Double-Heterojunction Photoanode with Enhanc - ACS Publications

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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*,† †

School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon 440-746, Korea Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States § Department of Materials Science & Engineering and Energy Systems Research, Ajou University, Suwon 443-749, Korea ∥ School of Mechanical Engineering, Hanyang University, Seoul 133-791, Korea ‡

S Supporting Information *

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 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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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-

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 © 2016 American Chemical Society

Received: October 8, 2016 Accepted: December 19, 2016 Published: December 19, 2016 1479

DOI: 10.1021/acsami.6b12782 ACS Appl. Mater. Interfaces 2017, 9, 1479−1487

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ACS Applied Materials & Interfaces

Figure 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) Cross-sectional 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.

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 singleabsorber) inevitably requires external bias to achieve an unassisted/bias-free water splitting reaction, which is originated from the thermodynamic voltage requirements (1.23 V) and the associated overpotential 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

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 (∼320 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.

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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 (Figure 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 (Figures S1 and 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 Figure 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 (Figure 1b). The cross-sectional SEM image (Figure 1c) 1480


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Figure 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.

Figure 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 parentheses. 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 Figure S7 and the Experimental Section for details).

measurements (Figure 1d and Figure S3). First, the valence band maximum (VBM) positions were obtained from UPS measurements (Figure S3). The calculated VBM positions of SnO2, WO3, and BiVO4 were 4.13, 3.36, and 2.44 V vs RHE, respectively (Figure 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 (Figure 1d). Importantly, a staggered band alignment of the CBM and VBM positions indicates a type-II heterojunction relationship among

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, Figure 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) 1481


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originating from the formation of a type-II heterojunction, as reported previously.34 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,1.23 V (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 (Figure 1d and Figure 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,1.23 V.49−51 We also observed that the BiVO4/SnO2/WO3 generated a higher Jph,1.23 V 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. Figure 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 parentheses) corresponded well with the Jph,1.23 V 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 (Figure S7).52,53 As shown in Figure 3d, the TPH photoanode shows the highest ηtransfer and ηtransport values at 1.23 VRHE among the five photoanodes. The η transfer values of the BiVO 4 /SnO 2 (∼77%), BiVO 4 / WO3(∼80%), BiVO4/SnO2/WO3 (∼81%), and BiVO4/WO3/ SnO2 (∼85%) photoanodes were quite similar to each other. Because all 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/nonsurface 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 (Figure 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 spectroscopy (EIS) analysis (Figure 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 layer.

the three layers, thus, facilitating electron/hole transport/ transfer in between the layers. 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) (Figure 2). As shown in the cross-sectional TEM image (Figure 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, reduce the internal light scattering, enhancing light transmittance at longer wavelengths. The crystal structure and crystallinity of each layer were further characterized by highresolution TEM analysis at spots A, B, and C, shown in Figure 2a. First, as shown in Figure 2b, the SnO2 nanoparticles (∼10− 15 nm in size) are interconnected well and exhibit nanoporosity. The high-resolution TEM images (inset of Figure 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’ (Figure 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 highresolution TEM image confirms its high crystallinity and monoclinic phase (Figure 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 (Figure 2d). In addition, the EDS line scan (Figure S5) and elemental mapping results (Figure 2e) 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 Figure 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 planar-type 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 (Figure 3a, see the Experimental Section for the preparation of each sample). Figure 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, 1.23 V) 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,1.23 V and Vonset compared to the BiVO4/ SnO2 photoanode, which is because of the better light absorption (by WO 3 ) and efficient charge separation 1482


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Figure 4. Optical properties of the BiVO4/WO3/SnO2 TPH photoanode. (a) Light absorption (= 100 − R − T) and (b) transmittance spectra. The parentheses 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.

Figure 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 a two-electrode configuration. PSC#1, single junction PSC; and PSC#2, serially connected small area PSCs for obtaining a high open-circuit 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%.

Next, the light absorption, reflectance, and transmittance of the TPH photoanode were measured and compared with the other four photoanodes (Figure 4a, b and Figure S9). As shown in Figure 4a, 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 1483


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photoanodes, PSC#1 and PSC#2 show short circuit current density values of ∼15.1 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. Figure 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% (Figure S12). Therefore, the photocurrent values correspond to theoretical STH efficiencies of ∼3.5 and 4.5%, respectively. Bias-free 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 Figure 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 Figure 5c, might arise from a high dark current density (Figure 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 BiVO4-based photoanodes,55 our CoOx/TPH photoanode shows a good photocurrent stability for at least 2 h (see Figure 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

contrast, the transmittance shows a large difference, especially in the longer wavelength region (≳500 nm) (Figure 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 (Figure S10 and Figure 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 (Figure S11). As shown in Figure S11a, the Jph,1.23 V 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 (Figure S11b, c) 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, Figure 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-organic−inorganic halide (CH3NH3PbI3) PSC tandem PEC device was fabricated, and its bias-free PEC water splitting performance was evaluated (Figure 5). As shown in Figure 5a, the CoOx/TPH photoanode and the PSC were 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 subcells in series. The schematic structure and basic solar cell performances of these two PSCs are summarized in Figure S13. Figure 5b compares the photocurrent density (at 1.23 VRHE) vs transmittance (at 550 nm) of our CoOx/TPH and other best performing 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

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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 WO 3 and WO 3 /SnO 2 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 ∼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 1484


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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 (SP-200, 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 91150 V, 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

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.

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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 of H2O2 (34%, EP, Duksan), followed by heated on a hot-plate (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·H2O, 98%, Acros Organics) and 0.221 g of vanadyl acetylacetonate (≥99%, SigmaAldrich) 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 (Figure 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 mpTiO2/FTO substrate. During spin-coating, diethyl ether was slowly dropped on the spinning CH3NH3PbI3/mp-TiO2/FTO substrate. The spin-coated 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-tertbutylpyridine, and 17.6 μL of Li-TFSI 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 Figure S13. The illuminated area, which acts as a second

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12782. 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 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +82-31-219-2468. Fax: +82-31-219-1613. *E-mail: [email protected]. Tel: +82-31-290-7403. Fax: +8231-290-7410. ORCID

In Sun Cho: 0000-0001-5622-7712 1485


Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT and 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 (NRF-2015R1C1A1A01053785).

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