Band Gap Engineering of Oxide Photoelectrodes: Characterization of

Article pubs.acs.org/JPCC

Band Gap Engineering of Oxide Photoelectrodes: Characterization of ZnO1−xSex Marie A. Mayer,*,†,‡ Kin Man Yu,† Derrick T. Speaks,†,‡ Jonathan D. Denlinger,§ Lothar A. Reichertz,† Jeffrey W. Beeman,† Eugene E. Haller,†,‡ and Wladek Walukiewicz*,† †

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States § Advanced Light Source Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡

S Supporting Information *

ABSTRACT: No single material or materials system today is a clear choice for photoelectrochemical electrode applications. Generally, materials with narrow, well-aligned band gaps are unstable in solution and stable materials have band gaps that are too wide to efficiently absorb sunlight. Here, we demonstrate the narrowing of the ZnO band gap and combine a variety of electrical, spectroscopic, and photoelectrochemical methods to explore the opportunities for this so-called highly mismatched alloy in photoelectrochemical water splitting applications. We find that the conduction band edge of ZnO1−xSex is located at 4.95 eV below the vacuum level (0.5 V below the hydrogen evolution potential). Soft X-ray emission and absorption spectroscopies confirm that the previously observed ∼1 eV reduction in the ZnO band gap with the addition of selenium result from the formation of a narrow Se-derived band. We observe that this narrow band contributes to photocurrent production using applied bias incident photon to current efficiency measurements at an electrochemical junction. Electrical measurements, electrochemical flat band, and photocurrent measurements as a function of x in ZnO1−xSex alloys indicate that this alloy is a good candidate for an oxide/silicon tandem photoelectrochemical device because of the natural band alignment between the silicon valence band and the ZnO1−xSex conduction band. We observe that the photocurrent onset in preliminary ZnO1−xSex/silicon diode tandem devices is shifted toward spontaneous hydrogen production compared to ZnO1−xSex films grown on sapphire. With these findings, we hope that our method of band gap engineering oxides for photoelectrodes can be extended to devise better materials systems for spontaneous solar water splitting.

1. INTRODUCTION Sustainable fuels derived from solar energy are attractive both as an alternative to fossil fuels in transportation and as storage of solar energy.1 Hydrogen in particular can be produced by photoelectrochemical water splitting with few reaction intermediates. In general, an appropriate photoelectrochemical device must (1) efficiently absorb the solar spectrum, (2) have band edges positioned so that electrons (holes) can be transferred to initiate hydrogen reduction (water oxidation) reaction, (3) remain resistant to corrosion during long-term operation, and (4) have ample catalytic activity such that the reaction occurs at a reasonable rate.2−4 To date, no lightabsorbing material with successful conversion of photons to electrical carriers has demonstrated catalytic properties equivalent to photoinactive catalysts.5,6 Therefore, we believe it is appropriate to seek an absorber/converter layer separately from catalysts for use in composite systems.7 Although significant progress has been made since the original observation of photoelectrochemical water splitting,8 no single material or materials system is a clear choice for the absorber © 2012 American Chemical Society

applications today. Generally, materials with narrow, wellaligned band gaps, such as III−V3,9 semiconductors, are unstable in solution. Stable materials, typically oxides,8,10,11 have band gaps that are too wide to efficiently absorb sunlight. Stabilizing III−V materials in solution and narrowing the band gaps of stable materials12 are both reasonable approaches for engineering a photoelectrochemical material. Independently, it has been shown that the substitution of anion species with an isovalent atom of considerably different electronegativity in a group III−V or II−VI compound semiconductor results in the formation of a localized defect level.13 At alloy-like compositions, an anticrossing interaction between the localized states and the extended band states of the host semiconductor matrix results in a drastic restructuring of the electronic bands of the alloys. The final band structure of these highly mismatched alloys (HMAs) is determined by the Received: May 8, 2012 Revised: June 19, 2012 Published: June 28, 2012 15281

dx.doi.org/10.1021/jp304481c | J. Phys. Chem. C 2012, 116, 15281−15289

The Journal of Physical Chemistry C

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In our previous work, we showed that the substitution of a small fraction of Se for O in ZnO is analogous to the case of As in GaN, and we observed a roughly 1 eV upward shift of the valence band in ZnO1−xSex alloys.26 Quantitative modeling indicated that the shifted valence band edge originates from a valence band anticrossing (VBAC) interaction between the valence band of ZnO and the Se-induced defect states at 0.9 eV above the valence band. The VBAC coupling parameter of 1.2 eV was determined by fitting of the optical absorption curves. Here, we explore ZnO1−xSex as a potential photoanode by measuring the band alignment and photoresponse of this ternary oxide alloy. We integrate solid-state physics and electrochemistry by employing electrical and spectroscopic techniques in addition to electrochemical measurements to obtain the absolute positions of the band edges. We observe excellent correlation between the electronic structure measured with these techniques and the location of the band edges obtained from photoelectrochemical measurements. Finally, we demonstrate an opportunity for four photon spontaneous water splitting by using a tandem,9 dual semiconductor (D4)27 photoanode of ZnO1−xSex grown directly on silicon.

location of the original defect energy level, the strength of the coupling between the localized and the extended states, and the alloy composition.13 For example, in a prototypical HMA, GaN1−xAsx, a small percentage of substitutional N in GaAs results in a large downward shift of the conduction band and a reduction of the band gap.14 Equivalently, introduction of a less electronegative anion, such as As into GaN, creates a localized level roughly 0.7 eV above the original valence band of GaN.14 At higher As concentrations, the localized levels form a band through an anticrossing interaction with the valence band of the GaN matrix. The net effect is the reduced band gap of the resulting GaN1−xAsx alloy with the fundamental absorption edge determined by electron transitions from the top of the Asderived band (highest occupied states) to the bottom of the conduction band (lowest unoccupied states). As illustrated in Figure 1, the valence bands of common oxide photoanodes lie almost 2 eV below the water oxidation

2. METHODS 2.1. Synthesis and Characterization. In this work, we measure the absolute band edge positions of ZnO and ZnO1−xSex alloys using a variety of experimental methods including synchrotron X-ray spectroscopy, pinning of the Fermi level with native defects, and photoelectrochemical flat band measurements. ZnO alloy samples were grown with pulsed laser deposition (PLD) on polished c-plane sapphire. Pressed powder targets with varying compositions of ZnO and ZnSe were used.26 The PLD chamber was pumped down to a base pressure of either 5 × 10−5 or 6.2 × 10−6 Torr. The pulse frequency for all depositions was 5 Hz. The growth temperatures ranged from 300 to 400 °C. The target-to-substrate distance was 5 cm. The 248 nm KrF laser fluences were about 3−4 J/cm2 unless otherwise noted. A detailed description of the PLD growth process is given elsewhere.28 Measurements with X-ray diffraction (XRD) revealed textured films oriented in the hexagonal 0001 (c-plane) direction as established by the sapphire substrate. This oriented texturing persisted when the films were grown on 111 Si substrates. The texturing was found to increase as a function of Se composition, but no evidence of crystals in other orientations was observed. No pure Se phases were observed in any ZnO1−xSex films using XRD, transmission electron microscopy, or Raman scattering, and no phase separation was present in the films used in this paper. Compositions and thicknesses were measured using Rutherford backscattering spectrometry (RBS). Samples grown under similar conditions in two different pulsed laser deposition chambers and laboratories showed equivalent behavior. All samples were n-type, and the Fermi level was well above the Sederived band. The carrier concentration was measured by the Hall effect using a 0.6 T permanent magnet and pressed-on In contracts in the van der Pauw configuration. 2.2. Band Edge Measurements: Defect Induced Fermi Level Stabilization. It has been shown previously that incorporation of a large concentration of amphoteric native defects pins the Fermi energy at the Fermi level stabilization energy (EFS) that is located at 4.9 eV below the vacuum level.29 Particle irradiation has been used to introduce point defects into semiconductors as a method to determine the location of

Figure 1. The band edge alignments of several oxides considered for use in photoelectrochemical cells. Si is shown for comparison. The oxide conduction bands are quite low and very close to the Si valence band. The scale is given with respect to vacuum.

potential while the conduction band edge is close to the hydrogen reduction potential.15,16 The standard hydrogen electrode scale at a pH of 0 can be obtained from the vacuum scale by subtracting 4.5 and by dividing by the charge on an electron, e.17 (In this work, we rely upon both the vacuum scale and the electrochemical scale; although many values are reported in terms of energies, this relationship can be applied to convert between the scales. Energy (voltage) differences on a single scale can be related by dividing energy values by e.) In particular, ZnO with reported values of the electron affinity ranging from 4.25 to 6 eV18−20 has a conduction band edge close to the hydrogen reduction potential. ZnO is known to be stable in pH 8−12,21,22 but it is inactive in visible light because it has a band gap of 3.3 eV.23 Indeed, several groups have explored ZnO or ZnO/GaN-based systems for photoelectrochemical water splitting.4,24,25 It remains very appealing to be able to narrow the band gap of an oxide in a controlled fashion by shifting only the valence band upward. 15282



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the band edges by pinning the bulk Fermi level at EFS29,30 According to the amphoteric defect model,31 the stabilization of the Fermi energy is a direct consequence of the changing character (donor or acceptor) of the native defects depending on the location of the Fermi energy. Assuming parabolic bands, the band edges relative to the Fermi level can be calculated from the measured carrier concentration and from using the equation

and Ag/AgCl reference. The electrolyte used was a pH 9 phosphate buffer (Hydrion) which was chosen on the basis of the reported stability of ZnO in pH 8−12.22 Previous measurements on other semiconducting systems suggest that relationships between the semiconductor band edges and the redox couple energies persist as a function of pH.35 The solution was purged with nitrogen for any experiments requiring charge transfer from the semiconductor to the solution. No catalysts were used. Pressed-on indium front contacts were used for contacts to photoelectrochemical anodes grown on sapphire. A glass beaker with a 0.325 cm diameter hole held the solution over the sample. The metal contact was isolated from solution with a silicone O-ring (O-rings West) compressed by clamping the beaker to a wooden base. Using the illuminated open circuit potential (IOCP) and photocurrent onset methods,2,36 we measured the flat band potential (VFB) of ZnO1−xSex with 0 < x < 0.13 to obtain the positions of the band edges with respect to the hydrogen and oxygen redox potentials. The IOCP was recorded using the open circuit potential function in the Framework software for a Gamry Reference 600 potentiostat. First, the open circuit potential of the sample in the dark was measured, and then the sample was exposed to high intensity white radiation from a xenon lamp (Solar Light Co.). For each sample, the power of the lamp was increased until the IOCP was saturated at the reported value. Under these conditions, photoexcited carriers near the interface accumulate to oppose the built-in electric field flattening the semiconductor bands. Flat band measurements that rely on a photoresponse do not require modeling the semiconductor and the Helmholtz layer as two capacitors in series, which can be quite erroneous for novel materials with unknown surface behaviors or structures.36 We make the standard assumption that, under high-intensity light, the semiconductor bands flatten completely. Depending upon the role of surface states and radiative recombination, this approximation could lead to a measured band edge position that is 100−200 meV lower than the actual value. The IOCP measures the sample Fermi level with respect to the reference electrode rather than the conduction band edge.37 The conduction band edge was found from this value with eq 1 using the Hall effect carrier concentration. The photocurrent production as a function of applied voltage versus the reference electrode was measured. The light source was a 150 W xenon lamp calibrated to 1 sun using a commercially available power meter. Ideally, when the working electrode bias is sufficient to flatten the semiconductor bands, electrons can be transferred to the electrolyte. As a result, the onset of a detectable electrical current should occur when the bias on the working electrode versus the Ag/AgCl is equal to VFB. However, this measurement is expected to report slightly lower conduction band edges (with respect to vacuum) because of poor kinetics that also require that an overpotential.37 Anodic three electrode voltage sweeps from the dark open circuit potential were performed using the Gamry Framework software. Using the method described by Butler,36 the photocurrent onset was obtained by a linear fit of the photocurrent squared as a function of voltage versus the reference. These values roughly corresponded to an onset threshold of 1 μA/cm2 in this material system. Spectrally dependent photocurrent measurements were performed in order to determine that photons within the visible energy spectrum could be extracted and to correlate these effects with the Se composition, x. Measurements were

∞

∫

nF = Nc

0

x1/2 1 + exp⎡⎣x −

EF ⎤ kT ⎦

dx (1)

The conduction band structure of ZnO and effective mass of 0.2932 are well established. Knowledge of the conduction band edge (CBE) and valence band edge (VBE) with respect to EFS coupled with the established value of EFS at 4.9 eV with respect to vacuum allows for the determination of the actual band edge positions with respect to the vacuum level.29 To measure EFS with respect to the ZnO band edges, we irradiated ZnO films using Ne+ ions with energies of 150 and 50 keV to ensure constant damage through 200 nm thick films. The irradiation was carried out in steps until the carrier concentration was saturated, and therefore, EFermi = EFS. 2.3. Band Edge Measurements: Synchrotron X-ray Spectroscopy. To directly measure the band edge position shifts with the introduction of Se, we used a combination of soft X-ray emission (SXE) and X-ray absorption (XAS) spectroscopies.33 XAS and SXE directly probe the partial density of states of the conduction band and valence band, respectively.14,34 SXE and XAS spectra provide a direct measurement of the relative energy positions of the VBE and CBE in semiconductor materials. The creation of a core hole can result in additional states near the Fermi level and a corresponding slight downward shift of the CBE about a tenth of an eV. Here, we use SXE and XAS to probe only the difference between the two bands. In this work, the oxygen K edge at around 530 eV was investigated at room temperature at the Advanced Light Source (ALS) on beamline 8.0.1. XAS was detected with the total fluorescence yield detection mode, and SXE was measured using the Tennessee/Tulane grating spectrometer with a total energy resolution of 0.6 eV. An elastic emission peak in the SXE spectra found near threshold excitation was used for calibration of the SXE detector energy to the XAS monochromator energy. Unless otherwise noted, the sample polar angle was set at 45° to the synchrotron beam. The excitation energy was then tuned to the onset of the CB, and the resultant SXE due to electronic transitions from the upper VB to the O K shell was recorded with a wavelength-dispersive detector. The SXE spectra are scaled such that second-order Zn L3 and L2 emission peaks at 505 and 516 eV, respectively, are at constant height. The XAS spectra are normalized to a unitary step height from below threshold to far above (∼570 eV). The relative intensity scaling between SXE and XAS spectra is arbitrary. 2.4. Photoelectrochemical Characterization. We used flat band potential measurements and photocurrent spectroscopy for in situ photoelectrochemical evaluation. Unless otherwise noted, electrochemical measurements were done using a three-electrode electrochemical cell with a semiconductor working electrode, platinum foil counter electrode, 15283



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Si p−n junctions were fabricated from Si(111) wafers by the following process: a lightly doped n-type Si wafer (n ∼ 1016 cm−3) was B+ implanted at 33 keV with 1 × 1015 cm−2 ions on the front to form a p−n junction. The back of the wafer was separately implanted with P+, 1 × 1015 cm−2 ions at 33 keV to form the n+-doped ohmic back contact. The implanted wafer was annealed at 940 °C for 20 s to activate the dopants. The backs were vapor etched with HF before the evaporation of Pd/ Au back contacts. The front surfaces of the diodes were protected with apiezon wax, and then the diodes were immersed in HNO3:HF (20:1) for 30 s. This was found to be necessary to eliminate edge leakage in the diodes along cleavage surfaces. After the slow etch, the wax was removed from the surface with warm xylenes, and the surface was cleaned with a one minute oxygen plasma clean (PlasmaPreenII-862). All Si-grown samples were HF vapor etched for 1 min immediately before deposition. Several silicon p−n junctions were processed one step further in order to add Al/Ag front contacts for photovoltaic evaluation. Power conversion efficiencies of these solar cells were found to depend on the contact placement, but opencircuit voltages (VOC) about 0.3 V under 1 sun were obtained. The measured VOC was reduced to 0.2 V when the incident lamp light first passed through a 578 nm long pass filter to simulate the operating conditions of the solar cell as the bottom section of the tandem ZnO1−xSex/Si device. The short-circuit current with the filter was 68% of that without the filter. We expect the Si solar cells to have slightly better carrier extraction with the oxide film as the front contact instead of Al/Ag lithographically patterned lines because the ZnO1−xSex film would provide complete surface coverage.

recorded using a three electrode electrochemical cell with a semiconductor working electrode, platinum foil counter electrode, and a Ag/AgCl reference electrode in a pH 9 phosphate buffer electrolyte (Hydrion). A 150 W xenon lamp served as the excitation source. The light passed through a monochromator with a 600 grooves/mm grating and a 2 mm slit width (wavelength resolution of approximately 4 nm). Light exiting the monochromator was chopped3 at 155 Hz and passed through a 578 nm long pass filter for wavelengths greater than 600 nm to eliminate second harmonics. A Stanford Research Lock-in amplifier was connected to the current (I) readout of a Gamry Reference 600 potentiostat and was controlled through a custom Labview program that fixed the signal gain at a single value for the duration of the scan. Under these conditions, the observed photocurrent remained constant with sequential measurements taken at 1 Hz over 5 s. The initial photocurrent response decreased over several seconds because of carrier traps at surface states;10,38 chopping assured that the initial photocurrent response was recorded at all wavelengths. The appropriate current to voltage converter (I/ E) range on the potentiostat was chosen to maximize the signal-to-noise ratio of the signal output. A 1.5 V versus Ag/ AgCl bias ensured adequate band bending for charge-carrier extraction. Although the normalized measurements showed the same qualitative photocurrent trend as a function of wavelength under 0.5, 1.0, and 1.5 V (Figure 1 of the Supporting Information), the 1.5 V data is reported because the signal-tonoise ratio was roughly equivalent in all samples. While the magnitude of photocurrent for each sample was different, all samples showed current in the middle to upper portion of the I/E range. The instrument output spectrum as measured by a calibrated silicon reference diode was removed from the unprocessed spectra. Because the purpose of these measurements was to examine the photocurrent response as a function of wavelength rather than the absolute value of the photocurrent, the spectra were normalized such that the maximum value was 1. Normalization removed any lingering, nonmonotonic contributions from current transients, applied bias, and mobility (which will be discussed in depth below) which were convoluted with compositionally sensitive information in the raw, absolute spectra. The ZnO1−xSex samples were stable for the duration of all electrochemical testing. Each sample was removed from the electrolyte and was retested to verify reproducibility of the results (and measurements were found to be comparable, generally within ±100 mV). No damage to the samples was visible by eye or optical microscope. No quantitative corrosion testing was undertaken because, to guarantee that the catalytic activity was identical in all samples, no catalyst was employed for the tests reported here. To accurately assess the long-term stability of these (or any) materials during photoelectrochemical operation, a catalyst ought to be present to ensure that carrier transfer to the solution occurs quickly. Similarly, the use of a catalyst would increase the measured photocurrents reported here. 2.5. Fabrication of ZnO1−xSex/Si Tandem Devices. The results reported here are from layers of ZnO1−xSex grown on p−n junctions fabricated from (111)Si39 wafers. The structural and electrical properties of ZnO1−xSex films on Si were first evaluated by growing films on intrinsic Si and were found to be similar to films grown on sapphire. ZnO1−xSex films grown on conductive (3.5 × 10−3 Ω cm) p-type Si were used to test the ohmic contact between the film and p-type Si.

3. RESULTS AND DISCUSSION 3.1. Defect Induced Fermi Level Stabilization. The locations of semiconductor band edges are the key parameters determining suitability of a semiconductor material for solar water splitting. Although vacuum and solid-state techniques do not account for any potential drops across the Helmholtz layer when these materials are immersed in solution, these measurements are important to design solid−solid interfaces in heterostructure devices and are often correlated with in situ electrochemical measurements.40,41 A bulk technique relying on the introduction of native defects through irradiation as described in section 2.2 was used to determine the position of the ZnO conduction band edge with respect to vacuum. As shown in Figure 2, the saturation electron concentration after irradiation in ZnO was found to be 5 × 1018 cm−3. By eq 1, EFS is located at only 0.04 eV above the conduction band edge. Therefore, we concluded that the conduction band edge of ZnO is 4.95 eV below the vacuum level. This value is within the broad range, 4.25−6 eV, of previously reported electron affinities that were determined using several techniques including the Kelvin method42 and ultraviolet photoelectron spectroscopy.43 Later in this work, we show that this value is also in excellent agreement with values measured electrochemically. This method cannot be used on ZnO1−xSex films because of the low electron mobility of ZnO1−xSex films after irradiation that makes the electron concentration obtained by Hall effect unreliable. 3.2. Synchrotron X-ray Spectroscopies. We used synchrotron X-ray spectroscopy to directly measure the band structure of both the conduction and the valence bands after replacing a small fraction of O atoms in ZnO with Se. 15284



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Figure 4. X-ray emission spectroscopy (SXE) and X-ray absorption spectroscopy (XAS) data to probe the valence and conduction bands, respectively. The inset shows the full spectrum, while the large portion of the figure is a close-up of the near band edge region.

Figure 2. To determine the saturation concentration of electrons donated by native defects, ZnO samples were irradiated with Ne+ ions in an ion implanter. The free-electron concentration as measured by Hall effect is shown as a function of the irradiation dose. The dashed green line shows the saturation value for which the Fermi level is equivalent to the Fermi stabilization energy, EFS.

onset of the XAS spectra (CBE) aside from a slight broadening as the disorder in the films increased. Conversely, the SXE spectra thresholds showed a slight broadening and monotonic energy shift to the right with Se concentration. This is consistent with the hybridization of a localized Se level and the ZnO valence band. A linear extrapolation of the ZnO SXE leading edge and XAS threshold provided estimates of the VB maximum and CB minimum to be at 528.9 and 530.2 eV, respectively, consistent with a bulk ZnO band gap of 3.3 eV. Alternatively, the high-energy tail in the VB spectra for the sample with x = 0.28 extends almost to 528 eV, which is consistent with a shift of the valence band edge to the Sederived band at ∼1 eV above the VB maximum of the ZnO matrix as shown in Figure 2. The different SXE threshold line shapes confirm the VBAC upward shift of the valence band obtained from a theoretical interpretation of the optical absorption coefficient in our previous work.26 One also notices the replacement of sharp features, such as the peak in ZnO at 533 eV, by smooth profiles within the XAS spectra as x increases. This is exhibited in Figure 5. This behavior is attributed to an increase in disorder as a function of the film selenium content.14 For x > 0.17, the long-range periodicity of the film structures deteriorated reflecting the transition from textured to amorphous films. 3.3. Photoelectrochemical Flat Band Measurements. Using the illuminated open circuit potential (IOCP) and photocurrent onset methods,2,36 we measured the flat band potential (VFB) of ZnO1−xSex with 0 < x < 0.13 and obtained the positions of the band edges with respect to the hydrogen and oxygen redox potentials. The x’s in Figure 6a represent the raw IOCP data measured as described in section 2.4. Since the IOCP is a measurement of the sample Fermi level rather than the actual band edges, the value of the conduction band edge can be obtained from the a calculation of the difference between the Fermi energy and the conduction band edge. Using eq 1, the Fermi level was found to extend from 0.1 eV below the conduction band edge (for n = 1017 cm−3) to 0.5 eV above the conduction band edge (for n = 3 × 1020 cm−3). The squares in Figure 6a mark the actual conduction band edges for the ZnO1−xSex samples. Because the band gap (defined as the transition from the highest occupied state to the

Previously, we used theoretical fitting of the optical absorption coefficient to discern how the addition of Se to ZnO alters the band structure.26 On the basis of this work, Figure 3 shows a

Figure 3. Calculated E vs k dispersion relation for ZnO0.95Se0.05 on the basis of the band structure determined by Mayer et al.26

calculation of the dispersion relationship (energy vs k) for ZnO0.95Se0.05 approximating the valence band of ZnO as a single band rather than three separate p orbital derived bands.26,44 The initial Se state at 0.9 eV above the valence band and the ZnO valence band are shown as dashed lines. The Se-derived states and the valence band undergo anticrossing with a coupling constant of 1.2 eV45 to form the two separate bands (solid lines). As discussed above, the Se-derived band acts as the top of the valence band, which results in a band gap roughly 1 eV less than that of ZnO. Figure 4 shows SXE and XAS spectra for ZnO and ZnO0.91Se0.09. The main figure shows a wide energy range illustrating the various intensity normalizations, while the inset focuses on the highest occupied (valence band) and the lowest unoccupied states (conduction band). We measured films with Se content up to x = 0.28 and observed little change in the 15285



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Figure 5. X-ray absorption spectroscopy for several values of x in ZnO1−xSex. The broadening of features, such as the peak at 533 eV, as a function of x demonstrates that the crystal symmetry breaks down at higher values of x.

lowest unoccupied state) in all ZnO1−xSex samples with x < 0.15 is 2 eV or larger,26 the valence bands are well below the oxygen evolution potential. For comparison, we also measured the flat band potential using the photocurrent onset method. The measured photocurrent onsets are shown as red squares in Figure 6a. They were found to be about 0.2 eV below those measured by the IOCP method, as expected from kinetic considerations, but they show the same qualitative trend as a function of Se content. Despite the contributions of the electrochemical system, the CBE measured by the IOCP method shows excellent agreement (within 0.2 eV) with the measurements of the ZnO conduction band edge using the Fermi stabilization by the irradiation method as shown in Figure 6b. The alignment of the scales was obtained as described in section 1. In general, the CBE of these ZnO1−xSex alloys is about 0.5 eV too low to spontaneously split water. However, in addition to the upward shift of the valence band edge, alloying with Se results in a slight upward shift of the CBE consistent with the change of the electron affinity from 4.95 eV in ZnO to only 3.7 eV in ZnSe. 3.4. Photocurrent Production and Mobility. Spectrally resolved photocurrent measurements are shown in Figure 7. There was a monotonic redshift of the photocurrent onset with ZnO1−xSex composition from 3.3 eV in pure ZnO to 1.8 eV in the ZnO0.87Se0.13 sample. An increase in the low-energy density of states with increasing Se content was also apparent. Additionally, in the x = 0.07 and x = 0.13 samples, a second current onset after a plateau was observed at about 3.3 V roughly corresponding to the energy where the matrixlike band is expected from the band anticrossing model (Figure 2). These measurements confirm that the Se-derived band does contribute to photocurrent production at longer wavelengths and that the highly mismatched alloys are able to convert a larger portion of the solar spectrum to electrical carriers than the unaltered oxide. The spectra were normalized to their maxima for objective comparison of the influence of Se composition on the spectrally dependent behavior, since the

Figure 6. (a) Flat band measurements using open-circuit potential and photocurrent onset measurements and the calculated conduction bands. (b) Agreement between the band edge measured within the framework of the amphoteric defect model and the photoelectrochemical method.

magnitude of the photocurrent is influenced by the electron mobility independently of Se content,28 as well as the applied bias used for the measurements. It is known that the absolute magnitude of the broad band photocurrent depends on the sample mobilities46 and on the carrier recombination rates as well as on the band gap of a photoelectrochemical material. In many cases, including our own, these three properties are either directly or indirectly dependent variables making it difficult to separate the effect of each on the photocurrent. The dark current and the photocurrent using a white light source were recorded as a function of applied bias for ZnO1−xSex photanodes. Threeelectrode I/V measurement results for several samples with different Se compositions and mobilities are shown in Figure 8. The ZnO reference sample measured here had a mobility of 23 15286



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these films makes it unreasonable to expect mobility to be a monotonic function of carrier concentration, resistivity, or any single structural defect. Previous work has shown that the Se composition does not directly limit the mobility.28 The mobility plotted as a function of x (Figure 2 of the Supporting Information) shows no apparent relationship. Photocurrent measurements indicate that optimizing the mobility is important to the optimization of ZnO1−xSex alloys for use as photoelectrodes. 3.5. Tandem ZnO1−xSex/Si Devices. In our studies, we found that the electron affinities of ZnO and ZnO1−xSex with small values of x are 4.95 eV locating the CBE of these materials below the hydrogen reduction potential at 4.5 eV on the vacuum scale. This prevents the alloy from functioning as a single gap photoanode for a two photon/electron device (S2)27 for the spontaneous production of hydrogen (and oxygen). However, this oxide system is a candidate for a dual band gap device that uses four photons (D4 device27) for evolution of two hydrogen/oxygen molecules. This is analogous to a tandem solar cell that captures two photons of different energy to produce a single electron−hole pair. Two possible configurations for this type of device have been suggested: one that is a single tandem electrode and the other that combines two materials with different band gaps to act as separate anodes and cathodes. The first, known as a Turner cell, uses a photovoltaic cell in series with the photoelectrochemically active layer to provide the excess voltage required to drive the oxygen (hydrogen) evolution reaction by connecting an n-type (ptype) layer through an Ohmic contact to a p−n (n−p) junction. Typically, this contact is made via a tunnel junction.9 The band diagram for such a device is shown in Figure 9. In the

Figure 7. Spectrally dependent photocurrent measurements of ZnO1−xSex samples. A redshift in the photocurrent onset as well as an increase in the density of states with x can be observed in the data.

Figure 8. Three-electrode photocurrent under 1 sun illumination as a function of applied voltage vs Ag/AgCl. The magnitude of the photocurrent is nonmonotonic with both the mobility and the Se composition, x.

cm2/(V s) and showed a lower photoresponse than the ZnO1−xSex alloys. The largest photoresponse and greatest fill factor were observed in a ZnO0.87Se0.13 sample with a mobility of 6 cm2/(V s). However, the photocurrent response was not only a function of Se content; the second best performing sample had x = 0.09 and mobility of 24 cm2/(V s), which caused it to outperform the x = 0.16 sample with a mobility of only 5 cm2/(V s). This higher mobility sample also outperformed a sample of nearly identical composition (x = 0.08) with a mobility only half as large. The measurements shown here exemplify observations made on over 20 ZnO1−xSex samples all of which indicated a photoresponse that appeared to be a function of both mobility and composition. Depending on the PLD growth conditions, a wide variety of mobilities within ZnO thin films have been reported ranging from 1 up to 150 cm2/(V s).47 The ZnO reference samples used in this study had mobilities ranging from 5 to 50 cm/(V s), while the ZnO1−xSex alloy mobilities ranged from 2 to 20 cm2/(V s). Unlike in crystalline Si, the polycrystalline nature of

Figure 9. Band diagram of a tandem D4 water splitting device consisting of a Si solar cell and ZnO1−xSex photoactive layer. The natural band alignment of the Si valence band and the ZnO1−xSex conduction band creates an Ohmic contact between p-Si and nZnO1−xSex.

second configuration, a single n-layer is connected to a single player such that the p-layer would drive the cathodic hydrogen reduction and the oxygen evolution would occur at the n-type photoanode.27 Si is a good choice for a photocathode48 because of its cathodic durability, appropriate band alignment, and ease of manufacture, but a suitable oxide remains to be found. The band positions in Figure 1 show that the energetic values of the ZnO conduction band edge and of the Si valence band edge are very close. Therefore, if the n-type oxide alloy is grown directly on a Si p-type layer, an electron in the ZnO conduction 15287



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band can easily recombine with a hole excited in the Si valence band, while the remaining hole and electron can be transferred to the electrolyte to evolve products. By changing the Se composition, one can optimize the band gap of the oxide for such a tandem system. Our fabrication of these devices is described in section 2.5. Figure 10 shows that for currents up to 20 mA/cm2, the contact between the oxide film and the p-type Si was near

Figure 11. Illuminated two electrode current vs voltage curve for a ZnO 1−x Se x film on a silicon p−n junction. A film grown simultaneously on a sapphire substrate is provided as a reference. There is a nearly 0.5 V shift toward spontaneous water splitting when the film is grown on a solar cell in place of an inactive substrate. Figure 10. Current voltage measurements across an n-type ZnO1−xSex on p-type Si junction. The near-linear behavior indicates an Ohmic contact.

excellent agreement with that measured electrochemically. Using X-ray emission spectroscopy, we confirm that alloying of ZnSe with ZnO indeed shifts the valence band upward. Finally, we evaluate ZnO1−xSex as a photoelectrochemical anode and show that the addition of small percentages of Se significantly enhances the photoelectrochemical performance of ZnO. The demonstration of a D4 ZnOSe/Si device confirms that, without special preparation, there is a natural Ohmic contact at the noxide/p-Si interface, which simplifies device fabrication. An oxide-based alloy such as ZnO1−xSex is a promising absorber for a multicomponent, monolithic photoelectrochemical cell. With these findings, we hope that our method of band gap engineering oxides for photoelectrodes can be extended to devise better materials systems for spontaneous solar water splitting.

Ohmic (linear fit R2 = 0.98) with overall contact resistances ranging from 40 to 80 Ohm/cm2. Although this is larger than optimized tunnel junction resistances,49 the bulk oxide and bulk Si as well as nonideal back contacts made of InGa alloy contributed to this high resistance. As the data points in Figure 6a show, the use of rudimentary Si diodes as substrates for an oxide alloy shifted the flat band potential by 0.3−0.4 V toward spontaneous hydrogen evolution. No such change was observed in ZnO1−xSex samples grown on intrinsic or p-type silicon. The results of a two-electrode I/V measurement between the ZnO0.9Se0.1 semiconductor working anode and the platinum counter electrode are shown in Figure 11. The photocurrent onset for the reference ZnO1−xSex layer on sapphire was very near zero, which was more so than expected from flat band measurements. The discrepancy may be due to evolution of products other than hydrogen/oxygen evolution, which would be reduced by including an oxygen evolution catalyst. Regardless, the presence of an underlying Si p−n junction shifted the photocurrent onset 0.5 V negative suggesting that the photoelectrochemical cell was able to output power in addition to performing a chemical reaction. Our p−n Si diodes had significant variation and low output power compared to commercially available Si solar cells mainly because of nonoptimized doping. We believe that by using commercialgrade Si solar cells, the flat band voltage should surpass the hydrogen evolution potential.

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ASSOCIATED CONTENT

S Supporting Information *

Measurements of the open-circuit potential and electrical properties of ZnO1−xSex is provided. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.M.); W_Walukiewicz@ lbl.gov (W.W.). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

4. CONCLUSION On the basis of the results presented here, we conclude that the oxide alloy ZnO1−xSex does not meet all of the requirements for a single gap (S2) device. However, it is a promising material for potential use as a photoelectrochemical anode in conjunction with silicon or an alternative p-type cathode in a D4 configuration. A measurement of the absolute position of the ZnO band edges using different experimental techniques is in

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Todd Deutsch and Dr. John Turner at the National Renewable Energy Laboratory for invaluable discussion about semiconductor electrochemistry and Bill Hansen and Erin Ford at LBNL for their fabrication advice. 15288



Article

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This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. M.A. Mayer thanks Intel Corporation and the Department of Defense (NDSEG) for fellowship support.

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Band Gap Engineering of Oxide Photoelectrodes: Characterization of

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