In Operando Deconvolution of Photovoltaic and Electrocatalytic Performance in Water Splitting Photocathodes

Many present-day investigations of water splitting photoelectrodes are based on buried p–n junctions, which usually offer an improved photovoltage and therefore a higher solar-to-hydrogen efficiency in tandem photoelectrochemical cells. In this work, we demonstrate that the dual working electrode (DWE) technique enables the measurement of the surface potential of water splitting buried-junction photocathodes under operation, enabling the deconvolution of the photovoltaic and electrocatalytic performance in operando . Consequently, we can access properties of the buried p–n junction independent of the surface kinetics, and gain information related to the charge transfer through the electrode/electrolyte interface independent of the photovoltaic properties. Moreover, the DWE technique provides a clearer understanding of the photocathode degradation mechanism during stability tests. Two p–n junction-based photocathodes are investigated in this work: a pn + -Si/TiO 2 photocathode as model system, and the application of the developed method to the emerging material system Cu 2 O/Ga 2 O 3 /TiO 2 . a for


Main Text
Photoelectrochemical (PEC) water splitting has been recognized as a promising avenue for harvesting renewable hydrogen fuel from inexhaustible solar energy and water. [1][2][3] In order to increase the efficiency of this process and thereby lower the cost of the resulting hydrogen, a given semiconductor absorber (or pair of absorbers) is optimized to maximize the photocurrent, the photovoltage, and fill factor (FF), so that a maximum of the incident solar power is converted into chemical energy. 4 One approach to achieve larger open-circuit voltages (V oc ) is using so-called "buried junctions", whereby a p-n junction underneath the semiconductor-electrolyte junction generates the photovoltage. 5,6 In such a situation, the V oc is decoupled from the semiconductorelectrolyte interface and the increased band bending of the p-n junction can significantly enhance electron-hole pair separation. 7 Furthermore, one can use semiconductors that are prone to degradation in water through the use of protection layers between the semiconductor photoabsorber and the catalyst, which stabilizes the absorber.
The configuration of a buried junction photocathode can be modeled as a series combination of a p-n junction photoabsorber and surface catalyst (pn/cat). 6 The efficiency of a pn/cat photocathode is largely determined by the intrinsic properties of the buried p-n junction. 8 Although it is possible to measure the photovoltage generated by the semiconductor in a solution containing a chemical redox couple, 9 the resistive losses of the protective overlayer and the surface catalyst under hydrogen evolution cannot be determined in this fashion. The semiconductor-catalyst and catalyst-electrolyte interfaces also play a critical role in the overall performance of the photelectrodes. Issues such as charge transport in the protective layer, the nature of the semiconductor-catalyst contact (Ohmic or Schottky-type), as well as the chemical reactions occurring at the catalyst-electrolyte interface are intertwined. 10,11 Therefore, it is highly desirable to develop an experimental technique that can not only evaluate the PEC performance but also simultaneously provide an understanding of these different interfaces during PEC operation. By diagnosing, for example, the stability of the p-n junction versus the stability of the catalyst, one can identify the points of failure in real PEC devices under operation, and thus identify targets for improvement.
In the 1970s, Nakato, Pinson and Wilson reported that n-GaP and n-TiO 2 photoanodes coated with thin gold films showed a photovoltaic effect, representing early examples of in situ measurements of the surface potential. [12][13][14] Recently, the Boettcher group further developed this dual-working-electrode (DWE) technique to study a photoanode-catalyst interface. 15 Here, we adopt a similar approach to deconvolute the different aspects of buried junction photocathodes. The second working electrode is able to monitor the surface potential of a water splitting electrode in operando. The hidden J-V curve of the buried p-n junction can be revealed independently of the surface kinetics by measuring the difference in voltage between the backside and the surface of the photocathode (ΔV). A TiO 2 -protected pn + -Si junction photocathode was chosen as a platform to develop this method. Three main questions are tackled in this article: (1) the extraction of solar cell-like J-V curves from water splitting photocathodes under operation; (2) the different mechanisms for degradation of the performance; and (3) the interfacial properties of the TiO 2 /Pt junction. We then apply the technique to a Cu 2 O-based photocathode to diagnose the underlying causes of performance loss in this system.

Interface Energetics of the pn + Si/TiO 2 /Pt(ed) photocathode
A pn + -Si photocathode with 100 nm-thick ALD-TiO 2 protective layer was chosen as a model system for the development of the technique. A schematic diagram of the DWE setup is depicted in Fig. 1a. WE1 is used for controlling the back contact potential (V1) of the photocathode. WE2 is connected to the photocathode surface and kept at open circuit during PEC measurements to directly probe the surface potential (V2) in relation to the reference electrode. When illuminated, electron-hole pairs are generated and separated by the built-in electric field across the p-n junction, bringing photoelectrons to the surface while sweeping holes to the back contact. Consequently, a photovoltage is created across the pn + -Si homojunction. Considering that both the n-type Si layer and TiO 2 are highly doped, 16 the width of the space charge region of both n + -Si and TiO 2 is very narrow and ensures an Ohmic contact. The Pt catalyst also forms an Ohmic contact with the TiO 2 layer. 9 As a result, the measured potentials of V1 and V2 directly give the energetic positions of the quasi-Fermi level of holes and electrons, respectively. The difference between V1 and V2, denoted ΔV, is the output voltage of the pn + Si junction (see also Fig. 1b). V2 represents the energy level of the surfaceaccumulated electrons (Fig. 1a) and reaches a plateau when V1 is swept into reverse bias (Fig. 1c). The value of the surface potential in the plateau region is related to the resistance at the TiO 2 /Pt/electrolyte catalytic interface, and V2 is a direct measure of the overpotential for hydrogen generation at this interface. After the Pt catalyst was electrodeposited onto the TiO 2 surface, the conventional current density-back contact potential (J-V1) curve of the pn + Si/TiO 2 /Pt(ed) photocathode was obtained in 0.5 M H 2 SO 4 with a linear sweep from positive to negative potential (Fig. 1b). Under one sun illumination, the pn + Si/TiO 2 /Pt(ed) photocathode exhibits an onset potential for water reduction of ~0.5 V RHE . As V1 becomes more negative, the photocurrent density increases and eventually saturates to 25 mA cm -2 at V1 = -0.2 V RHE . WE2 enables the in situ measurement of surface potential V2 during the sweep of V1. Supplementary Fig. S3 presents V2 and ΔV values as a function of V1 with and without illumination.
A hidden J-ΔV curve, analogous to the current-voltage characteristic of a PV cell, can then be extracted and is plotted in Fig. 1b. The V oc and J sc are 475 mV and 24.6 mA cm -2 , respectively (the characteristics are also listed in Supplementary Table S1). A significant loss of fill factor is observed when comparing the J-V1 and J-ΔV curves, which derives from the additional series resistances in a PEC cell versus a PV cell, namely the TiO 2 /catalyst junction resistance, the overpotential of the catalyst required for driving a chemical reaction, and the solution resistance. 17 In essence, the J-ΔV curve shows the best possible fill factor that can be achieved by the J-V1 curve. In practice, a real PEC J-V1 curve will always have a smaller fill factor due to the catalyst overpotential as well as the series resistances mentioned above. Moreover, because the solution resistance does not change in the strongly acidic solution, the difference in fill factor between J-V1 and J-ΔV curves represents only a change in the electron transfer kinetics through the TiO 2 /Pt/electrolyte interface.
In order to more clearly visualize the effect of the surface potential on the current, a stepwise chronoamperometry measurement was carried out whereby the potential of V1 was stepped every 30 s and both V2 and the photocurrent were recorded (Fig. 1c). When V1 is positive of ~0.5 V RHE , V2 remains at a constant distance (constant ΔV, see also Supplementary Fig. S3) and nearly zero photocurrent is recorded. Substantial cathodic photocurrents appear at V1 = 0.4 V RHE , where V2 is more negative than 0 V RHE (with ΔV now starting to shrink). After the onset potential, although V2 continues to move negatively with each step, ΔV shrinks further as the photocurrent increases. Ultimately, both V2 and the photocurrent become constant, even as V1 becomes more negative, eventually entering a reverse bias-type regime. Supplementary Fig. S4 depicts band energy diagrams under several conditions of applied bias, and Supplementary Fig. S5 gives a detailed discussion of the relationship between photovoltage and the onset potential.

Stability of the p + nSi/TiO 2 /Pt(ed) photocathode
A critical issue for photoelectrodes is the long-term stability. A standard procedure for assessing the stability is to carry out a chronoamperometry experiment under a static back contact potential and to compare the J-V1 behavior before and after the stability test. This type of analysis, however, is relatively limited because the underlying degradation mechanisms are inaccessible. The decrease in PEC performance can be due to several factors. Firstly, the H 2 -evolving catalyst may be deactivated, poisoned or dislodged from the electrode surface. Secondly, the p-n junction may produce a reduced output V oc , due to partial photocorrosion and increased recombination. These changes in the semiconductor material also result in lower photocurrent densities and fill factor.
In order to characterize the degradation mechanism in the Si photocathode, we performed a 2 h stability test by holding V1 at 0 V RHE , a typical value for these types of test in the literature. 8 Fig. 2a shows the J-V1 and J-ΔV curves before and after the 2 h stability test. Compared with the initial J-V1 scan, the scan after the 2 h shows similar onset potential and slightly decreased saturation photocurrent, but a remarkably poorer fill factor. As the J-ΔV curves remain the same, it is immediately apparent that the problem relates to the catalyst and not to the buried junction. Fig. 2b depicts how the surface potential V2 and photocurrent density change over time under a static back contact potential of 0 V RHE . Over 2 h, the photocurrent density drops from ~23 to ~20 mA cm -2 , while V2 steadily shifts to more negative values. Fig. 2c clearly shows the negative shift of the surface potential following the stability test, which indicates that higher overpotential is needed in order to achieve a similar current density. A poor contact between the surface and the catalyst (TiO 2 /catalyst) as well as worsening kinetics at the catalyst/electrolyte interface (e.g. surface poisoning) will result in a higher overpotential for the catalytic interface. 18 Pt was then re-deposited onto the electrode surface (Fig. 2c,d). Due to the fact that the fill factor is completely restored upon re-platinization, we can confirm that neither a degradation in the p-n junction of the silicon nor resistive losses through e.g. formation of a silicon oxide layer are responsible for the change in the J-V1 curve. The degradation likely results from desorption of the Pt nanoparticles, as has been previously observed for electrodeposited platinum on ALD TiO 2 . 19 When the ALD TiO 2 was replaced by a thin metallic Ti film, the Pt catalyst binding was much more robust over a 2 h stability measurement ( Supplementary Fig. S7).

Investigation of the TiO 2 /Pt junction
For the pn + -Si photocathodes described above, the Pt catalyst was electrodeposited onto either the TiO 2 or Ti metal surface as nanoparticles with a size range of ~10-30 nm ( Supplementary Fig. S8a). This non-continuous catalyst morphology may be unfavorable for efficient extraction of the surface electrons, resulting in a poor fill factor of the J-V1 curve. 20 Therefore, we investigated a nominally 2 nm-thick Pt film with nearly full coverage on the TiO 2 layer by sputter coating (denoted pn + Si/TiO 2 /Pt(sp)). The Pt deposited in this way makes the surface slightly rough ( Supplementary Fig. S8b). Fig. 3a compares the J-V1 curves of pn + Si/TiO 2 /Pt(sp) and pn + Si/TiO 2 /Pt(ed) photocathodes under one sun illumination. Sputtered Pt exhibits a similar onset potential and improved fill factor, but much reduced photocurrent densities due to the optical transmission loss through the 2 nm-thick Pt film (Supplementary Fig  S9). For better comparison between the sputtered and electrodeposited samples, we also measured the J-V1 curve of the pn + Si/TiO 2 /Pt(sp) at an increased light intensity to achieve a similar photocurrent density, plotted in green. The green curve exhibits an earlier onset potential despite having the same V oc as the Pt(ed) curve, suggesting a better catalytic activity of sputtered Pt over electrodeposited Pt. Support for this hypothesis is shown by comparing their individual catalytic activities towards H 2 generation when deposited on FTO slides (Fig S10). Additionally, pn + Si/TiO 2 /Pt(sp) always shows an enhancement in the fill factor the in J-V1 curves, reflecting the smaller TiO 2 /Pt/electrolyte interfacial resistance for the TiO 2 /Pt(sp) as compared to the TiO 2 /Pt(ed). In the case of a conformal Pt film, electron transfer is more probable as the catalyst surface area is increased, which is also indicated by the much more positive V2 value in pn + Si/TiO 2 /Pt(sp), shown in Supplementary Fig. S11. For example, to reach the same saturation photocurrent, a ~130 mV overpotential is required for pn + Si/TiO 2 /Pt(sp) but ~200 mV for pn + Si/TiO 2 /Pt(ed). What we have already hypothesized by the performance of the different Pt on FTO is confirmed by the determination of the J-∆V curves. At similar saturation photocurrents the J-∆V of pn + Si/TiO 2 /Pt(sp) is essentially identical with the pn + Si/TiO 2 /Pt(ed) (see the green and red dashed curves) while the J-V1 curve shows a clearly higher FF for the device with sputtered Pt.
Thus far, we have developed the DWE technique with a model pn + Si/TiO 2 /Pt(ed) photocathode, with which we can gain a deeper insight into the PEC process and the photocathode stability. Next, we applied this technique to the emerging material ALD TiO 2 -protected Cu 2 O to demonstrate the generality of the technique. 19,21 An n-type Ga 2 O 3 was used as a buffer layer between the Cu 2 O and TiO 2 overlayer because this interlayer ensures a positively shifted onset potential, compared to that of n-Al:ZnO (AZO) (Supplementary Fig. S12). 22,23 Fig. 4a schematically depicts the multilayer structure of the Cu 2 O/Ga 2 O 3 /TiO2 photocathode. In a similar fashion as for the silicon photocathodes described previously, a second working electrode was introduced to probe the surface potential V2, prior to deposition of the Pt catalyst by sputtering. PEC measurements were performed in a pH 5 phosphate/sulfate solution. Fig. 4b displays the J-V1 and J-ΔV curves. A positive onset potential of ~0.9 V RHE is observed in the J-V1 curve, and at V1 = 0 V RHE , the photocurrent density is 3.9 mA cm -2 . The onset potential is much more positive than that from both Cu 2 O/ZnO and Cu 2 O/AZO photocathodes, reflecting the larger photovoltage generated by the Cu 2 O/Ga 2 O 3 junction. In the case of the J-ΔV curve, the V oc , J sc and fill factor are 836 mV, 4.0 mA cm -2 and 36.1%, respectively. Compared with reported Cu 2 O/Ga 2 O 3 solar cells in the literature, the V oc and fill factor values are comparable, but the J sc is lower due to light absorption by the Pt catalyst. 23,24 Still, resistance at the TiO 2 /Pt/electrolyte interfaces contribute to the fill factor loss between the J-V1 and J-ΔV curves. The fill factor loss is not very significant. As the J-ΔV is basically mirroring the J-V1 curve, this indicates that the buried Cu 2 O/Ga 2 O 3 junction is responsible for the shape of the J-V1 curve of Cu 2 O/Ga 2 O 3 /TiO 2 /Pt(sp) and not the catalytic activity of the Pt catalyst. When using an electrodeposited Pt the PEC system exhibits a lower fill factor, showing that the TiO 2 /Pt/electrolyte resistance indeed can also limit J-V1 performance, as shown in Supplementary Fig. S15. We further carried out a stepwise test on the Cu 2 O/Ga 2 O 3 /TiO 2 /Pt(sp) photocathode under illumination. Results and discussion are provided in Supplementary Fig. S16 and S17 and confirm our statements above. To study the stability of the Cu 2 O/Ga 2 O 3 /TiO 2 /Pt(sp) photocathode, a 2 h chronoamperometric measurement was performed under illumination at V1 = 0 V RHE . Fig.  4b shows the comparison of the J-V1 curves before and after a 2 h stability test. The onset potential shows a negative shift of nearly 120 mV, although the photocurrent density remains similar. Fig. 4b also provides the initial J-ΔV curve and the one after the 2 h stability test. An obvious decrease in V oc is evident, from 836 mV to 743 mV, while the J sc shows a slight increase from 4.0 to 4.1 mA cm -2 (Supplementary Table  S1). Compared with the pn + -Si photocathode, the Cu 2 O/Ga 2 O 3 photocathode shows a much faster degradation of the photovoltaic performance. For the silicon system, J-ΔV remained constant while the J-V1 changed (Fig. 2). In the Cu 2 O/Ga 2 O 3 case, J-ΔV has changed while J-V1 remains similar (retains a similar photocurrent and fill factor). We thus attribute the loss of performance in this system to the Cu 2 O/Ga 2 O 3 heterojunction, which could be due to slight photocorrosion that increases recombination in the system, or due to intrinsic instability of the Cu 2 O/Ga 2 O 3 junction.

Conclusions
We have developed a novel application of the DWE technique that is able to probe the surface potential of a water splitting photocathode under operation. This technique has been demonstrated as a universal method to systematically investigate independently the photovoltaic and electrocatalytic properties of catalyst-modified buried junction photocathodes. A pn + Si/TiO 2 /Pt photocathode was first fabricated as a platform to model the DWE system. By means of surface potential measurements, the intrinsic properties of the buried p-n junction were studied, and the hidden J-V curve of a photovoltaic cell was extracted. Additionally, the fill factor loss between J-V1 and J-ΔV curves was identified as a parameter that characterizes the TiO 2 /Pt/electrolyte interface, where the morphology of the catalyst plays an important role. Furthermore, the PEC performance degradation mechanism was discussed. We have demonstrated that the stability of underlying p-n junctions in buried junction photocathodes can be characterized in operando. Finally, we applied the DWE technique to a Cu 2 O/Ga 2 O 3 /TiO 2 /Pt photocathode, where it was found that the large V oc generated by the Cu 2 O/Ga 2 O 3 heterojunction degraded significantly within two hours of stability testing. As new material combinations are synthesized for PEC measurements, the DWE electrode technique enables a rapid diagnosis of the cause of degradation in these systems, while also obtaining the PV characteristics of these new junctions without the need to construct separate PV cells.

Methods
Si wafer cleaning. (111)-oriented Si wafers (thickness ~0.5 mm) used for DWE fabrication were purchased from PrimeWafers. The p-Si substrate was lightly boron-doped (~2×10 16 cm -3 ) and a 2 μm-thick n-type surface layer (polished) was doped with phosphorus (~2×10 19 cm -3 ). These wafers were cut into 2.5 × 1 cm 2 pieces and sonicated sequentially in acetone, ethanol and MilliQ water (18 MΩ) for 10 minutes each. Next, a two-step deep cleaning was accomplished by using a 5:1:1 mixture of H 2 O:NH 4 OH:H 2 O 2 , followed by a 5:1:1 mixture of H 2 O:HCl:H 2 O 2 , both at 50 °C for 10 minutes, in order to completely remove organic and inorganic contaminants. The native oxide layer was etched away by dipping the wafer pieces in 2% HF for 30 s. The samples were then rinsed with deionized water and dried under a stream of nitrogen, and then placed immediately into the ALD chamber for deposition of TiO 2 onto the n + -Si surface.

Cu 2 O plate preparation.
A Cu 2 O plate was prepared via oxidation of a high purity Cu plate (99.9999%), using a method adapted from the literature. 25 The Cu plate (0.1 mm thick) was first cut into small pieces (~2 × 2 cm 2 ) and heated from room temperature to 1050 °C (ramp: 17 °C per min) and kept at this temperature for 1 h under Ar flow (1 L min -1 ). Next, air was introduced into the furnace and held for 3 h. The gas environment was then switched back to Ar for another 3 h of annealing at the same temperature. After cooling down to room-temperature under Ar, the as-prepared Cu 2 O plate was dark red when held up to the light.
Atomic layer deposition of TiO 2 and Ga 2 O 3 . TiO 2 on Si wafer and Ga 2 O 3 -TiO 2 on Cu 2 O plate were deposited by atomic layer deposition (ALD) using a Picosun R200 tool. Before the deposition, the sample was rinsed with deionized water and dried under a stream of N 2 . The samples were then placed inside the ALD chamber, which was already heated to 120 °C. Tetrakis(dimethylamino)titanium (Sigma-Aldrich) and H 2 O were used as the precursor for Ti and O, respectively. The Ti precursor was heated to 85 °C and a 1.6 s pulse was used (with software boost function), followed by a 6.0 s N 2 purge. H 2 O was held at room temperature and a 0.1 s pulse was used, followed by a 6.0 s N 2 purge. To reach 100 nm of thickness for TiO 2 , 1860 cycles were used. Measurement of the thickness of ALD-TiO 2 deposited on a piece of Si witness wafer was carried out by ellipsometry (alpha-SE, J.A. Woolam Co.), and fitted with a model for transparent films. The cross sectional SEM image ( Supplementary Fig. S1) shows that the ALD-TiO 2 protective layer is conformably coated on the Si wafer.
For the Ga 2 O 3 layer, bis(μ-dimethylamino)tetrakis-(dimethylamino)digallium (STREM, 98%) was used as Ga precursor. The ALD chamber temperature was kept at 160 °C during deposition. The Ga precursor was held at 150 °C and a 2.5 s pulse was used (with software boost function), followed by a 7.0 s N 2 purge. The H 2 O was held at room temperature, and a 0.1 s pulse was used, followed by a 4.0 s N 2 purge time. To deposit a 20 nm-thick Ga 2 O 3 thin film, 250 cycles were used. In order to avoid ALD growth on the back side of the sample (potentially leading to shunting problems), teflon tape was used to cover the back side of the Si wafers and Cu 2 O plates during the ALD process. Supplementary Fig. S13 present the Cu 2 O/Ga 2 O 3 /TiO 2 multilayered structure.

Fabrication of Si-based DWE.
After ALD TiO 2 deposition, the working electrode 1 (WE1) contact was made to the back side of the p-type silicon by scratching the wafer, applying Ga-In eutectic (Aldrich) and attaching copper foil (Aldrich). A layer of epoxy resin (Loctite Epoxide-resin EA 9461) was then used to cover and glue the electrode to a glass microscope slide, with a certain portion of the TiO 2 surface left uncovered for use as the electrochemical active area. For making the front contact, a 20 nm-thick Au layer was sputtered (Safematic CCU-010) onto the epoxy as well as a small part of the exposed TiO 2 . A copper wire was connected to the Au layer with Ag paint (Ted Pella, Inc.) on top of the epoxy, as a connection for the second working electrode (WE2). Finally, the front contact was protected from the electrolyte by masking it with a second epoxy layer. Supplementary Fig. S2 shows an optical photograph and the structure scheme of an asfabricated pn + Si/TiO 2 DWE.

Fabrication of Cu 2 O-based DWE.
After the deposition of the Ga 2 O 3 -TiO 2 overlayer, a 100 nm-thick Au layer was then sputtered onto the back side of the Cu 2 O plate (the front side was protected with teflon tape), followed by connecting an Ag wire with Ag paint as WE1. Epoxy resin was then used to cover the whole back side of the electrode to provide protection and enhance the stiffness of the Cu 2 O plate. WE2 was connected to the ALD-TiO 2 surface using the same method as for the Si-based DWE, described above.
Platinum catalyst deposition. For some samples, Pt catalyst was deposited onto the as-prepared DWE via galvanostatic electrodeposition from a 1 mM H 2 PtCl 6 aqueous solution, denoted as Pt(ed). A constant current of -0.85 μA cm -2 was applied to the back contact (WE1) for 15 min. For other samples, nominally 2 nm-thick Pt catalyst was deposited by sputter coating, denoted as Pt(sp).
Characterization. The morphologies of electrodeposited Pt and sputtered 2 nm-thick Pt film, and the cross-sectional scanning electron microscopy (SEM) images of pn + Si/TiO 2 and Cu 2 O/Ga 2 O 3 photoelectrodes were obtained with a Zeiss Supra 50 VP scanning electron microscope. The polycrystalline structure of the Cu 2 O plate is revealed by the X-ray diffraction (XRD) pattern ( Supplementary Fig. S14), using a Rigaku Smartlab diffractometer with Cu Kα radiation. UV-VIS spectra of the Pt(ed) and Pt(sp) on FTO slides were recorded with a Shimadzu UV-3600Plus UV/Vis/NIR spectrometer equipped with an integrating sphere.
Photoelectrochemical Measurements. Photoelectrochemical measurements were performed in a four-electrode configuration using a BioLogic SP-300 bipotentiostat. The reference electrode was Ag/AgCl (0.197 V vs. NHE) and a Pt wire served as the counter electrode. All electrode potentials were converted into RHE scale: at room temperature, V RHE = V NHE + 0.059 × pH = V Ag/AgCl + 0.059 × pH + 0.197. Before the measurements, the electrolyte was sparged with N 2 for at least 10 min to remove dissolved oxygen. Simulated one sun illumination (100 mW cm -2 ) was provided by a 150 W Xe-lamp with AM 1.5 G filter from LOT Oriel, and the intensity was calibrated with a standardized silicon diode from PV Measurements (USA). 0.5 M H 2 SO 4 and 0.1 M pH 5 phosphate solution (containing 0.5 M Na 2 SO 4 ) were used for Si-based and Cu 2 O-based DWE experiments, respectively. Linear sweep voltammograms (LSVs) were collected by sweeping the back contact potential (V1). V1 stepwise measurements were performed by potential step chronoamperometry (CA). Each V1 potential step had a duration of 30 s. The stability tests of the samples were performed for 2 h with V1 held at 0 V RHE under one sun illumination. During all measurements, the second working electrode was kept at open circuit to record the surface potential (V2) against the reference electrode.
Faradaic efficiencies. The Faradaic efficiencies of the photocathodes were measured in a gas-tight three-compartment cell in a threeelectrode configuration, with an Ag/AgCl reference electrode and a Pt wire counter electrode. The photocathodes were covered with epoxy to fix the active area to ~0.08 cm 2 . The measurement was performed in the same electrolyte as in the PEC measurements (described above). The electrolyte was stirred and constantly sparged with Ar gas at a rate of 20 ml/min. The gas outlet from the cell was connected to a 450-GC Gas Chromatograph Bruker Daltonics GmbH for gas analysis. One LSV scan was first performed for choosing a suitable V1 potential for the Faradaic efficiency tests. During the measurement, the exposed area of the photocathode was illuminated with a white-light LED. The intensity of the light was calibrated to reach a similar photocurrent density as obtained under simulated one sun illumination, as described above.
Data availability. The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Supplementary Table S1 J-V parameters extracted from the J-ΔV behavior of the water splitting photocathodes. We expect the pn + -Si junction samples from the same wafer to generate the same V oc . The small V oc variation among pn + Si/TiO 2 /Pt(ed), pn + Si/Ti/Pt(ed), pn + Si/TiO 2 /Pt(sp) is likely due to the different light intensity as a result of light absorbtion by the catalyst and overlayers, or perhaps passivation and recombination at the Si/overlayer interface (through pinholes). The power conversion efficiency (η) is given for completeness, and is defined as:

J-ΔV
where P in of one sun illumination is the incident illumination power density (100 mW cm -2 ). For the pn + Si/TiO 2 /Pt(sp) sample, an increased light intensity (P in = ~137 mW cm -2 ) was also used.