SnO2 Double-Heterojunction Photoanode with Enhanced

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BiVO4/WO3/SnO2 Double-heterojunction Photoanode with Enhanced Charge Separation and Visible-transparency for Bias-free Solar Water-Splitting with a Perovskite Solar Cell Ji Hyun Baek, Byeong Jo Kim, Gill Sang Han, Sung Won Hwang, Dong Rip Kim, In Sun Cho, and Hyun Suk Jung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12782 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 19, 2016

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ACS Applied Materials & Interfaces

BiVO4/WO3/SnO2 Double-heterojunction Photoanode with Enhanced Charge Separation and Visible-transparency for Bias-free Solar Water-Splitting with a Perovskite Solar Cell Ji Hyun Baek,a Byeong Jo Kim,a Gill Sang Han,b Sung Won Hwang,c Dong Rip Kim,d In Sun Cho,*c and Hyun Suk Jung *a a

School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon 440-746, Korea

b

Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States c

Department of Materials Science & Engineering and Energy Systems Research, Ajou University, Suwon 443-749, Korea

d

School of Mechanical Engineering, Hanyang University, Seoul 133-791, Korea

*Corresponding Authors: In Sun Cho Tel: +82-31-219-2468; Fax: +82-31-219-1613; Email: [email protected]

Hyun Suk Jung Tel: +82-31-290-7403; Fax: +82-31-290-7410; Email: [email protected]

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Abstract Coupling dissimilar oxides in heterostructures allows the engineering of interfacial, optical, charge separation/transport and transfer properties of photoanodes for photoelectrochemical (PEC) water splitting. Here, we demonstrate a double-heterojunction concept based on a BiVO4/WO3/SnO2 triple-layer planar heterojunction (TPH) photoanode, which shows simultaneous improvements in the charge transport (~93 % at 1.23 V vs. RHE) and transmittance at longer wavelengths (> 500 nm). The TPH photoanode was prepared by a facile solution method: a porous SnO2 film was first deposited on a fluorine-doped tin oxide (FTO)/glass substrate followed by WO3 deposition, leading to the formation of a double layer of dense WO3 and a WO3/SnO2 mixture at the bottom. Subsequently, a BiVO4 nanoparticle film with was deposited by spin coating. Importantly, the WO3/(WO3+SnO2) composite bottom layer forms a disordered heterojunction, enabling intimate contact, lower interfacial resistance, and efficient charge transport/transfer. In addition, the top BiVO4/WO3 heterojunction layer improves light absorption and charge separation. The resultant TPH photoanode shows greatly improved internal quantum efficiency (~80 %) and PEC water oxidation performance (~3.1 mA/cm2 at 1.23 V vs. RHE) compared to the previously reported BiVO4/WO3 photoanodes. The PEC performance was further improved by a reactive-ion etching treatment and CoOx electrocatalyst deposition. Finally, we demonstrated a bias-free and stable solar water-splitting by constructing a tandem PEC device with a perovskite solar cell (STH: ~3.5 %).

Keywords: Double-heterojunction photoanode; BiVO4/WO3/SnO2; charge transport; transmittance; tandem PEC device

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Introduction Increasing global environmental deterioration is becoming a serious concern, leading to an exponential increase in scientific interest in renewable energy as a technology to replace fossil fuels. Photoelectrochemical (PEC) water splitting, which directly converts sunlight into hydrogen fuels, is a promising renewable energy technology.1-5 The PEC device consists of two photoelectrodes, the photoanode and the photocathode, where electrons/holes are generated by solar light absorption. The generated electrons and holes then participate in the water reduction and oxidation reactions, respectively.1 A major advantage of the PEC systems is that they involve relatively simple process steps (by mimicking photosynthesis in nature) compared to many other H2 production systems. However, for the construction of commercially viable PEC devices, one of the critical challenges is the design/development of photoanodes with a sufficiently high efficiency and stability.1, 6-7 Until now, a number of materials including TiO2,8-10 WO3,11-12 Fe2O3,13-14 CuWO4,15 BiVO4,16-18 Bi2WO6,19-20 TaON,21-22 and Ta3N423 have been used as photoanodes. However, the PEC performance of these single absorber materials is largely limited because of charge recombination occurring in the bulk, interface, and surface, leading to low charge transport/transfer efficiencies, and thus, lowering the internal quantum efficiency (IQE).18, 24-25 Recently, coupling of two materials, e.g., BiVO4/WO3, Fe2O3/WO3, TiO2/BiVO4, CuWO4/WO3, BiVO4/Sb:SnO2, and layered double hydroxide (LDH)/BiVO4 to form a type-II heterojunction26-29 or semiconductor/electrocatalyst junctions30-31 has been demonstrated to be a viable means of improving the PEC performance by simultaneously enhancing the charge transport and the transfer efficiencies. In particular, BiVO4/WO3 has been extensively studied because of its suitable band alignment and band gap energies.26-27, 32-33 Hong et al.34 and Fujimoto,35 constructed a layer-by-layer composite photoanode of BiVO4/WO3, which was found to be more active (high photocurrent generation and H2/O2 gas evolution) than either WO3 or BiVO4 single-component photoanodes. More recently, a number of nanostructures such as nanowires, nanorods, helix, and inverse opal structures with the BiVO4/WO3 heterojunction have been studied, which demonstrated enhanced photocurrent density by further improving light absorption.27, 36-37 On the other hand, a conventional PEC device (a single-absorber) inevitably requires external bias to achieve an unassisted/bias-free water splitting reaction, which is originated from the thermodynamic voltage requirements (1.23 volts) and the associated over-potential to split water.1, 38-39 A tandem PEC system consisting of a photovoltaic/photoelectrode (PV/PEC) device or photoanode/photocathode design can achieve the bias-free solar water splitting.40-42 For the PV/PEC device, the PV cell absorb transmitted sunlight and then generate enough voltage to assist bias-free water splitting reaction.43 Recently, perovskite solar cells (PSCs) have shown an unprecedented increase in solar-to-electrical power conversion efficiency, the highest being ~21%.44 PSCs with a high output voltage make it possible to achieve highly efficient bias-free water splitting using a tandem PEC device.43, 45 Here, we fabricated a double-heterojunction photoanode based on BiVO4/WO3/SnO2 triple-layer (TPH) via a facile solution method and demonstrate its impacts for bias-free PEC water-splitting. We found that the incorporation of a porous SnO2 layer below the BiVO4/WO3 film enables intimate 3



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contact and a double heterojunction, thereby, promoting electron transport and reducing interfacial charge recombination compared to BiVO4/WO3. Additionally, the resultant TPH photoanode shows a relatively high transmittance because of its low thickness (~300 nm), which is advantageous for coupling with a solar cell to form a tandem PEC device. The overall PEC performance was further optimized by surface etching using reactive-ion etching (RIE) and by electrocatalyst (CoOx) deposition. Finally, we demonstrated bias-free PEC water splitting by coupling the TPH photoanode with a PSC in series, i.e., a tandem configuration, which surpasses the performance of previously reported BiVO4/PSCs-based tandem structures.

Results and Discussion Characterization of BiVO4/WO3/SnO2 TPH photoanode The BiVO4/WO3/SnO2 TPH photoanode was fabricated using a sol-gel spin-coating method (Fig. 1a). First, a porous SnO2 nanoparticle film with an average thickness of ~180 nm was deposited on a fluorine-doped tin oxide (FTO)/glass substrate using a SnO2 precursor solution (0.2 M, 9 times, 450 °C/1 h), followed by coating of a WO3 precursor solution (0.1 M, 5 times, 500 °C/1 h). Next, a BiVO4 film with a thickness of ~110 nm was deposited, followed by annealing at 500 °C for 2 h in air. The average thicknesses per coating-cycle and the optimal coating thickness of each layer were predetermined to obtain the best PEC performance (Fig. S1 & S2). The fabricated BiVO4/WO3/SnO2 TPH photoanode is greenish-yellow in color and has a high transparency to visible light, as shown in the last image of Fig. 1a. The X-ray diffraction (XRD) pattern of the TPH photoanode matched well with that of tetragonal SnO2 (JCPDS: #77-0450), monoclinic WO3 (JCPDS: #83-0950), and monoclinic BiVO4 (JCPDS: #83-1699), indicating successful formation of SnO2, WO3, and BiVO4 layers in the TPH photoanode (Fig. 1b). The cross-sectional SEM image (Fig. 1c) revealed that a triple-layer of BiVO4 (110 nm), WO3 (30 nm), and SnO2 (180 nm) with a nanoparticle morphology was formed on the FTO/glass substrate, and the layers were in intimate contact with each other without any voids and cracks. Here, it should be noted that the thickness of the WO3 layer in the TPH photoanode (~30 nm) was much lower than the expected thickness (~80 nm, Fig. S1b), which is due to the infiltration of WO3 solution into the porous SnO2 layers during the spin-coating process. Next, band alignment of SnO2, WO3, and BiVO4 was determined from ultraviolet photoelectron spectroscopy (UPS) measurements (Fig. 1d & Fig. S3). First, the valence band edge (VBM) positions were obtained from UPS measurements (Fig. S3). The calculated VBM positions of SnO2, WO3, and BiVO4 were 4.13, 3.36, and 2.44 V vs. RHE, respectively (Fig. S3b–c). All the obtained results corresponded well with previous reports.28, 46 Based on these results and optical band gap, the band alignments of the fabricated TPH photoanode were constructed (Fig. 1d). Importantly, a staggered band alignment of the CBM and VBM positions indicates a type-II heterojunction relationship among the three layers, thus, facilitating electron/hole transport/transfer in between the layers.

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The detailed microstructure, interfacial characteristics, and elemental distributions of the TPH photoanode were examined using a transmission electron microscope (TEM) equipped with an energy dispersive spectroscope (EDS) (Fig. 2). As shown in the cross-sectional TEM image (Fig. 2a), three layers with a total thickness of ~320 nm were identified. At the bottom, the SnO2 layer forms a mesoporous nanoparticle film with an average thickness of ~180 nm, which is in intimate contact with the surface of the FTO/glass substrate, while the WO3 layer forms a uniform and dense film (~30 nm in thickness) on top of the SnO2 layer. In the case of BiVO4, a granular morphology with an average particle size of ~50 nm was observed, and it is an intimate contact with the underlying dense WO3 layer. Notably, the intimate contact and good adhesion between the three layers, without any voids and cracks, reduces the internal light scattering, enhancing light transmittance at longer wavelengths. The crystal structure and crystallinity of each layer were further characterized by high-resolution TEM analysis at spots A, B, and C, shown in Fig. 2a. First, as shown in Fig. 2b, the SnO2 nanoparticles (~10–15 nm in size) are interconnected well and exhibit nanoporosity. The high-resolution TEM images (inset of Fig. 2b) clearly show that the SnO2 nanoparticles are highly crystalline and have a tetragonal rutile phase. Additionally, the selected area electron diffraction (SAED) pattern obtained near spot ‘A’ (Fig. S4) indicates that WO3 infiltrates the SnO2 layer, thus, forming a composite of SnO2 and WO3. On the other hand, the middle WO3 layer exhibited a band-like dense film. The high-resolution TEM image confirms its high crystallinity and monoclinic phase (Fig. 2c). Interestingly, some parts of the WO3 layer form a single crystal film with a vertical direction of [012], normal to the FTO surface. For the top BiVO4 layer, the TEM and high-resolution TEM images confirm that it has a high crystallinity and a monoclinic phase (Fig. 2d). In addition, the EDS line scan (Fig. S5) and elemental mapping (Fig. 2e) results clearly show a distinct separation of each layer. Significantly, elemental W is present together with Sn at the bottom SnO2 layer, confirming that a composite layer of porous SnO2 and infiltrated WO3 is formed. Therefore, as shown in Fig. 2f, the fabricated TPH photoanode structure can be denoted as BiVO4/WO3/(WO3+SnO2), in which the WO3 and WO3+SnO2 layers form a bulk heterojunction, thus, being beneficial for charge transport and transfer.

PEC performance and optical characterizations Next, the PEC performance of the TPH photoanode was measured and compared with other planartype heterojunction photoanodes reported previously, i.e., BiVO4/WO3, BiVO4/SnO2, BiVO4/SnO2/WO3, and BiVO4 to elucidate the impact of the addition of the bottom SnO2 layer (Fig. 3a, see the Experimental section for the preparation of each sample). Fig. 3b compares the PEC photocurrent-potential (J-V) curves of the five photoanodes. First, the BiVO4/SnO2 photoanode exhibits a much higher photocurrent density at 1.23 VRHE (Jph, 1.23V) and a lower onset potential (Vonset) compared to BiVO4 alone, indicating that the CBM is lower for SnO2 than for BiVO4, i.e., possibly forming a type-II heterojunction such that it improves charge transport and transfer.47 Second, the BiVO4/WO3 photoanode shows a large improvement in the Jph,1.23V and Vonset compared to the BiVO4/SnO2 photoanode, which is because of the better light absorption (by WO3) and efficient charge separation originating from the formation of a type-II heterojunction, as reported previously.34 5



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According to a recent paper by L. Zhou et al., however, Sb-doped SnO2/BiVO4 is favorable than the WO3/BiVO4 because it can prevent excessive W-doping into the BiVO4.48 More interestingly, our BiVO4/WO3/SnO2 TPH photoanode shows a much higher Jph,1.23V (3.1 mA/cm2) compared to the BiVO4/SnO2/WO3 photoanode (2.3 mA/cm2), which is attributed to the staggered band alignment of BiVO4/WO3/SnO2 photoanode forming the double-heterojunction (Fig. 1d and Fig. S6). According to previous reports, a very thin SnO2 middle layer in between the BiVO4 and WO3 layers or the BiVO4 and FTO/glass acts as a hole mirror that reduces recombination at the interfaces, improving Jph,1.23V.49-51 We also observed that the BiVO4/SnO2/WO3 generated a higher Jph,1.23V than the BiVO4/WO3 and BiVO4/SnO2 photoanodes. However, our result indicates that adding the SnO2 layer at the bottom, i.e., the BiVO4/WO3/SnO2 structure is more advantageous for improving the PEC performance, especially photocurrent generation. Fig. 3c shows the incident photon to current conversion efficiencies (IPCEs) of the five photoanodes, which shows a similar trend as the J-V results, and the integrated photocurrent density values (shown in parenthesis) corresponded well with the Jph,1.23V values. In particular, the BiVO4/WO3/SnO2 TPH photoanode outperforms the other four photoanodes and shows a maximum IPCE of ~70 % (corresponding to an internal quantum efficiency of ~80 %) and an integrated photocurrent density of ~3.1 mA/cm2. In order to elucidate the impact of the SnO2 layer in the TPH photoanode, we measured the charge transfer (ηtransfer) and transport efficiencies (ηtransport) of the five photoanodes using a hole scavenger method (Fig. S7).52-53 As shown in Fig. 3d, the TPH photoanode shows the highest ηtransfer and ηtransport values at 1.23 VRHE among the five photoanodes. The ηtransfer values of the BiVO4/SnO2 (~77%), BiVO4/WO3(~80%), BiVO4/SnO2/WO3 (~81%), and BiVO4/WO3/SnO2 (~85%) photoanodes were quite similar to each other. Since all the four photoanodes are terminated with the BiVO4 layer that shows little change in morphology and surface area (because of the same spin-coating procedure), ηtransfer, which relates to the surface catalytic activity for water oxidation, shows little difference. The ηtransfer value of BiVO4 alone is quite low, which may be connected to bulk/non-surface related limitation. In contrast, for ηtransport, the BiVO4/WO3/SnO2 TPH photoanode shows the highest value of ~93%, which is even higher than the BiVO4/SnO2/WO3 photoanode (~84%). These results indicate that adding the SnO2 layer at the bottom is more effective in improving the electron/hole transport, which is due to the cascade band-alignment of the three layers by adding the SnO2 layers below the WO3 (Fig. 1d). For the TPH photoanode, the double heterojunction interfaces with intimate contact (both BiVO4/WO3 and WO3/SnO2) improve charge transport and reduce interfacial charge recombination. In addition, the composite layer of WO3 and SnO2, which forms a bulk heterojunction (i.e., disordered heterojunction) with the middle WO3 layer, might be further advantageous in minimizing the electron migration distance to the FTO surface. The electrochemical impedance (EIS) analysis (Fig. S8) of the BiVO4/WO3/SnO2 and BiVO4/SnO2/WO3 photoanodes revealed that the TPH structure has a smaller charge transport resistance (R2), supporting the improvement in the charge transport properties by the bottom SnO2 layers. Next, the light absorption, reflectance, and transmittance of the TPH photoanode were measured and compared with the other four photoanodes (Fig. 4a-b and Fig. S9). As shown in Fig. 4a, 6


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the BiVO4/WO3 photoanode shows a larger light absorption (~68%) than the BiVO4/SnO2 (~57%) and BiVO4 (~59%) photoanodes, which is attributed to the WO3 layer. Notably, the BiVO4/SnO2/WO3, BiVO4/SnO2, and the TPH photoanodes exhibited comparable light absorption (~62–69%). This result indicates that the incorporation of the SnO2 layer has little impact on the light absorption properties. In contrast, the transmittance shows a large difference, especially in the longer wavelength region (> ~500 nm) (Fig. 4b). The BiVO4/SnO2, BiVO4/WO3, and BiVO4/SnO2/WO3 photoanodes show a comparable transmittance of ~63% at 550 nm, whereas the TPH photoanode has a transmittance that is 1.2 times higher (~73%). This high transparency of the TPH photoanode at longer wavelengths originates from the optical path differences caused by the difference in the refractive index of the three layers as well as reduced internal light scattering due to intimate contact and lower porosity/voids. A similar result on the increase in the transparency was reported by adding SiO2 in the BiVO4 film.54 The optical simulation result obtained by the Finite-difference time-domain (FDTD) method (Fig. S10 & Fig. 4c) supports the fact that the TPH photoanode has higher transparency at longer wavelengths compared to the BiVO4/SnO2/WO3 photoanode because of the refractive index differences in the three layers. Therefore, together with the improved charge transport properties of the TPH photoanode, the presence of the bottom SnO2 layer increases the optical transparency at longer wavelengths, which will be advantageous when it is combined with a solar cell for a bias-free tandem PEC device. The PEC performance of the TPH photoanode was further optimized by a surface RIE and CoOx oxygen evolution electrocatalyst (OEC) deposition before combining with a PSC (Fig. S11). As shown in Fig. S11a, the Jph,1.23V increases to 3.3 mA/cm2, and more impressively, the Vonset reduced to ~0.3 VRHE (cathodic shift ~350 mV) after the RIE etching and CoOx deposition. The charge transfer efficiency and EIS measurements (Fig. S11b & S11c) confirm that the CoOx deposition improves the surface catalytic activity by reducing the surface charge recombination.55 Additionally, the CoOx/TPH photoanode shows a relatively good photocurrent stability for 120 min. Furthermore, the O2 evolution measurement of CoOx/TPH photoanode (measured at 1.0 VRHE, Fig. S12) showed a faradaic efficiency of ~94.2 % (corresponds to a STH of ~3.8 %), which is similar to the previous reports. However, after 60 min measurement, the faradaic efficiency and photocurrent density decreased slightly (~10 % degradation).

BiVO4/WO3/SnO2 TPH photoanode-PSC tandem PEC device Using the best-performing TPH photoanode (with RIE and CoOx OEC), a photoanode-organicinorganic halide (CH3NH3PbI3) PSC tandem PEC device was fabricated, and its bias-free PEC water splitting performance was evaluated (Fig. 5). As shown in Fig. 5a, the CoOx/TPH photoanode and the PSC was assembled in series, where the CoOx/TPH photoanode absorbs sunlight at wavelengths below ~500 nm and the rest of the solar light with wavelengths above ~500 nm is absorbed by the PSC. Here, we fabricated two types of PSCs: PSC#1-conventional single junction and PSC#2-specially designed to yield a high Voc (>2 V) by connecting two sub-cells in series. The schematic structure and basic solar cell performances of these two PSCs are summarized in Fig. S13. Fig. 5b compares the photocurrent density (at 1.23 VRHE) vs. transmittance (at 550 nm) of our CoOx/TPH and other best performing 7



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BiVO4-based photoanodes reported recently,37, 42, 45, 56-58 indicating that our TPH photoanode has a high figure of merit in terms of photocurrent generation and transparency (at 550 nm). This high figure of merit will be beneficial in transmitting more sunlight through the photoanode, and thus, increasing the light absorption in the PSC, which maximizes the photocurrent density at the operating potential for the tandem PEC device.54 With the CoOx/TPH photoanodes, PSC#1 and PSC#2 show short circuit current density values of ~15.1 mA/cm2 and ~9.0 mA/cm2 respectively, which are the highest values compared to the previously reported values with PSCs,41, 43, 45, 59 thus, indicating that there is potentially much room for improving the PEC performance further for a tandem PEC device. Fig. 5c shows the superimposed J-V curves of the PSCs and TPH photoanode with intersection points of 3.1 mA/cm2 (at 1.0 V with PSC#1) and 3.9 mA/cm2 (at 1.8 V with PSC#2). From the O2 evolution measurement, the faradaic efficiency was calculated to be ~94.2% (Fig. S12). Therefore, the photocurrent values correspond to theoretical STH efficiencies of ~3.5% and 4.5% respectively. Biasfree solar water-splitting performance of the tandem device was measured in a 0.5 M phosphate buffer solution (pH 7.0) under a simulated solar light illumination (100 mW/cm2). As shown in Fig. 5d, the CoOx/TPH photoanode-PSC#2 tandem device generates a stable photocurrent density of ~3.1 mA/cm2, which corresponds to a STH of ~3.5 % in the given time period (~15 min). The lower STH compared to the expected values (~4.5%), from Fig. 5c, might arise from a high dark current density (Fig. S11a) at the high potential and/or high contact resistance in the series connection. The transient photocurrent upon illumination stems from the excess charge generation in the PSC that should equilibrate with the current generated in the CoOx/TPH photoanode. Similar to previous studies on the stability of BiVO4based photoanodes,55 our CoOx/TPH photoanode shows a good photocurrent stability for at least 2 h (See Fig. S11d). However, the photocurrent stability of our tandem device gradually decreased after 20 min of operation. This originates from the well-known instability issue in CH3NH3PbI3 -based perovskite solar cells,43, 60-61 in which the intrinsic instability of the MAPbI3 and/or UV light-induced degradation are major problems. Therefore, for improving the stability of the TPH-PSC tandem device, further improvements in the design of the tandem structure to effectively screen UV light and/or use of more stable perovskite materials (e.g., (NH2CH=NH2)PbI3) for the PSC are needed.62-63

Conclusion In summary, we have fabricated a BiVO4/WO3/SnO2-based double-heterojunction photoanode on an FTO/glass substrate by using a facile sol-gel spin-coating process. A porous SnO2 film was first deposited on the FTO/glass substrate followed by WO3 deposition, which generates double-layers consisting of a dense WO3 and WO3/SnO2 mixture at the bottom. Subsequently, a BiVO4 nanoparticle film was deposited by spin coating. SEM and TEM characterizations revealed that three distinct layers that were in intimate contact and contained no pores and voids were successfully formed, with WO3 infiltrating the bottom SnO2 layers. Moreover, UPS measurements confirmed the staggered band-edge positions in the three layers. The resultant TPH photoanode showed improved charge transport/transfer efficiency as well as high transmittance at longer wavelengths (> 500 nm) compared to the BiVO4/WO3 and BiVO4/SnO2/WO3 photoanodes, showing an impressive PEC performance (photocurrent density of 8


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~3.1 mA/cm2, IPCEmax of ~70%, and charge transport efficiency of ~90%) compared to the previously reported planar-type BiVO4/WO3 photoanodes. The PEC performance of the TPH photoanode was further optimized by a RIE treatment and CoOx electrocatalyst deposition. Finally, we demonstrated a bias-free solar water splitting (STH of ~3.5%) by constructing a tandem PEC device with a PSC. We believe that the multilayered heterojunction approach, i.e., stacking/combining dissimilar materials that have suitable band edge positions in a multilayer provides favorable opportunities to tune the interfacial, optical, and charge transport/transfer properties of photoanodes, and thus, will impact the development of tandem PEC devices with a high STH efficiency.

Supporting Information. Optimization of coating cycles, Ultraviolet photoelectron spectroscopy (UPS), TEM-EDS line scan, Charge transfer/transport efficiency measurements, Electrochemical impedance analysis, FDTD optical simulation, Measurement of O2 evolution (faradaic efficiency), Structure and performance of perovskite solar cell and SEM images before/after PEC stability test.

Acknowledgements This work was supported by National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contracts NRF2012M3A7B4049986 (Nano-Material Technology Development Program), NRF-2016M3D1A1027664 (Creative Materials Discovery Program) and NRF-2014R1A4A1008474 (Basic Research Lab Program). This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF2015R1C1A1A01053785).

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Experimental section Preparation of TPH photoanode The BiVO4/WO3/SnO2 TPH photoanodes were prepared by a sol-gel spin-coating method. For the preparation of the precursor solution, 2-methoxyethanol (2ME, ≥99%, Sigma Aldrich) was used as the main solvent because it has high solubility and stability toward most inorganic precursors.64-65 For the SnO2 coating solution (0.2 M), 0.701 g of tin (IV) chloride pentahydrate (SnCl4⋅5H2O, 98%, Sigma Aldrich) and polyethylene glycol (M.W. = 6000, 1 g) were sequentially dissolved in 10 mL 2ME with ultrasonication for 20 min. For the WO3 coating solution (0.1 M), 1.25 g of tungstic acid (H2WO4, 99%, Sigma Aldrich) was dissolved in 25 mL H2O2 (34%, EP, Duksan), followed by heated on a hotplate (150 °C) until the solution volume was reduced to 5 mL, and then 45 mL of 2ME was added and mixed by ultrasonication for 1 h. For the BiVO4 coating solution (0.2 M), 0.404 g of bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O, 98%, Acros Organics) and 0.221 g of vanadyl acetylacetonate (≥ 99%, Sigma Aldrich) were dissolved in a solvent mixture of acetic acid (≥ 99.99%, Sigma Aldrich): 2ME acetyl acetone (≥ 99%, Sigma Aldrich) = 1:4:1 v/v) with magnetic stirring (30 min) and ultrasonication (30 min). Using the above coating solution, each film was deposited by a spin coating method: FTO/glass substrate (Pilkington, TEC-8), 2500 rpm/40 s, intermediate annealing after each coating = 350 °C/5 min, final annealing of SnO2, WO3, and BiVO4 = 450 °C/1 h, 500 °C/1 h, and 500 °C/2 h, respectively. The average film thickness per coating cycle was predetermined (SnO2: ~21 nm/cycle, WO3: ~17 nm/cycle, BiVO4: ~19 nm/cycle), and the optimal film thickness of each layer was obtained by measuring the PEC J-V curves (Fig. S2). RIE was performed for the surface treatment of the photoanodes (50 W, 10 s, RIE5000, SNTEK). For the CoOx electrocatalyst deposition, 0.146 g of cobalt nitrate hexahydrate (>99%, Sigma-Aldrich) was dissolved in 2ME (10 mL), and then, it was spin-coated (2000 rpm, 40 s, 3 times) and annealed at 350 °C/1 h. Fabrication of PSC Patterned FTO/glass substrates (Pilkington, TEC15) were cleaned for 15 min with acetone, ethanol, and DI water in an ultrasonic bath. First, a compact hole-blocking layer of TiO2was spin-coated on the substrate (3000 rpm/20 s) using 0.15 M titanium diisopropoxide bis (acetylacetonate) (75 wt.% in isopropanol) in anhydrous 1-butanol, and then, it was baked at 130 °C for 5 min. A porous TiO2 layer was prepared on top of the TiO2/FTO/glass substrates using a diluted TiO2 paste (Dyesol 18NRT TiO2 paste in ethanol, volume ratio of ethanol: paste = 4.5 : 1) and the film was annealed at 500 °C for 1 h. The resulting mp-TiO2/FTO substrate was immersed in a 0.04 M TiCl4 solution at 70 °C for 15 min, rinsed with DI water, and then, annealed at 500 °C for 30 min. In order to prepare the CH3NH3PbI3 solution, 461 mg of PbI2, and 158 mg of CH3NH3I were dissolved in 600 mL of DMF and 75 mL of DMSO solution at 25 °C. The dissolved solution was filtered using 0.45 µm PTFE filters, and then, the precursor solution was spin-coated on the prepared mp-TiO2/FTO substrate. During spin-coating, diethyl ether was slowly dropped on the spinning CH3NH3PbI3/mp-TiO2/FTO substrate. The spincoated film was then dried at 65 °C for 1 min and at 130 °C for 20 min to crystallize CH3NH3PbI3.66 Subsequently, a hole-transport layer was formed via spin coating using a spiro-MeOTAD solution, which consisted of 72 mg of spiro-MeOTAD, 28.8 µL of 4-tert-butylpyridine, and 17.6 µL of Li-TFSI 10


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solution (720 mg of Li-TFSI in acetonitrile) in 1 mL of chlorobenzene. The Au electrodes were deposited on the PSCs by thermal evaporation. The substrate size and the device active area were 2 × 2 cm2 and 0.14 cm2, respectively. For high Voc, PSCs were designed by directly connecting single PSCs, as can be seen in Fig. S13. The illuminated area, which acts as a second absorber of the tandem device, was equivalent to the working area of the photoanode. Material characterization The film morphology and thickness were characterized using a field-emission scanning electron microscope (FE-SEM, JSM-7600F, Jeol). Cross-sectional TEM, STEM, and EDS images of the samples were obtained using a high resolution TEM (HR-TEM, JEM-2100F, Jeol), and the TEM samples were obtained using a focused ion beam system (FIB, SMI3050TB, SII). The crystal structure and the phases of samples were investigated using X-ray diffraction (D8 advance, Bruker). The valence band positions of the samples were analyzed using UPS (ESCALAB250, Thermo Scientific). The transmittance (T) and reflectance (R) spectra of the samples were measured using a UV–vis spectrometer (Cary 5000, Agilent Technologies), and the absorption (A) was calculated from these measurements using the formula: A = 100 – T – R. Photoelectrochemical measurements PEC measurements (J-V and J-t curves) of the photoanodes were performed using a potentiostat (SP200, Biologics) under simulated solar light (AM1.5G, 100 mW/cm2) using a solar simulator (Model 94306A, Class AAA, Oriel). A standard three-electrode cell with a Ag/AgCl reference electrode (3 M NaCl, E° = 0.199 V), photoanodes as a working electrode, and a Pt coil counter electrode were employed. Before measurement, the solar simulator intensity was calibrated with a reference silicon diode (Mode 91150V, Newport). The illuminated area of the working electrode was 0.196 cm2, defined by a mask. For the electrolyte, either a 0.5 M phosphate buffer solution (pH 7.0) or a 0.5 M phosphate buffer solution with H2O2 solution (~2 vol.%) was used. During the PEC measurement, the electrolyte was deaerated by argon purging to remove the dissolved oxygen. For a typical J-V measurement, a potential scan rate of 50 mV/s was used and the potentials were recorded with correction using the Nernst relation: ERHE = EAg/AgCl + 0.059 × pH + 0.199 where EAg/AgCl is the applied bias potential and 0.199 V is the reference potential of Ag/AgCl with respect to the RHE scale. EIS measurements were performed in the same three electrode configuration using the same potentiostat under 1 sun illumination. The IPCEs were measured at 1.23 VRHE using a specially designed IPCE system for solar cells (QEX7, PV measurements), using the three electrode configuration. A 75-W xenon lamp equipped with a monochromator (CM-110, 1/8, SP Spectra Product) was used to generate a monochromatic beam. The incident light intensity was calibrated using a standard silicon photodiode. The charge transport and transfer efficiencies were estimated as functions of the applied potential by using H2O2 as a hole scavenger under AM 1.5G simulated solar light illumination.53

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Fig. 1. Preparation and characterizations of BiVO4/WO3/SnO2 triple-layer planar heterojunction (TPH) photoanode. (a) Sol-gel deposition process. (b) XRD pattern of the TPH photoanode. (c) Crosssectional SEM image of the optimized TPH photoanode showing intimate contact between layers without any voids. (d) Band alignment of the SnO2, WO3, and BiVO4 films. The conduction band-edge and valence band-edge positions were estimated from the ultraviolet photoelectron spectroscopy (UPS) and optical band gap measurements, respectively.

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Fig. 2. TEM characterizations of BiVO4/WO3/SnO2 TPH photoanode. (a) Cross-sectional TEM and (b–d) magnified TEM images at (b) spot A, (c) spot B, and (d) spot C from (a). The insets show the high-resolution TEM images (scale bar = 2 nm). (e) Bright-field STEM image and corresponding EDS elemental mapping images. It was confirmed that WO3 penetrated into the SnO2 porous layer, forming a mixed layer of WO3 and SnO2 at the bottom, and the rest of the WO3 forms a dense layer in the middle. (f) Schematic image showing the structure of the final TPH photoanode.

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Fig. 3. Comparison of photoelectrochemical (PEC) performance. (a) Structures of five photoanodes (i.e., BiVO4, BiVO4/WO3, BiVO4/SnO2, BiVO4/SnO2/WO3, and BiVO4/WO3/SnO2). (b) Representative photocurrent-potential (J-V) curves and (c) IPCEs of five photoanodes measured at 1.23 VRHE for water oxidation (pH 7 phosphate buffer electrolyte, simulated solar light (AM1.5G, 100 mW/cm2) illumination). The photocurrent density values at 1.23 VRHE, and the integrated photocurrent density values are shown in parenthesis. The TPH photoanode outperforms previously reported BiVO4/WO3 and BiVO4/SnO2/WO3 photoanodes. (d) Charge transport and transfer efficiencies for water oxidation (obtained at 1.23 VRHE, See Fig. S7 and the experimental section for details).

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Fig. 4. Optical properties of the BiVO4/WO3/SnO2 TPH photoanode. (a) Light absorption (=100-RT), and (b) Transmittance spectra. The parenthesis shows the transmittance value at 550 nm. (c) Transmittance spectra obtained by FDTD simulation. Experimental and calculation results shows a higher transmittance of the TPH photoanode at 500 nm comparing to other types of photoanodes.

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Fig. 5. BiVO4/WO3/SnO2 TPH photoanode/perovskite solar cell tandem device. (a) Schematic drawing of the TPH photoanode and perovskite solar cell (PSC) tandem device. (b) Comparison of photocurrent density (at 1.23 VRHE) vs. transmittance (at 550 nm) of BiVO4-based photoanodes. (c) J-V curves of the best performing TPH photoanode (with CoOx electrocatalyst) and PSCs (under the TPH photoanode). The J-V curve of the TPH photoanode was measured with 2-electrode configuration. PSC#1: single junction PSC and PSC#2: serially-connected small area PSCs for obtaining a high opencircuit voltage (~2.1 V). The intersection (~ 3.9 mA/cm2) represents the expected operating point of the stand-alone water splitting device. (d) Photocurrent density and calculated solar-to-hydrogen conversion (STH) efficiency as a function of time for the tandem cell (TPH/PSC#2), demonstrating a STH of ~3.5 %.

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References 1. Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S., Solar Water Splitting Cells. Chem. Rev. 2010, 110 (11), 6446-6473. 2. Ohashi, K.; McCann, J.; Bockris, J. M., Stable Photoelectrochemical Cells for the Splitting of Water. Nature 1977, 266, 610-611. 3. Khaselev, O.; Turner, J. A., A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting. Science 1998, 280 (5362), 425-427. 4. Li, Y.; Zhang, J. Z., Hydrogen Generation from Photoelectrochemical Water Splitting Based on Nanomaterials. Laser Photonics Rev. 2010, 4 (4), 517-528. 5. Hisatomi, T.; Kubota, J.; Domen, K., Recent Advances in Semiconductors for Photocatalytic and Photoelectrochemical Water Splitting. Chem. Soc. Rev. 2014, 43 (22), 7520-7535. 6. Khan, S. U.; Al-Shahry, M.; Ingler, W. B., Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2. science 2002, 297 (5590), 2243-2245. 7. Swierk, J. R.; Mallouk, T. E., Design and Development of Photoanodes for Water-Splitting Dye-Sensitized Photoelectrochemical Cells. Chem. Soc. Rev. 2013, 42 (6), 2357-2387. 8. Wang, G.; Wang, H.; Ling, Y.; Tang, Y.; Yang, X.; Fitzmorris, R. C.; Wang, C.; Zhang, J. Z.; Li, Y., Hydrogen-Treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11 (7), 3026-3033. 9. Park, J. H.; Kim, S.; Bard, A. J., Novel Carbon-Doped TiO2 Nanotube Arrays with High Aspect Ratios for Efficient Solar Water Splitting. Nano Lett. 2006, 6 (1), 24-28. 10. Cho, I. S.; Logar, M.; Lee, C. H.; Cai, L.; Prinz, F. B.; Zheng, X., Rapid and Controllable Flame Reduction of TiO2 Nanowires for Enhanced Solar Water-Splitting. Nano Lett. 2013, 14 (1), 24-31. 11. Su, J.; Feng, X.; Sloppy, J. D.; Guo, L.; Grimes, C. A., Vertically Aligned WO3 Nanowire Arrays Grown Directly on Transparent Conducting Oxide Coated Glass: Synthesis and Photoelectrochemical Properties. Nano Lett. 2010, 11 (1), 203208. 12. Kim, J. K.; Shin, K.; Cho, S. M.; Lee, T.-W.; Park, J. H., Synthesis of Transparent Mesoporous Tungsten Trioxide Films with Enhanced Photoelectrochemical Response: Application to Unassisted Solar Water Splitting. Energy Environ. Sci. 2011, 4 (4), 1465-1470. 13. Sivula, K.; Le Formal, F.; Grätzel, M., Solar Water Splitting: Progress Using Hematite (α-Fe2O3) Photoelectrodes. ChemSusChem 2011, 4 (4), 432-449. 14. Ling, Y.; Wang, G.; Wheeler, D. A.; Zhang, J. Z.; Li, Y., Sn-Doped Hematite Nanostructures for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11 (5), 2119-2125. 15. Yourey, J. E.; Bartlett, B. M., Electrochemical Deposition and Photoelectrochemistry of CuWO4, a Promising Photoanode for Water Oxidation. J. Mater. Chem. 2011, 21 (21), 7651-7660. 16. Kim, T. W.; Choi, K.-S., Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343 (6174), 990-994. 17. Abdi, F. F.; Firet, N.; van de Krol, R., Efficient BiVO4 Thin Film Photoanodes Modified with Cobalt Phosphate Catalyst and W-Doping. ChemCatChem 2013, 5 (2), 490-496. 18. Rettie, A. J.; Lee, H. C.; Marshall, L. G.; Lin, J.-F.; Capan, C.; Lindemuth, J.; McCloy, J. S.; Zhou, J.; Bard, A. J.; Mullins, C. B., Combined Charge Carrier Transport and Photoelectrochemical Characterization of BiVO4 Single Crystals: Intrinsic Behavior of a Complex Metal Oxide. J. Am. Chem. Soc. 2013, 135 (30), 11389-11396. 19. Hill, J. C.; Choi, K.-S., Synthesis and Characterization of High Surface Area CuWO4 and Bi2WO6 Electrodes for Use as Photoanodes for Solar Water Oxidation. J. Mater. Chem. A 2013, 1 (16), 5006-5014. 20. Zhang, L.; Baumanis, C.; Robben, L.; Kandiel, T.; Bahnemann, D., Bi2WO6 Inverse Opals: Facile Fabrication and Efficient Visible-Light-Driven Photocatalytic and Photoelectrochemical Water-Splitting Activity. Small 2011, 7 (19), 27142720. 21. Abe, R.; Higashi, M.; Domen, K., Facile Fabrication of an Efficient Oxynitride TaON Photoanode for Overall Water Splitting into H2 and O2 under Visible Light Irradiation. J. Am. Chem. Soc. 2010, 132 (34), 11828-11829. 22. Higashi, M.; Domen, K.; Abe, R., Fabrication of Efficient TaON and Ta3N5 Photoanodes for Water Splitting under Visible Light Irradiation. Energy Environ. Sci. 2011, 4 (10), 4138-4147. 23. Hitoki, G.; Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Domen, K., Ta3N5 as a Novel Visible Light-Driven Photocatalyst (. LAMBDA.< 600 nm). Chem. Lett. 2002, 7, 736-737.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 20

24. Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S., Technical and Economic Feasibility of Centralized Facilities for Solar Hydrogen Production via Photocatalysis and Photoelectrochemistry. Energy Environ. Sci. 2013, 6 (7), 1983-2002. 25. Seitz, L. C.; Chen, Z.; Forman, A. J.; Pinaud, B. A.; Benck, J. D.; Jaramillo, T. F., Modeling Practical Performance Limits of Photoelectrochemical Water Splitting Based on the Current State of Materials Research. ChemSusChem 2014, 7 (5), 1372-1385. 26. Su, J.; Guo, L.; Bao, N.; Grimes, C. A., Nanostructured WO3/BiVO4 Heterojunction Films for Efficient Photoelectrochemical Water Splitting. Nano Lett. 2011, 11 (5), 1928-1933. 27. Rao, P. M.; Cai, L.; Liu, C.; Cho, I. S.; Lee, C. H.; Weisse, J. M.; Yang, P.; Zheng, X., Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/Shell Nanowire Photoanode for Photoelectrochemical Water Oxidation. Nano Lett. 2014, 14 (2), 1099-1105. 28. Resasco, J.; Zhang, H.; Kornienko, N.; Becknell, N.; Lee, H.; Guo, J.; Briseno, A. L.; Yang, P., TiO2/BiVO4 Nanowire Heterostructure Photoanodes Based on Type II Band Alignment. ACS Cent. Sci. 2016, 2 (2), 80-88. 29. Zhan, F.; Li, J.; Li, W.; Liu, Y.; Xie, R.; Yang, Y.; Li, Y.; Chen, Q., In Situ Formation of CuWO4/WO3 Heterojunction Plates Array Films with Enhanced Photoelectrochemical Properties. Int. J. Hydrogen Energy 2015, 40 (20), 6512-6520. 30. Tang, Y.; Wang, R.; Yang, Y.; Yan, D.; Xiang, X., Highly Enhanced Photoelectrochemical Water Oxidation Efficiency Based on Triadic Quantum Dot/Layered Double Hydroxide/BiVO4 Photoanodes. ACS Appl. Mater. Interfaces 2016, 8 (30), 19446-19455. 31. He, W.; Wang, R.; Zhang, L.; Zhu, J.; Xiang, X.; Li, F., Enhanced Photoelectrochemical Water Oxidation on a BiVO4 Photoanode Modified with Multi-Functional Layered Double Hydroxide Nanowalls. J. Mater. Chem. A 2015, 3 (35), 1797717982. 32. Pihosh, Y.; Turkevych, I.; Mawatari, K.; Uemura, J.; Kazoe, Y.; Kosar, S.; Makita, K.; Sugaya, T.; Matsui, T.; Fujita, D., Photocatalytic Generation of Hydrogen by Core-Shell WO3/BiVO4 Nanorods with Ultimate Water Splitting Efficiency. Sci. Rep. 2015, 5, 11141. 33. Kim, J. H.; Lee, J. S., BiVO4-Based Heterostructured Photocatalysts for Solar Water Splitting: a Review. Energy Environ. Focus 2014, 3 (4), 339-353. 34. Hong, S. J.; Lee, S.; Jang, J. S.; Lee, J. S., Heterojunction BiVO4/WO3 Electrodes for Enhanced Photoactivity of Water Oxidation. Energy Environ. Sci. 2011, 4 (5), 1781-1787. 35. Fujimoto, I.; Wang, N.; Saito, R.; Miseki, Y.; Gunji, T.; Sayama, K., WO3/BiVO4 Composite Photoelectrode Prepared by Improved Auto-Combustion Method for Highly Efficient Water Splitting. Int. J. Hydrogen Energy 2014, 39 (6), 2454-2461. 36. Shi, X.; Choi, I. Y.; Zhang, K.; Kwon, J.; Kim, D. Y.; Lee, J. K.; Oh, S. H.; Kim, J. K.; Park, J. H., Efficient Photoelectrochemical Hydrogen Production from Bismuth Vanadate-Decorated Tungsten Trioxide Helix Nanostructures. Nat. Commun. 2014, 5, 4775. 37. Ma, M.; Kim, J. K.; Zhang, K.; Shi, X.; Kim, S. J.; Moon, J. H.; Park, J. H., Double-Deck Inverse Opal Photoanodes: Efficient Light Absorption and Charge Separation in Heterojunction. Chem. Mater. 2014, 26 (19), 5592-5597. 38. Minggu, L. J.; Wan Daud, W. R.; Kassim, M. B., An Overview of Photocells and Photoreactors for Photoelectrochemical Water Splitting. Int. J. Hydrogen Energy 2010, 35 (11), 5233-5244. 39. Zhang, X.; Zhang, B.; Cao, K.; Brillet, J.; Chen, J.; Wang, M.; Shen, Y., A Perovskite Solar Cell-TiO2@BiVO4 Photoelectrochemical System for Direct Solar Water Splitting. J. Mater. Chem. A 2015, 3 (43), 21630-21636. 40. Jacobsson, T. J.; Fjallstrom, V.; Edoff, M.; Edvinsson, T., Sustainable Solar Hydrogen Production: from Photoelectrochemical Cells to PV-Electrolyzers and Back Again. Energy Environ. Sci. 2014, 7 (7), 2056-2070. 41. Gurudayal; Sabba, D.; Kumar, M. H.; Wong, L. H.; Barber, J.; Grätzel, M.; Mathews, N., Perovskite–Hematite Tandem Cells for Efficient Overall Solar Driven Water Splitting. Nano Lett. 2015, 15 (6), 3833-3839. 42. Shi, X.; Zhang, K.; Shin, K.; Ma, M.; Kwon, J.; Choi, I. T.; Kim, J. K.; Kim, H. K.; Wang, D. H.; Park, J. H., Unassisted Photoelectrochemical Water Splitting Beyond 5.7% Solar-to-Hydrogen Conversion Efficiency by a Wireless Monolithic Photoanode/Dye-Sensitised Solar Cell Tandem Device. Nano Energy 2015, 13, 182-191. 43. Kim, J. H.; Jo, Y.; Kim, J. H.; Jang, J. W.; Kang, H. J.; Lee, Y. H.; Kim, D. S.; Jun, Y.; Lee, J. S., Wireless Solar Water Splitting Device with Robust Cobalt-Catalyzed, Dual-Doped BiVO4 Photoanode and Perovskite Solar Cell in Tandem: A Dual Absorber Artificial Leaf. ACS Nano 2015, 9 (12), 11820-11829. 44. Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I., High-Performance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348 (6240), 1234-1237. 45. Chen, Y.-S.; Manser, J. S.; Kamat, P. V., All Solution-Processed Lead Halide Perovskite-BiVO4 Tandem Assembly for Photolytic Solar Fuels Production. J. Am. Chem. Soc. 2015, 137 (2), 974-981.

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46. Hong, S. J.; Lee, S.; Jang, J. S.; Lee, J. S., Heterojunction BiVO4/WO3 Electrodes for Enhanced Photoactivity of Water Oxidation. Energy Environ. Sci. 2011, 4 (5), 1781-1787. 47. Yin, J.; Huang, S.; Jian, Z.; Wang, Z.; Zhang, Y., Fabrication of Heterojunction SnO2/BiVO4 Composites Having Enhanced Visible Light Photocatalystic Activity. Mater. Sci. Semicond. Process. 2015, 34, 198-204. 48. Zhou, L.; Zhao, C.; Giri, B.; Allen, P.; Xu, X.; Joshi, H.; Fan, Y.; Titova, L. V.; Rao, P. M., High Light Absorption and Charge Separation Efficiency at Low Applied Voltage from Sb-Doped SnO2/BiVO4 Core/Shell Nanorod-Array Photoanodes. Nano Lett. 2016, 16 (6), 3463-3474. 49. Saito, R.; Miseki, Y.; Sayama, K., Highly Efficient Photoelectrochemical Water Splitting Using a Thin Film Photoanode of BiVO4/SnO2/WO3 Multi-Composite in a Carbonate Electrolyte. Chem. Commun. 2012, 48 (32), 3833-3835. 50. Liang, Y.; Tsubota, T.; Mooij, L. P. A.; van de Krol, R., Highly Improved Quantum Efficiencies for Thin Film BiVO4 Photoanodes. J. Phys. Chem. C 2011, 115 (35), 17594-17598. 51. Chatchai, P.; Murakami, Y.; Kishioka, S.-y.; Nosaka, A.; Nosaka, Y., FTO∕SnO2∕BiVO4 Composite Photoelectrode for Water Oxidation under Visible Light Irradiation. Electrochem. Solid-State Lett. 2008, 11 (6), H160-H163. 52. Dotan, H.; Sivula, K.; Grätzel, M.; Rothschild, A.; Warren, S. C., Probing the Photoelectrochemical Properties of Hematite (α-Fe2O3) Electrodes Using Hydrogen Peroxide as a Hole Scavenger. Energy Environ. Sci. 2011, 4 (3), 958-964. 53. Cho, I. S.; Lee, C. H.; Feng, Y.; Logar, M.; Rao, P. M.; Cai, L.; Kim, D. R.; Sinclair, R.; Zheng, X., Codoping Titanium Dioxide Nanowires with Tungsten and Carbon for Enhanced Photoelectrochemical Performance. Nat. Commun. 2013, 4, 1723. 54. Pilli, S. K.; Deutsch, T. G.; Furtak, T. E.; Turner, J. A.; Brown, L. D.; Herring, A. M., Light Induced Water Oxidation on Cobalt-Phosphate (Co-Pi) Catalyst Modified Semi-Transparent, Porous SiO2-BiVO4 Electrodes. Phys. Chem. Chem. Phys. 2012, 14 (19), 7032-7039. 55. Zhong, M.; Hisatomi, T.; Kuang, Y.; Zhao, J.; Liu, M.; Iwase, A.; Jia, Q.; Nishiyama, H.; Minegishi, T.; Nakabayashi, M.; Shibata, N.; Niishiro, R.; Katayama, C.; Shibano, H.; Katayama, M.; Kudo, A.; Yamada, T.; Domen, K., Surface Modification of CoOx Loaded BiVO4 Photoanodes with Ultrathin p-Type NiO Layers for Improved Solar Water Oxidation. J. Am. Chem. Soc. 2015, 137 (15), 5053-5060. 56. Abdi, F. F.; Han, L.; Smets, A. H. M.; Zeman, M.; Dam, B.; van de Krol, R., Efficient Solar Water Splitting by Enhanced Charge Separation in a Bismuth Vanadate-Silicon Tandem Photoelectrode. Nat. Commun. 2013, 4, 2195. 57. Zhao, X.; Luo, W.; Feng, J.; Li, M.; Li, Z.; Yu, T.; Zou, Z., Quantitative Analysis and Visualized Evidence for High Charge Separation Efficiency in a Solid-Liquid Bulk Heterojunction. Adv. Energy Mater. 2014, 4 (9). 58. Nair, V.; Perkins, C. L.; Lin, Q.; Law, M., Textured Nanoporous Mo:BiVO4 Photoanodes with High Charge Transport and Charge Transfer Quantum Efficiencies for Oxygen Evolution. Energy Environ. Sci. 2016, 9 (4), 1412-1429. 59. Qiu, Y.; Liu, W.; Chen, W.; Chen, W.; Zhou, G.; Hsu, P.-C.; Zhang, R.; Liang, Z.; Fan, S.; Zhang, Y.; Cui, Y., Efficient Solar-Driven Water Splitting by Nanocone BiVO4-Perovskite Tandem Cells. Sci. Adv. 2016, 2 (6). 60. Niu, G.; Guo, X.; Wang, L., Review of Recent Progress in Chemical Stability of Perovskite Solar Cells. J. Mater. Chem. A 2015, 3 (17), 8970-8980. 61. Rong, Y.; Liu, L.; Mei, A.; Li, X.; Han, H., Beyond Efficiency: the Challenge of Stability in Mesoscopic Perovskite Solar Cells. Adv. Energy Mater. 2015, 5 (20), 1501066. 62. Eperon, G. E.; Stranks, S. D.; Menelaou, C.; Johnston, M. B.; Herz, L. M.; Snaith, H. J., Formamidinium Lead Trihalide: a Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells. Energy Environ. Sci. 2014, 7 (3), 982988. 63. Lee, J. W.; Kim, D. H.; Kim, H. S.; Seo, S. W.; Cho, S. M.; Park, N. G., Formamidinium and Cesium Hybridization for Photo-and Moisture-Stable Perovskite Solar Cell. Adv. Energy Mater. 2015, 5 (20). 64. Schwartz, R. W.; Schneller, T.; Waser, R., Chemical Solution Deposition of Electronic Oxide Films. C. R. Chim. 2004, 7 (5), 433-461. 65. Dey, S. K.; Budd, K. D.; Payne, D. A., Thin-Film Ferroelectrics of PZT of Sol-Gel Processing. IEEE Trans. Sonics Ultrason. 1988, 35 (1), 80-81. 66. Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I., Solvent Engineering for High-Performance Inorganic–Organic Hybrid Perovskite Solar Cells. Nat. mater. 2014, 13 (9), 897-903.

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SnO2 Double-Heterojunction Photoanode with Enhanced

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