Absorber Tandem Water Splitting Using Perovskite

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Targeting Ideal Dual-Absorber Tandem Water Splitting Using Perovskite Photovoltaics and CuInxGa1−xSe2 Photocathodes Jingshan Luo,* Zhen Li, Shiro Nishiwaki, Marcel Schreier, Matthew T. Mayer, Peter Cendula, Yong Hui Lee, Kunwu Fu, Anyuan Cao, Mohammad Khaja Nazeeruddin, Yaroslav E. Romanyuk, Stephan Buecheler, S. David Tilley, Lydia Helena Wong, Ayodhya N. Tiwari, and Michael Grätzel

diffuse and intermittent nature requires that we combine an efficient harvesting strategy with an effective method for energy storage before large-scale utilization can be envisioned. Inspired by natural photosynthesis, converting solar energy directly into chemical fuels is considered as one of the most promising ways to solve this challenge.[2] Among the possible chemical fuels, hydrogen is the simplest form and it can be generated through sunlight-driven water splitting with semiconductor light absorbers in contact with aqueous solutions.[3] The thermodynamic potentials and kinetic demands of the water splitting reactions strictly define the bandgap energy and the band edge positions required of a semiconductor for photoelectrochemical (PEC) water splitting. Despite tremendous efforts in modifying existing materials and searching for new materials during the past several decades,[4] there is still no single material that meets all the requirements for efficient and durable water splitting under solar illumination.

Efficient sunlight-driven water splitting devices can be achieved by pairing two absorbers of different optimized bandgaps in an optical tandem design. With tunable absorption ranges and cell voltages, organic–inorganic metal halide perovskite solar cells provide new opportunities for tailoring top absorbers for such devices. In this work, semitransparent perovskite solar cells are developed for use as the top cell in tandem with a smaller bandgap photocathode to enable panchromatic harvesting of the solar spectrum. A new CuInxGa1−xSe2 multilayer photocathode is designed, exhibiting excellent performance for photoelectrochemical water reduction and representing a near-ideal bottom absorber. When pairing it below a semitransparent CH3NH3PbBr3-based solar cell, a solar-to-hydrogen efficiency exceeding 6% is achieved, the highest value yet reported for a photovoltaic–photoelectrochemical device utilizing a single-junction solar cell as the bias source under one sun illumination. The analysis shows that the efficiency can reach more than 20% through further optimization of the perovskite top absorber.

1. Introduction Solar energy is plentiful enough to fulfill all of mankind’s energy demands as a green and renewable source,[1] yet its

Dr. J. Luo, M. Schreier, Dr. M. T. Mayer, Dr. Y. H. Lee, Dr. M. K. Nazeeruddin, Dr. S. D. Tilley, Prof. M. Grätzel Laboratory of Photonics and Interfaces Institute of Chemical Sciences and Engineering School of Basic Sciences Ecole Polytechnique Fédérale de Lausanne (EPFL) 1015 Lausanne, Switzerland E-mail: [email protected] Dr. Z. Li, K. Fu, Prof. L. H. Wong Energy Research Institute at NTU (ERI@N) Nanyang Technological University Research Techno Plaza 50 Nanyang Drive, Singapore 637553, Singapore Dr. S. Nishiwaki, Dr. Y. E. Romanyuk, Dr. S. Buecheler, Prof. A. N. Tiwari Laboratory for Thin Films and Photovoltaics Empa–Swiss Federal Laboratories for Materials Science and Technology Ueberlandstrasse 129, 8600 Dübendorf, Switzerland

Dr. P. Cendula Institute of Computational Physics Zurich University of Applied Sciences (ZHAW) Wildbachstrasse 21, 8401 Winterthur, Switzerland K. Fu, Prof. L. H. Wong School of Materials Science and Engineering Nanyang Technological University (NTU) Block N4.1, Nanyang Avenue, Singapore 639798, Singapore Prof. A. Cao Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871, China

DOI: 10.1002/aenm.201501520

Adv. Energy Mater. 2015, 1501520

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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To overcome the dilemma between needing a bandgap large enough to generate sufficient driving force yet small enough to harvest a broad range of the solar spectrum, a multiple absorber tandem concept has been proposed.[5] By pairing two absorbers of different bandgaps, a greater fraction of solar photons can be utilized and the photovoltages produced by each can be additive toward meeting the electrolysis energy demand. Given the constraints of the incident solar radiation and the demand for photovoltages of 1.5–1.8 V to drive electrolysis, detailed balance analysis predicts that two-absorber water splitting efficiencies can be maximized when pairing 1.6–1.8 and 1.1 eV bandgap absorbers in a stacked configuration.[5c] Importantly, the success of this approach requires balanced photovoltage generation from each component commensurate with their Shockley– Queisser limits, on the order of 1.0 and 0.6 V for the ideal top and bottom absorbers, respectively.[5b] Only then can a tandem device achieve water splitting at high solar-to-hydrogen (STH) efficiency. Many variations of water splitting tandems have been demonstrated in recent years,[6] but so far only devices based on efficient III–V materials have approached the idealized balance of bandgaps and photovoltages toward this application.[7] Clearly, efficient photoabsorbing semiconductors are important for efficient photoelectrolysis, but achieving this using relatively inexpensive absorbers under one-sun illumination remains a challenge. Purely PEC approaches, employing a photocathode and photoanode both in direct contact with water, represent a common target for tandem water splitting, wherein a wireless, monolithic structure could conceivably provide an optimized balance among simplicity, cost, and efficiency.[5a,8] Rather few photoelectrodes have been demonstrated to exhibit the ideal bandgap–photovoltage balance explained above, and therefore two-absorber tandem efficiencies have remained modest.[9] Among materials for the small-bandgap photoelectrode, silicon has shown the most promise thus far.[10] Here, we present a new candidate PEC device based on CuInxGa1−xSe2 (CIGS), with remarkable photocathode performance approaching the ideal for use in a water splitting tandem. Though its water splitting properties have been less studied compared to Si, it represents a group of materials called copper chalcopyrites that have tunable bandgaps through variation of their compositions.[11]

The tunability of the bandgap offers great flexibility in building a tandem cell, and the possibility of solution synthesis makes them highly promising as a cost-effective absorber for solar water splitting.[12] More challenging is identifying a suitable wide-bandgap (1.6–1.8 eV) component, where most explored materials, primarily transition metal oxides, have bandgaps too wide or photovoltages too small, or both. The search is underway for photoelectrodes capable of producing the necessary voltage under standard conditions,[13] but the requirement for ≈1 V photovoltage at current densities sufficient for reasonable STH efficiency drastically limits the available candidates. Several recent reports have adopted the approach of using a photovoltaic (PV) cell to provide the necessary additional voltage toward PEC water splitting when paired with a photoanode.[14] Although many of these PEC–PV tandem approaches achieved standalone water splitting, their bandgap–photovoltage balances deviated from the ideal for a two-absorber tandem, limiting their theoretically-achievable efficiencies. Given that suitable small-bandgap photocathodes have been identified, the best chance for achieving ideal STH efficiency lies in pairing them with a wider-bandgap transparent PV cell. The emergence of solution-processed hybrid organic–inorganic perovskite PVs provides new opportunities for solar water splitting, particularly due to their large photovoltages.[14a,15] Importantly, it was discovered that their bandgaps and open circuit voltages can be tuned by varying their compositions.[16] Additionally, they can be made semi-transparent, paving the way for their incorporation as the top cell in a tandem device.[17] Herein we demonstrate the pairing of two types of transparent perovskite cell with a CIGS photocathode for efficient overall water splitting, as illustrated in Figure 1. With CIGS serving as a near-ideal small-bandgap photocathode, we investigated the influence of varied perovskite composition on the tandem performance, examining the interplay between transparency, spectral response, and photovoltage of each component. Our best PEC–PV tandem reached an STH efficiency over 6%, yet we predict that the development of bandgap-optimized transparent and efficient perovskite PVs could enable tandem efficiencies exceeding 20%.

Figure 1. General schematic and energy potential diagrams of the perovskite and CIGS tandem water splitting cell. a) Schematic diagram. DSA, dimensionally stable anode. b) Energy potential diagram.

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2.1. Transparent Perovskite PVs In order to use a perovskite PV cell as the top absorber in a tandem device, the cell must be semitransparent to allow the longer wavelength photons to pass. In conventional perovskite cells, a hole transport material (HTM) layer is present on top of the perovskite absorber layer, followed by a metal electrode deposited by evaporation. Both of these two layers reduce the device light transmittance, especially the opaque metal electrode. Efforts have been made to use silver nanowires or metal meshes to make semitransparent perovskite solar cells.[18] Here, we use carbon nanotube (CNT) networks as the contact, which have high conductivity, good transparency, and high chemical stability.[19] To further enhance the transparency, the devices were fabricated without an HTM layer, and to improve the fill factor of the device, gold finger electrodes were deposited on the surface. The bandgap of the top absorber is an important parameter of a tandem device, and both CH3NH3PbI3 and CH3NH3PbBr3 were investigated here to explore the effect of different absorbers on the device performance. Figure 2a depicts the J–V characteristics of the perovskite solar cells with different absorbers. The CH3NH3PbI3-based cell shows a short circuit current density (Jsc) of 17 mA cm−2 and an open circuit voltage (Voc) of 1.05 V, while the CH3NH3PbBr3-based cell shows much higher Voc of 1.4 V with a lower Jsc of 5.4 mA cm−2. The incident photon to current efficiency (IPCE) reveals the percentage of photons that the cell can convert into electrons. Both CH3NH3PbI3 and CH3NH3PbBr3 based cells have

relatively broad and flat IPCE response within their absorption range, corresponding to bandgaps of 1.5 and 2.3 eV, respectively (Figure 2b). Integration of the IPCE with the Air Mass 1.5 Global (AM 1.5G) solar spectrum yields the Jsc generated by the solar cell, yielding values of around 17 and 5.2 mA cm−2 for CH3NH3PbI3 and CH3NH3PbBr3 cells, respectively, which is in agreement with the J–V characteristics, Figure S1 (Supporting Information). The transmittance of these cells, an important factor determining how many photons the bottom cell could absorb, is shown in Figure 2c. Both cells show more than 40% transmission beyond their bandgaps, representing the fraction of incident light that will be transmitted to the bottom absorber in the tandem. In Figure 2d, a cross section microscopy image details the structure of a representative perovskite cell. Hysteresis is nearly negligible in this device structure, which might be due to the use of thick mesoporous TiO2 layer and the CNT network as a hole selective contact. Figure S2 (Supporting Information) shows the J–V curves of one of the Br based perovskite solar cell under forward and backward scan at a scan rate of 1 V s−1. These results show the balance between bandgap, photovoltage, photocurrent, and transmittance, parameters that are important to the ultimate tandem performance which could be tuned here by simply exchanging the halide component of the perovskite absorber materials.

2.2. CIGS Photocathodes With the semitransparent perovskite PV cells in hand, the next step was the development of a suitable bottom absorber

Figure 2. Photovoltaic, optical, and morphological characteristics of CH3NH3PbI3 and CH3NH3PbBr3 perovskite solar cells. a) J–V curves under dark and simulated AM 1.5G 100 mW cm−2 solar illumination. b) IPCE spectra. c) Transmission spectra; inset, photos of the samples. d) Cross section SEM image of a representative CH3NH3PbBr3 cell. The image was acquired in a region beneath one of the Au finger electrodes.




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2. Results

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Figure 3. Characterizations of CIGS photocathodes. a) SEM cross sectional image of the CIGS electrode. b) J–V curves of bare CIGS and with overlayer and catalyst coatings under simulated AM 1.5G 100 mW cm−2 solar illumination. c) Stability of CIGS photocathode with overlayer and Pt catalyst coatings under chopped simulated AM 1.5G 100 mW cm−2 solar illumination at 0 V versus RHE.

photoelectrode, for which CIGS (bandgap 1.1 eV) is a promising candidate. Though CIGS solar cells have been commercialized, there are only a few successful trials of using CIGS as photocathodes for solar water splitting,[11a] mainly due to its instability in the electrolyte. The pristine CIGS photocathode in direct contact with water shows a very late onset potential towards water reduction, a consequence of poor charge separation and a high overpotential for hydrogen evolution. To improve this, CdS and intrinsic ZnO were deposited on the surface to form a p–n junction. However, CdS and ZnO are unstable under water splitting conditions, especially when aggressive electrolytes are used. To enable the durable performance of the CIGS photocathode, we employed a similar protection strategy as our lab previously developed for Cu2O photocathodes with atomic layer deposited (ALD) TiO2 layer.[20] Figure 3a shows the cross section scanning electron microscopy (SEM) image of the CIGS photoelectrode after the coating of CdS, ZnO, and TiO2 layers. To further improve the charge transfer from the electrode surface to electrolyte, Pt catalyst was deposited on the surface by PEC deposition. This complete CIGS photocathode reached a plateau photocurrent density of 34 mA cm−2 and an onset potential of 0.6 V versus reversible hydrogen electrode (RHE), as illustrated in Figure 3b, making it one of the best performing photocathodes for water reduction. The disturbances in the J–V curve are due to the formation and release of hydrogen bubbles on the electrode surface. We also investigated the usage of MoSx as an alternative low cost electrocatalyst on the surface, and the same photocurrent density was achieved albeit with a later onset, which is in agreement with the catalytic activity differences between Pt and MoSx.[21] Interestingly, the amplitude of the disturbances of the J–V curves with Pt and MoSx catalysts is in correlation with their binding strength to the H2 molecule. With protection by ALD TiO2, the CIGS photocathode exhibited short-term stable water reduction in an acidic electrolyte (0.5 M H2SO4, pH 0) at 0 V versus RHE, Figure 3c. We acknowledge that even the CIGS photocathode is protected by a thicker TiO2 layer, the stability issue is still not solved. Figure S3a (Supporting Information) shows the extended long-term stability measurement of the same sample. After 1 h chopped illumination, the degradation of the sample starts to accelerate, this is due to the damage of the ZnO and CdS underlayers by the H+ ions, as ALD TiO2 layer is amorphous and permeable for protons. Following this, CIGS is exposed to the electrolyte, and

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the performance increases. Finally, CIGS is slowly corroded by the sulfuric acid and the performance decreases. Figures S3b and S3c (Supporting Information) show the J–V curves of the sample under chopped light illumination and the photos of the sample before and after stability measurement, respectively. It is worth noting that in order to inhibit the formation of a pH gradient, strong acid or base electrolyte should be used for long-term water splitting, though the sample is more stable is neutral pH condition.[22] During the stability measurement under simulated solar illumination, large amounts of hydrogen bubbles could be seen to evolve on the CIGS electrode surface. In order to correlate the photocurrent to the rate of hydrogen evolution, we performed in-line gas chromatography analysis of the evolved gas. Hydrogen was measured every 3 min during the 1 h stability measurement of a CIGS sample at 0 V versus RHE under continuous illumination, and the results are shown in Figure S2 (Supporting Information). The photocurrent measured by the potentiostat correlates well with the calculated photocurrent corresponding to the hydrogen generation, especially in the longer time scale, Figure S4a. Figure S4b shows the Faradic efficiency of each measurement. The low Faradic efficiency at the initial stage is due to the accumulation of hydrogen in the headspace of the measurement cell and the electrolyte. The decrease of the photocurrent from 400 to 600 s is due to the formation of a large bubble on the surface, which fully blocked the electrode surface from 580 s until it broke off. In the steady state (after 1800 s), we found hydrogen generation to be quantitative with respect to the measured photocurrent, confirming that practically all the electrons are going towards the HER. The instability of the sample, which continues in the steady state region, is therefore attributed to non-Faradaic processes, such as chemical corrosion or the loss of active catalyst. In summary, this configuration led to the successful realization of a high-performance CIGS photocathode which, importantly, exhibits the photovoltage necessary for use in a tandem device.

2.3. Constructing PEC–PV Tandem Water Splitting Devices Following the successful development of transparent perovskite PV cells and a CIGS photocathode, Figure S5a (Supporting Information), we combined all the components into devices for complete light-driven water splitting. The tandems




FULL PAPER Figure 4. Spectra response, J–V curves, and current density versus time characteristics. a) AM 1.5G photon flux and the maximum electron flux that the CIGS photocathodes can generate behind the perovskite solar cells, assuming 100% IPCE. b) J–V curves of the CH3NH3PbI3-based solar cell, DSA, and CIGS photocathode behind CH3NH3PbI3 solar cell. c) J–V curves of the CH3NH3PbBr3-based solar cell, DSA, and CIGS photocathode behind CH3NH3PbBr3 solar cell. d) Current density versus time curve of a standalone tandem device composed of CH3NH3PbBr3-based solar cell and CIGS photocathode under chopped simulated AM 1.5G 100 mW cm−2 solar illumination.

were assembled as depicted schematically in Figure S5b (Supporting Information), with the PV cell positioned against the front window of the PEC cell and wired to the photocathode for hydrogen evolution and to a dark anode for oxygen evolution. A commercially available dimensionally stable anode (DSA, Irand Ru-coated Ti plate) was used here, as there are no known Earth-abundant catalysts that are efficient for oxygen evolution and stable in acid. The J–V characteristic of the anode towards oxygen evolution in 0.5 M H2SO4 (pH 0) electrolyte is depicted in Figure S6 (Supporting Information). For the tandem device, the upper bound limit of the current density generated by the bottom electrode can be calculated based on the transmittance spectra of the perovskite top cell and the incident photon flux. Assuming a 100% IPCE for CIGS, the maximum current densities that the photocathode can possibly reach are around 10.7 and 15.9 mA cm−2 when CH3NH3PbI3 and CH3NH3PbBr3 are used as the top cell, respectively, Figure 4a. The operating photocurrent density (Jop) of the overall tandem device is determined by all components, including the perovskite solar cell, CIGS photocathode and dark anode. As they are connected together in series, all components are current matched, and the operating current can be predicted by plotting the J–V curves of the three components together. Among the three, the J–V curves of the CIGS photocathode and the DSA anode can be easily defined


against the RHE potential. Then the operating current density can be determined by sliding the J–V curve of the perovskite cell across the other two until the three components share the same current. The predicted operating point of each device is depicted in Figure 4b,c. The J–V curves of the CIGS photocathodes are measured with the perovskite solar cell in the front as filter, and their plateau current densities match well with that predicted in Figure 4a. The tandem device could deliver photocurrent densities of 2.1 and 5.1 mA cm−2 when CH3NH3PbI3 and CH3NH3PbBr3 are used as the top absorber, respectively, which corresponds to STH conversion efficiencies of 2.6% and 6.3%, respectively. The STH efficiency was calculated based on the ratio of Gibbs free energy of H2 produced to the solar power input with Equation (1),[14c] assuming a 100% Faradaic efficiency (ηF) for water splitting, where Psolar is the incident solar power

ηSTH =

J op ⎡⎣ mAcm −2 ⎤⎦ × 1.23[V] × ηF Psolar ⎡⎣ mWcm −2 ⎤⎦

(1)

To confirm the real performance of a standalone device, chronoamperometry was carried out on the illuminated tandem device with a CH3NH3PbBr3-based solar cell as the top electrode and with zero applied bias, and the result is illustrated in Figure 4d. Note that this is steady state performance, not affected by the



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Figure 5. Analysis of the STH efficiency. The theoretical maximum STH efficiency that can be achieved from a tandem device composed of two absorbers based on the Shockley–Queisser limit considering 500 mV overpotential for water splitting reactions and the current matching of both absorbers.

hysteresis of the perovskite solar cell. With a photocurrent density of around 5 mA cm−2, the device has a STH efficiency exceeding 6%, which is in good agreement with the prediction by the intersection of the J–V curves. The efficiency achieved here is the highest among the PEC–PV water splitting devices with only one single-junction solar cell as the bias source under one sun illumination, and there is still ample room for further improvement. Specifically, for the CH3NH3PbI3 based cell, the tandem efficiency is limited by the photovoltage and light transmittance, though it delivers high photocurrent density. For the CH3NH3PbBr3-based cell, the open circuit potential is higher than what is required, and the efficiency is limited by the smaller photocurrent density resulting from the larger bandgap. The optimal choice would be a mixed halide solar cell blending the large open circuit potential of the CH3NH3PbBr3 solar cell and the high photocurrent density of the CH3NH3PbI3 solar cell, targeting the ideal bandgap range for optimized tandem water splitting. The current device performance is limited by the nonideality of the perovskite component, with the CH3NH3PbBr3 cell producing too little photocurrent due to its narrow absorption range, and the CH3NH3PbI3 cell exhibiting too little transmittance and photovoltage. In order to gain insight into how efficient the present device configuration could perform in the ideal case, we performed a detailed analysis of the two absorber tandem system, Figure 5. The analysis is based on the Shockley–Queisser limit for each absorber, a 0.5 V overpotential for water splitting, perfect transmittance of subbandgap photons, and current matching of both absorbers (see supporting information for details).[23] Specifically, for a tandem cell with CIGS (bandgap 1.1 eV) as the bottom absorber, through varying the bandgap of the top absorbers, the idealized STH efficiency could approach 27%. This would depend on the realization of a ≈1.7 eV bandgap top absorber which is both transparent and efficient. While tunability of the hybrid perovskite bandgaps

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has been demonstrated across the range 1.5–2.3 eV,[24] an efficient device based on a 1.7 eV bandgap has yet to be accomplished. In our efforts to make a device from a 1.7 eV bandgap absorber, the cell failed to achieve the desired open circuit voltage. The realization of such a device, which should exhibit a Voc around 1.2 V, could allow the construction of water splitting tandems approaching the ideal balance of bandgaps and photovoltages, and is a goal for continued study. It is worth mentioning that this analysis is based on the diode ideality factor of 1, corresponding to a fill factor of ≈0.9 for the PV device. Experimentally for the tandem device, because of the fill factor, CH3NH3PbBr3 cell has better performance compared to CH3NH3PbI3 cell here. Additionally, in order to bring down the cost and to improve the scalability of the device, attentions must be paid to both the materials cost and the scalability of the techniques used in their fabrication. For example, Earth-abundant catalysts should be employed both on the photocathode and on the dark anode. There is a crucial need for abundant catalysts that are stable in the extreme pH solutions needed for efficient electrolysis, and research in this area is ongoing. Furthermore, the stability of the photoelectrode must be improved, and the viability of the passivation strategy should be examined in both acid and alkaline solutions. Indeed, this tandem device could operate equally well in basic solutions if proper photoelectrode stabilization was achieved, which would expand the catalogue of abundant anode catalysts which could be employed. Last, the development of perovskite PVs in efficiency, transmittance, and tunability in voltage will boost the efficiency in the future. Certainly, using these absorbers in a tandem PV configuration to drive dark electrodes is another feasible configuration for solar fuels generation. The field of photoelectrochemistry for water splitting, on the other hand, targets the goal of producing a simplified device that can, when immersed in water, directly perform efficient photoelectrolysis without the need for wires, external PVs, voltage converters, or separate electrolyzer units. Work of the nature reported in the present work, and almost all of the recent literature, represents progress in understanding different components that may one day prove suitable as part of such a complete PEC device. Here, the immersed CIGS photoelectrode represents one example.

3. Conclusion We have demonstrated a panchromatic tandem water splitting device comprising a semitransparent and high open circuit voltage perovskite solar cell and a state-of-the-art CIGS photocathode, achieving an STH conversion efficiency of 6%, which to the best of our knowledge is the highest value obtained so far for a PEC–PV water splitting device using only one single-junction solar cell as the bias source under one sun illumination. Analysis shows that the efficiency of more than 20% can be reached experimentally through further optimization of the device.

4. Experimental Section Perovskite Solar Cell Fabrication: Perovskite solar cells were fabricated on FTO glass substrates (TEC 15) covered with a 50 nm compact TiO2





(Praezisions Glas & Optik GmbH) to match the emission spectrum of the lamp to the AM 1.5G standard. Before each measurement, the exact light intensity was determined using a calibrated Si reference diode equipped with an infrared cutoff filter (KG-3, Schott). IPCE spectra were recorded as a function of wavelength under a constant white light bias of approximately 5 mW cm−2 supplied by an array of white-light-emitting diodes. The excitation beam coming from a 300 W xenon lamp (ILC Technology) was focused through a Gemini-180 double monochromator (Jobin Yvon Ltd) and chopped at approximately 2 Hz. The signal was recorded using a Model SR830 DSP Lock-In Amplifier (Stanford Research Systems). All measurements were conducted using a nonreflective metal aperture of 0.159 cm2 to define the active area of the device and avoid stray light to enter the cell from the sides. PEC Measurements: The PEC performance of the photocathodes was studied using an Ivium electrochemical workstation to acquire the photoresponse under simulated AM 1.5G illumination (100 mW cm−2) from a 450 W Xe-lamp (Lot-Oriel, ozone-free) equipped with an AM 1.5G filter (Lot-Oriel), calibrated with a silicon diode. Current–voltage measurements were carried out in a three-electrode configuration with the CIGS photocathodes as the working electrode, a Pt mesh as the counter electrode, and Ag/AgCl/sat. KCl as the reference electrode, in an electrolyte solution of 0.5 M H2SO4 at pH 0. A scan rate of 10 mV s−1 in the cathodic direction was used to acquire the data. IPCE measurements were performed under light from a 300 W xenon lamp with integrated parabolic reflector (Cermax PE 300 BUV) passing through a monochromator (Bausch & Lomb, bandwidth 10 nm FWHM) in three-electrode configuration at 0 V versus RHE. Comparison with a calibrated Si photodiode allowed the calculation of the IPCE. The stability of the electrodes was monitored by chronoamperometry at 0 V versus RHE under chopped and simulated AM 1.5 G illumination (100 mW cm−2). The oxygen evolution reaction activity of the DSA electrode was characterized by scanning from positive to negative potential at the RHE scale in a three electrode configuration at a scan rate of 1 mV s−1. The current of the standalone tandem water splitting cell was recorded by chronoamperometry without applying an external bias for different time periods under chopped AM 1.5G illumination. All measurements were conducted using the nonreflective metal apertures of 0.16 cm2 on perovskite solar cell to define the active area of the device and avoid light scattering through the sides. Faradaic Efficiency Measurements: Faradaic efficiency measurements were carried out as follows. Pt-catalyzed and TiO2-protected CIGS samples were fixed into a gas tight PEC cell, filled with 0.5 M H2SO4 which was kept under constant stirring. The photoelectrode was polarized at 0 V versus RHE using a potentiostat (Iviumstat, Ivium) and illuminated through a quartz window (Edmund Optics). The active area is defined by the epoxy with the glue gun, which is ≈0.2 cm2. A short Pt wire was used as counter electrode in the same compartment. Simulated AM 1.5G illumination calibrated to 1 sun intensity was supplied by a 450 W Xe light source (LOT Oriel) combined with an AM 1.5 filter (LOT Oriel). Ar gas (99.9999%, Carbagas) was sparged into the electrolyte at a constant rate of 5.00 (±0.5%) mL min−1, set by means of a mass flow controller (Bronkhorst HIGH-TECH). The resulting product gases passed through the sample loop of a gas chromatograph and analysis was carried out at an interval of 3 min. The chromatograph (Trace ULTRA, Thermo Scientific) was equipped with a ValcoPLOT column (FS, Molsieve 5A, 30 m, 0.53 mm, 20 um film, Vici) and a PDD detector (Vici). Helium (99.9999%, Carbagas) was used as carrier gas. A certified hydrogen standard (Carbagas) was used to calibrate the measurement. The measured molar flow of hydrogen was compared to the observed photocurrent density, yielding the faradaic efficiency. Due to the initial accumulation of hydrogen in the measurement cell’s headspace and electrolyte, a steady state is only reached after a few measurements.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.



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layer to which a 300 nm mesoporous TiO2 layer was superimposed. The compact and mesoporous TiO2 layers were screen printed using the BL-1 and diluted-30NRD pastes from Dyesol, respectively. After screen printing, each layer was sintered at 500 °C for 30 min. Before perovskite deposition, the TiO2 substrates were treated with 50 × 10−3 M TiCl4 solution for 30 min. Perovskite deposition followed the procedures of previously reported solvent engineering method.[25] For CH3NH3PbI3 deposition, 1.2 M of CH3NH3I and PbI2 were dissolved in a mixed solvent of γ-butyrolactone (GBL) and dimethyl sulfoxide (DMSO) with volume ratio of 7:3. The precursor solution was spin coated at 1000 rpm for 20 s, then increasing to 5000 rpm for 10 s. At the seventh second of the second spin coating step, 500 µL of toluene was added to induce fast crystallization of the perovskite. The substrates were annealed at 100 °C for 10 min after spin coating. For deposition of the CH3NH3PbBr3 perovskite, the precursor solution contained 1.2 M of CH3NH3Br and PbBr2, and a smaller amount of toluene (50 uL) was used during the second spin coating step for the morphology control. Transparent, free-standing CNT films synthesized from chemical vapor deposition were used to fabricate transparent perovskite solar cell.[17b] The CNT film was laminated on perovskite surface. Polymethyl methacrylate (PMMA) solution (10 mg mL−1 in toluene) was spin coated on CNT film to increase CNT adhesion and provide extra protection to perovskite absorber. Finally, gold finger electrodes were deposited on PMMA using vacuum evaporation to reduce the series resistance and improve the fill factor of the solar cells. CIGS Photocathode Fabrication: The CIGS photocathode was fabricated with a modified procedure as reported before.[26] CIGS layers were grown by coevaporation from elemental effusion cells in a highvacuum chamber (base pressure ≈10−8 hPa) equipped with an additional effusion cell for NaF. The so-called NaF postdeposition treatment was carried out by evaporating NaF (≈2 nm min−1 for 20 min) in the presence of Se onto the finished CIGS layer at a substrate temperature of about 350 °C. The average [Ga]/([In]+[Ga]), [Cu]/([Ga]+[In]) and film thickness of the CIGS layers were in the range of 0.38–0.40, 0.79– 0.85 and 2.3–2.5 µm, respectively, as determined by means of X-ray fluorescence measurements. The 50–80 nm thick CdS buffer layer was grown by chemical bath deposition (CBD) from an aqueous solution of Cd acetate, thiourea, and ammonium hydroxide heated to 70 °C on a water bath under slow stirring. The samples were then washed with high-purity water and dried with an ionized N2 gun, before an annealing step at 180 °C in air for 2 min. An i-ZnO with thickness of around 50–60 nm was deposited by RF magnetron sputtering to passivate CdS from air. The TiO2 protecting layer was deposited by atomic layer deposition (Savannah 100, Cambridge Nanotech) at 150 °C using tetrakis-dimethylamino titanium (TDMAT) and H2O2 as the Ti and O precursors, respectively. To ensure appreciable vapor pressure, TDMAT was heated to 75 °C. Typically, 1700 cycles deposition gives 100 nm TiO2 film. To enhance the kinetics of the water reduction reaction, platinum (Pt) catalyst was galvanostatically photodeposited on the surface from a solution of 1 × 10−3 M H2PtCl6 in deionized water at a current density of −8 µA cm−2 for 15 min.[20b] MoSx catalysts were deposited by consecutive cyclic voltammograms under Xe lamp illumination with an aqueous solution of 0.2 × 10−3 M (NH4)2MoS4 in 0.1 M NaClO4 at a pH of 6.8. The cyclic voltammograms were performed between 0.2 and 1.7 V versus RHE at a scan rate of 50 mV s−1, starting and ending at the cathodic potential.[21a] Material Characterizations: The transmission spectra of perovskite solar cells were measured by a Shimadzu UV-3600 UV–vis–NIR spectrophotometer (Shimadzu) through the Au finger electrode with a metal aperture of 0.16 cm2 size. The morphology of the CIGS films was characterized using a high-resolution scanning electron microscope (ZEISS Merlin). Solar Cell Performance Measurements: The current–voltage characteristics were recorded by applying an external potential bias to the cell while recording the generated photocurrent with a digital source meter (Keithley Model 2400). The light source was a 450 W xenon lamp (Oriel) equipped with a Schott K113 Tempax sunlight filter

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Acknowledgements The authors thank Dr. Néstor Guijarro and Prof. Kevin Sivula for the discussion and use of their spectrophotometer. J.L. would like to thank EPFL Fellowship co-funded by Marie Curie from the European Union's Seventh Framework Programme for research, technological development, and demonstration under Grant Agreement No. 291771, PECDEMO project co-funded by Europe’s Fuel Cell and Hydrogen Joint Undertaking (FCH JU) under Grant Agreement No. 621252, and NanoTera NTF project (TANDEM) for financial support. M.T.M. and M.G. acknowledge the Swiss Federal Office for Energy (PECHouse project). P.C. thanks Swiss National Science Foundation (SNSF) NRP70 project no. 154002. L.Z and L.H.W acknowledge the funding from National Research Foundation (NRF), Singapore through the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) CREATE program. Received: July 29, 2015 Revised: September 17, 2015 Published online:

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