of the TiO2 photoelectrode was then not limited by the low oxygen evolution rate from the photooxidation of water. The best membrane achieved an external quantum efficiency (EQE) of 1.02%. Please consider for calculations of the quantum efficiency that this membrane was illuminated from two sides, which doubles the number of available photons, and that no photon losses due to reflection and transmission (sample cell) have been considered so far. Figure 6 shows the mass signals of deuterium, nitrogen, and carbon dioxide of three selected membrane experiments, immediately after connection of the mass spectrometer capillary with the measurement cell. For the reactor, sealed without a PEC membrane (curve a, Figure 6), the D2 and CO2 signals remain constant over the whole measurement at 2.0 × 10-13 and 4.0 × 10-13 A, respectively. The increase of the N2 signal from 2.0 × 10-11 to 1.0 × 10-8 A is caused by the N2 atmosphere present in the glovebox, in that the sample cells were sealed before. This N2 signal intensity is the same for all experiments and therefore proves the reproducibility of the quantitative determination of the main gas component. Cells equipped with a TiO2/Ti/Cu(In,Ga)Se2/CdS/ Nb0.03Ti0.97O1.84 membrane (curves b and c, Figure 6) show a clear increase of the D2 and CO2 mass signals of more than 1 order of magnitude to values of about 5 × 10-11 and 2.5 × 10-12 A, respectively. In acidic solutions slightly higher D2 and CO2 formation rates were observed than in alkaline solutions (curves b and c, Figure 6, and Table 2). A general overview of

Figure 6. Deuterium (D2), nitrogen (N2), and carbon dioxide (CO2) mass signals measured at a PEC membrane free reactor (a), a PEC membrane working in alkaline solution (b; 1 M Na2CO3), and a PEC membrane working in acidified aqueous solution (c; 0.62 M D2SO4). Each sample cell contained D2O as the solvent and 0.8 M deuterated methanol (D3COD) as the sacrificial electron donor. The label “septumpenetration” shows the time point where the sample cell was connected with the mass spectrometer.

the D2 formation rates from different experiments is given in Table 2. Figure 6 also shows that the increase of the D2 signal in experiments with photoelectrochemical membranes is accompanied by an increase of the CO2 mass signal. This is a clear indicator of the desired parallel action of photooxidation and photoinduced reduction reactions at the anodic and cathodic PEC membrane sides, respectively. In general, the CO2 mass signal is lower than the D2 mass signal. This is caused by the much better solubility of carbon dioxide in aqueous solution relative to that of hydrogen (H2, 0.0016 g L-1; CO2, 1.5 g L-1; 298 K, 1 bar53) and by a lower ionization probability of CO2 in the mass spectrometer. In Figure 7, the I-V curve of a Nb0.03Ti0.97O1.84-modified Cu(In,Ga)Se2 solar cell and a TiO2-P25 photoelectrode of similar


J. Phys. Chem. C, Vol. 113, No. 49, 2009 20987 TABLE 3: D2 Mass Signal of Several Test Samples That Were Prepared and Analyzed Similarly to Those of the Membrane Experiments Described Beforea sample

D2 MS signal/pA

Ti/TiO2 Ti/TiO2/CdS Ti/Cu(In,Ga)Se2/CdS/Pt TiO2/Ti/Cu(In,Ga)Se2/CdS/Nb0.03Ti0.97O1.84-Pt

0.25 0.4 5.8 54.3

a The clear increase in D2 formation for complete membranes relative to their tested single compounds gives strong support for a concerted two-photon absorption process in complete membranes and the synergetic activity between the TiO2 and Cu(In,Ga)Se2 semiconductors.

Figure 7. I-V curve of a Nb0.03Ti0.97O1.84-deposited Cu(In,Ga)Se2 solar cell (cell area 0.50 cm2, AM 1.5, sun simulator, MPP ) maximum power point) plotted together with the potential-dependent photocurrents of a Ti/TiO2-P25 photoelectrode, typically used in the photoelectrochemial membranes, in 0.62 M deuterated sulfuric acid (D2SO4) with and without 0.88 M deuterated formic acid (DCOOD) and methanol (D3COD). Please note the log scale of the y axis and the dotted line of a dark CV of a TiO2-based photoelectrode typically observed in the experiments. (Note: V/NHE in the case of photoelectrodes.)

thickness and under similar light conditions compared to those of the tandem membrane experiments described above are plotted together. Similar to the analysis of Figure 2, the photocurrent of the TiO2-P25 photoelectrode still fits with the precaution that the solar cell has to function near its Voc voltage of 0.55 V for all three investigated electron donors. Again with formic acid as the electron donor, the best photocurrents were achieved. The photocurrent density increased from 0.12 mA cm-2, observed for pure/single acidified water, to 0.46 mA cm-2 for aqueous formic acid solution (0.55 V/NHE). Figure 7 shows that the membrane works in principle with pure water as the electron donor too, but a 3-4 times reduced photocurrent density relative to that with the use of organic electron donors and a therefore strongly reduced hydrogen formation rate would result. The influence of the photoelectrode dark current on the membrane experiments can easily be neglected (jdark ) 0.003 mA cm-2, 0.55 V/NHE). Slightly lower photocurrent densities were observed with deuterated methanol as the electron donor (jphoto ) 0.35 mA cm-2, 0.55 V/NHE). This described order in the electron donor related photocurrent density correlates well with the data presented in Table 2, where the highest D2 formation rates were observed with formic acid as the electron donor too. The basics for the differences in methanol and formic acid oxidation on TiO2 photoelectrodes can be read in detail in papers by Mills,4 Fox,9 Anderson,33 Bideau,34 and Salvador et al.35 The experimentally observed D2 formation of several membrane experiments is listed in Table 2, where sample Y0 is the reference sample cell without any photoelectrochemical membrane and the other samples, Y11, Y12, and Y22, represent typical D2 values that were observed for runs in alkaline and in acidified solvents. The highest hydrogen formation rates were observed after a change was made from alkaline to acidic solutions and from methanol to formic acid as the electron donor. The value of 11.16 µmol h-1 cm-2 stands in line with an external quantum efficiency of 1.02%. As mentioned before, for this calculation a two-sided photon flux has to be considered and the result is not corrected for photon losses caused by reflection and transmission (sample cell). A consideration of the relatively small absorption range of TiO2-P25 (200-412 nm of the AM1.5 simulated spectra, Eg(rutile) ) 3.0 eV, Eg(anatase) ) 3.2 eV52) allows the external quantum efficiency to increase to 4.48%.

In the last right column of Table 2, the D2 volume produced in a 15 min illumination time is correlated to the amount of TiO2-P25 powder used. The values range from 2795 µmol h-1 g-1 for membranes used in methanol containing alkaline solutions to 6682 and 7255 µmol h-1 g-1 for membranes Y12 and Y22 with methanol and formic acid as the electron donor in acidified electrolyte, respectively. Although the mass-related D2 formation rates of Table 2 are affected by some extrapolation error, they show a clear improvement relative to the results published in the literature for platinized, powderlike TiO2 experiments, where methanol was used as the sacrificial electron donor too (540 µmol h-1 g-1).12 The same is valid in comparison to other powdery and platinized hydrogen-evolving photocatalysts, such as indium-based tantalum(V) oxide (InTaO4; 480 µmol h-1 g-1),14 tantalum oxynitride (TaON; 120 µmol h-1 g-1),10 and bismuth- and tantalum-based niobium(V) oxide (BiTaNbO4; 600 µmol h-1 g-1).14 The TiO2/Ti/Cu(In,Ga)Se2/ CdS/Nb0.03Ti0.97O1.84 PEC membranes produce significantly higher volumes of hydrogen. This clearly shows the positive interaction between the photocatalyst and Cu(In,Ga)Se2 solar cell, as described in the Introduction. The successful three-time use of membrane Y12 (Table 2) shows that well-prepared photoelectrochemical membranes can function continuously and relatively stably according to the hydrogen formation rate. In addition, it shows that the hydrogen evolution is not a result of potential-induced or photoinduced corrosion reactions. Due to the fact that corrosion in aqueous solution at certain potentials and under illumination with light can be a danger for the solar cell absorber layer Cu(In,Ga)Se254-56 and the CdS buffer layer,2,57 this point and its relation to the D2 formation were investigated in more detail. The results of these control experiments are given in Table 3. Each of the following control experiments was done in the same way as already described for the investigations of complete TiO2/Ti/Cu(In,Ga)Se2/CdS/Nb0.03Ti0.97O1.84 membranes. First, a pure Ti/TiO2 layer was investigated, made by an identical preparation procedure and with a similar surface area compared to those of the TiO2-P25 layers used in the membranes. It showed no clear D2 formation (0.25 pA m/z 4 signal, Table 3) after the 15 min illumination period with UV/vis light at all. Only a small increase of the CO2 mass signal was observed. This important observation excludes the hypothesis that the observed oxidation and reduction reactions take place at the TiO2-P25 photoelectrode surface only. In a second experiment, a CdS layer of thickness similar to that used in the buffer layer deposition during the Cu(In,Ga)Se2 solar cell preparations was deposited on a new Ti/TiO2 substrate. The so far unprotected yellow shiny CdS film (Ti/TiO2/CdS photoelectrode) was stable in the dark between 0.1 and 1.25 V/NHE, whereas in the case of UV light illumination a fast photodegradation in the same

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potential region could be observed visually and by measuring a continuously sinking photocurrent (from 2.7 to 1.6 mA cm-2 in 30 s and to 0.25 mA cm-2 in 100 s), but the amount of produced hydrogen was not significantly higher than in the first experiment with the pure Ti/TiO2 sample (Table 3). Nevertheless, an all-time protection of the CdS buffer layer in the solar cell is therefore an important prerequisite for an optimally functioning membrane. Even after the complete cathodic membrane half-site in the form of a Ti/Cu(In,Ga)Se2/CdS/Pt substrate was tested (including a platinization!), the D2-correlated mass signal (m/z 4) was more than 9 times lower than for the majority of complete TiO2/Ti/Cu(In,Ga)Se2/CdS/Nb0.03Ti0.97O1.84 tandem membranes (5.80 pA relative to 54.30 pA for Y22). These observations give strong support for the synergetic interaction between both photoactive semiconductors. The hydrogen formation of the complete membrane was significantly higher than the sum of its single elements. The fact that the membrane Y12 showed similar D2 formation rates three times (Table 2) shows further that the Nb0.03Ti0.97O1.84 layer can successfully protect the selenide and sulfide layers of the membrane. 4. Discussion While many efforts have been made to successfully demonstrate light-induced water splitting and hydrogen generation, the effort presented here is the first that mimics the two-step excitation of the photosynthetic unit by using a monolithic assembly of inorganic layers in direct contact with water (Figure 1). Nonintentionally doped TiO2, which is a known water splitting photocatalyst, is present at the anodic membrane surface, and a newly developed niobium-doped TiOx layer protects the Cu(In,Ga)Se2 and CdS layers below as well as enables with the help of catalyzing Pt islands the reduction of protons to hydrogen at the cathodic ones. The necessary driving force to provide the enthalpy change and to overcome the potential for water splitting is generated by the two-step photoexcitation within the TiO2 and integrated Cu(In,Ga)Se2/ CdS solar cell layers. As a consequence, the TiO2/Ti/Cu(In,Ga)Se2/CdS/Nb0.03Ti0.97O1.84 membrane setup will allow in the future an advantageous decoupling from several efficiency limits present so far for typical powderlike water splitting photocatalysts. By use of a single light absorber, the photocatalyst valence band edge has to fit to the redox potential of the given electron donor and its conduction band edge has to simultaneously fit to the redox potential of the given electron acceptor. As a consequence, the band gap energy of the photocatalyst that is active in the photoinduced water splitting and hydrogen evolution typically exceeds values of 2.8 eV (UV region of the solar AM1.5 spectrum), whereas systems with a two-step photoexcitation can use semiconductors with much lower band gap energies, still fulfilling the band edge/redox potential relations. The tests with isolated compounds of the membrane (Table 3) and a complete TiO2/Ti/Cu(In,Ga)Se2/CdS/Nb0.03Ti0.97O1.84 membrane with a hydrogen formation significantly higher than the sum of its single elements clearly show the presence of a concerted two-photon absorption process and the synergetic interaction between both photoactive semiconductors. The increase in hydrogen formation relative to that of powderlike TiO2 photocatalysts observed within the membrane experiments can be explained by the polarization of the TiO2-P25 layer. The photovoltage supplied by the Cu(In,Ga)Se2 solar cell bends the TiO2 valence and conduction band positions, which finally reduces the energy barriers for charger transfer reactions and increases the charge carrier separation probability.

Neumann et al. The presented PEC membrane directly dips into water, and in its first modification no proton-conductive membrane separates the chambers for oxygen and hydrogen evolution. Doing so, the yield in hydrogen production would instantaneously increase for two reasons. First, the back reaction between oxygen and hydrogen in the experimental cell would be minimized. At present, the formation of water droplets at the inner wall of the experimental cell during the illumination time could be observed several times, without a significant temperature increase that would be high enough to evaporate the aqueous electrolyte. Second, a use of an alkaline electrolyte for the oxidative working membrane site and an acidified electrolyte for the reductive working membrane site would be advantageous for a shift of the semiconductor energy bands. This can finally reduce the necessary polarization energy for the water splitting reaction. Since the two outer faces of the PEC membrane are made up of functionalized titanium oxide-based material, potentially stable in alkaline and acidic aqueous solution and under illumination with UV/vis light, the efficiency of the photooxidation reaction is limited to the near-UV light region of the sun spectrum. This subsequently determines the final hydrogen formation rate of the membrane. 5. Conclusion All together, the experiments based on a combination of the well-known photocatalyst TiO2 with a surface-modified thin film Cu(In,Ga)Se2 solar cell show a clear positive and synergetic interaction between both semiconductors. Similar to the plant photosynthesis unit and its combined function between the photosystems P680 and P700, the charge transfer reactions and the charge carrier separation in the photooxidative working TiO2P25 layer are enhanced due to the photovoltage supplied by the Cu(In,Ga)Se2 solar cell. The newly developed Nb0.03Ti0.97O1.84 front contact of the Cu(In,Ga)Se2 solar cell serves its role as a protection layer and reactive interface that allows the reduction of protons to hydrogen. The overall hydrogen formation observed for complete TiO2/Ti/Cu(In,Ga)Se2/CdS/Nb0.03Ti0.97O1.84 membranes was more than 1 order of magnitude higher than for Ti/TiO2, Ti/TiO2/CdS, and Ti/Cu(In,Ga)Se2/CdS/Pt reference samples and reached values about of 11.16 µmol of H2 h-1 cm-2 and 7255.0 µmol h-1 g-1 of TiO2-P25 photocatalyst, respectively. Nevertheless, a photocatalyst more active in the visible light region is urgently needed, because this still is the bottleneck for a more efficient hydrogen evolution with the TiO2/Ti/ Cu(In,Ga)Se2/CdS/Nb0.03Ti0.97O1.84 membrane. The highest H2 formation was observed with formic acid as the electron donor in acidified aqueous solutions. This evidences the advantage of single-step electron extraction compared to multielectron extraction from electron donors. At the present time, the membrane external quantum efficiency in hydrogen formation is about 1.02%, but a consideration of the small absorption range of the so far used TiO2P25 photocatalyst would allow the EQE to increase to 4.48%. An additional increase in efficiency is expected for a use of separated oxygen and hydrogen formation chambers, a better adjusted light management, and a serial connection of several Cu(In,Ga)Se2 solar cells. The results shown with the alternative TCO layer Nb0.03Ti0.97O1.84 encourage an ongoing development and an investigation of ZnO alternatives as optical contact materials for a potential use in solar cells, thin film applications, and hydrogen-fuel-generating devices. A more detailed knowledge of the interfacial reactivity and reactions and of the stability

Photoelectrochemical Membrane for H2 Production and restructuring of surfaces will have to be elaborated for a better tuning of photochemically active devices, which is one of the preconditions toward a solar hydrogen fuel technology. Acknowledgment. Mr. Chr. A. Kaufmann together with his colleagues of the solar energy department SE 3 (HZB) are acknowledged for the preparation of the Cu(In,Ga)Se2 solar cells and the discussions within this topic. Mr. W. Bohne and Mr. P. R. Vo¨lz are acknowledged for ERDA measurements and technical support, and Mr. S. Brunken is thanked for the introduction to the temperature-dependent Hall measurements. References and Notes (1) Pleskov, Y. V. Solar Energy ConVersion; Springer-Verlag: Berlin, 1990. (2) Memming, R. Semiconductor Electrochemistry; Wiley-VCH-Verlag: New York, 2001. (3) Gra¨tzel, M. Nature 2001, 414, 338. Nowotny, J.; Bak, T.; Nowotny, M. R.; Sheppard, L. R. Int. J. Hydrogen Energy [Online] 2006. http:// dx.doi.org/10.1016/j.ijhydene.2006.09.004. (4) Mills, A.; Le Hunte, S. J. Photochem. Photobiol., A 1997, 108, 1. (5) Develin, R. M. Baker, A. V. Photosynthesis; VNR-Verlag: Bonn, Germany, 1971. (6) Hoganson, C. W.; Babcock, G. T. Science 1997, 277, 1953. Baldwin, M. J.; Pecoraro, V. L. J. Am. Chem. Soc. 1996, 118, 11325. McEvoy, J. P.; Brudvig, G. W. Phys. Chem. Chem. Phys. 2004, 6, 4754. (7) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (8) Nozik, A. J.; Memming, R. J. Phys. Chem. 1996, 100, 13061. (9) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (10) Hara, M.; Hitoki, G.; Takata, T.; Kondo, J. N.; Kobayashi, H.; Domen, K. Catal. Today 2003, 78, 555. (11) Hwang, D. W.; Lee, J. S.; Li, W.; Oh, S. H. J. Phys. Chem. B 2003, 107, 4963. (12) Zou, Z.; Ye, J.; Arakawa, H. Chem. Mater. 2001, 13, 1765. (13) Kato, H.; Asakawa, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125, 3082. Kato, H.; Kobayashi, H.; Kudo, A. J. Phys. Chem. B 2002, 106, 12441. (14) Zou, Z.; Arakawa, H. J. Photochem. Photobiol., A 2003, 158, 145. (15) Neumann, B.; Bogdanoff, P.; Sakthivel, S.; Kisch, H.; Tributsch, H. J. Phys. Chem. B 2005, 109, 16579. Tributsch, H.; Neumann, B. Int. J. Hydrogen Energy 2007, 32, 2679. (16) Khan, S. U. M.; Akikusa, J. J. Phys. Chem. B 1999, 103, 7184. (17) Sartoretti, S. J.; Ullmann, M.; Alexander, B. D.; Augustynski, J.; Weidenkaff, A. Chem. Phys. Lett. 2003, 376, 194. (18) Santato, C.; Ullmann, M.; Augustynski, J. J. Phys. Chem. B 2001, 105, 936. (19) Solarska, R.; Santato, C.; Sartoretti, S. J.; Ullmann, M.; Augustynski, J. J. Appl. Electrochem. 2005, 35, 715. (20) Dhere, N. G.; Jahagirdar, A. H. Thin Solid Films 2005, 480/481, 462. Dhere, N. G. Sol. Energy Mater. Sol. Cells 2007, 91, 1488. (21) Gra¨tzel, M. Chem. Lett. 2005, 34, 8. (22) Gao, X.; Kocha, S.; Frank, A. J.; Turner, J. A. Int. J. Hydrogen Energy 1999, 24, 319. (23) Rocheleau, R. E.; Miller, E. L.; Misra, A. Sol. Energy 1998, 12, 10. (24) Licht, S.; Wang, B.; Mukerji, S.; Soga, T.; Umeno, M.; Tributsch, H. J. Phys. Chem. B 2000, 104, 8920. (25) Klenk, R. Thin Solid Films 2001, 387, 135. (26) Bohne, W.; Lindner, S.; Ro¨hrich, J.; Strub, E. Surf. Interface Anal. 2004, 36, 1089. (27) Kaufmann, C. A.; Neisser, A.; Klenk, R.; Scheer, R. Thin Solid Films 2005, 480, 515. (28) Smarsly, B.; Grosso, D.; Brezesinski, T.; Pinna, N.; Boissiere, C.; Antonietti, M.; Sanchez, C. Chem. Mater. 2004, 16, 2948. Brezesinski, T.; Fischer, A.; Iimura, K.-I.; Sanchez, C.; Grosso, D.; Antonietti, M.; Smarsly, B. AdV. Funct. Mater. 2006, 16, 1433.

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