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High-performance a-Si/c-Si heterojunction photoelectrodesfor photoelectrochemical oxygen and hydrogen evolution

Hsin-Ping Wang, Ke Sun, Sun Young Noh, Alireza Kargar,Meng-Lin Tsai, Ming-Yi Huang, Deli Wang, and Jr-Hau He

Nano Lett., Just Accepted Manuscript ? DOI: 10.1021/nl5041463 ? Publication Date (Web): 09 Feb 2015

Downloaded from http://pubs.acs.org on February 18, 2015

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High-performance a-Si/c-Si heterojunction photoelectrodes for photoelectrochemical oxygen and hydrogen evolution Hsin-Ping Wang,?,? Ke Sun,? Sun Young Noh,? Alireza Kargar,? Meng-Lin Tsai,? Ming-Yi Huang,§ Deli Wang, ?,∥,?,* and Jr-Hau He?,?,* 15161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960

?Computer, Electrical and Mathematical Sciences and Engineering (CEMSE) Division, King Abdullah University of Science & Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ?Department of Electrical and Computer Engineering, University of California-San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States §Advanced Technology Department AU Optronic Corporation, Taichung, Taiwan, ROC ∥Materials Science and Engineering Program, University of California-San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States ?Qualcomm Institute, University of California-San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States *Corresponding author e-mail: jrhau.he@kaust.edu.sa; d.w.wang@ieee.org Abstract Amorphous Si (a-Si)/ crystalline Si (c-Si) heterojunction (SHJ) photoelectrochemical cells can serve as highly efficient and stable photoelectrodes for solar fuel generation. Low carrier recombination in the photoelectrodes leads to a high photocurrent and high photovoltage. Both SHJ photoanodes and photocathodes are designed for high efficiency oxygen and hydrogen evolution. The SHJ photoanode 2 ACS Paragon Plus Environment

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with sol-gel NiOx as the catalyst shows the current density of 21.48 mA/cm2 at the equilibrium water oxidation potential. The SHJ photocathode displays excellent hydrogen evolution performance with an onset potential of 0.640 V and a solar to hydrogen conversion efficiency of 13.26%, which is the highest ever reported for Si-based photocathodes. 15161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960

Keywords: Si heterojunction photoelectrodes, oxygen evolution, hydrogen evolution, oxygen conversion efficiency, hydrogen conversion efficiency 3 ACS Paragon Plus Environment

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Due to the steep increase in global energy demand and the serious effects of climate change, exploiting consecutive, eco-friendly, and cost-effective energy resources has become the top priority of research and development. The sun delivers energy to the earth in less than two hours sufficient to provide our annual energy consumption. Solar energy is such a large source of energy that it has attracted researchers to develop the technology needed to convert intermittently available solar energy into 15161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960

stable and storable fuels, for example by the splitting of water into hydrogen and oxygen by a combination of photovoltaic (PV) and electrolyzer technology. In order to achieve efficient self-biasing photoelectrochemical (PEC) systems, photoelectrodes are required that can supply a thermodynamic potential of 1.23 V for spontaneous overall water splitting (considering all losses, the band gap required for practical devices is about 1.6-2.4 eV)1 while at the same time to absorbing sufficient solar energy. Single materials with suitable band gaps for splitting water have usually been limited by insufficient absorption of the visible and infrared spectra. Also, for those materials with the right band gap, the conduction band or valence band of the photoelectrodes should properly aligned for water reduction or oxidation. Additionally, it is important to have stable PEC materials that are not prone to decomposition or corrosion. There is currently no single material which can be used for efficient solar water splitting. To overcome this problem, it is necessary to combine two or more materials with different band gaps to absorb a larger portion of the solar spectrum while simultaneously providing the necessary overall photovoltage to split water.2 Photocathode+photoanode configurations or tandem absorber+electrocatalysts configurations have the potential to be used in self-biasing integrated PEC water-splitting systems. It has been reported that two-photoabsorber configurations with a top-junction band gap of 1.65-1.8 eV and bottom-junction band gap of 4 ACS Paragon Plus Environment

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structures with enhanced performance, not only the light-harvesting ability but also the passivation problem have to be seriously considered. Note that hierarchical structures are promising candidates satisfying both requirements, with related applications now under investigation.25,37 Finally, the high ???/?? and ???‘–??‘??of our SHJ photocathodes with their pyramidal structure lead to a world-record high efficiency of 13.26% among all kinds of Si-based photocathodes. 15161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960

Because the same amount of incident light enters the SHJ PEC cells (the Pt catalyst was coated on the back sides of the cells), the JSC and VOC of the SHJ photocathodes are independent of the Pt thickness. The major difference in the photocathodic behavior is the FF, i.e., the catalytic ability. The catalytic activity of Pt reaches its highest value and then decreases slightly with an increase in thickness of the Pt, which is consistent with other researchers’ results.38-40 For a small amount of Pt (1-nm-thick Pt), the low catalytic ability might arise from insufficient catalyst loading and the non-crystalline nature of the material.39 A large fraction of Pt atoms are on the surface. The surface interaction between the Pt atoms and ITO layers restricts the Pt crystallization. With the increase in Pt loading, the Pt atoms assemble in a more spherical fashion in order to minimize the surface energy, which is of benefit to Pt crystallization and carrier transport. Further increasing the amount of Pt leads to a slight decrease in the slope of the photocathodic J-E curve perhaps due to the formation of grain boundaries arising from the assembly of Pt nanoparticles39 and lowering the surface to volume ratio (i.e., limited surface reaction sites), leading to a higher charge transfer resistance. The charge transfer resistance is investigated by electrochemical impedance spectroscopic (EIS) analysis (See Figure S7 in Supporting Information). The charge transfer resistance obtained from EIS can help to determine the active surface area and catalytic activity. The 2-nm-thick Pt shows the lowest 15 ACS Paragon Plus Environment

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charge transfer resistance, demonstrating the highest catalytic ability. The incident-photon-conversion efficiency (IPCE). The IPCE was measured as a function of the wavelength for the SHJ photocathode with 2-nm-thick Pt at 0 V vs. RHE (Figure 3b). The spot size (Aph = 0.1 mm × 0.2 mm) of the monochromatic light on the sample surface was much smaller than the sample size. This was considered in the IPCE calculation. The photocurrent was calculated using the equation below: 15161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960

????=????????, where ?? is the dark current and ??? is the current measured under light illumination; and ??? is the light spot size. IPCE=???????????????=???????? (3) ?????, ?where λ is the wavelength of incident light; ?????? is the photocurrent from the Si photodetector; ?? is the responsitivity of the Si photodetector provided by the supplier; and ???? is the external quantum efficiency of the Si photodetector. The IPCE of the SHJ PEC cells matches the EQE of the SHJ cells well. The IPCE rises quickly and reaches over 70% in the visible light region from 450 to 1000 nm with the highest value of 94% at 850 nm. The high IPCE over the broad-band wavelength range indicates the broad-band light absorption and efficient carrier transport in the SHJ cells and between the SHJ cell/ITO/Pt electrodes. Working pH range and stability. In order to demonstrate that the SHJ photocathode can efficiently work over a range of pH values, water reduction by the SHJ photocathode with 2-nm-thick Pt was measured under AM 1.5G illumination in acidic (1 M H2SO4), neutral (1 M PBS buffered Na2SO4) and basic (1 M NaOH) electrolyte solutions (Figure 3c). The characteristics of the photocathode are summarized in Table 4. Due to the buried junctions of the SHJ PEC cells, the photovoltage of the solar-driven hydrogen evolution is not dependent on the pH of the 16 ACS Paragon Plus Environment

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electrolyte.41 The photovoltage can be further confirmed by comparing the VOS of the 2-nm Pt-coated SHJ photocathode under illumination with the VOS of the 2-nm Pt-coated ITO film in the dark in the 1 M Na2SO4 and 1 M NaOH electrolyte, respectively (See Figure S8 in Supporting Information). In addition to the performance tests carried out in different pH electrolytes, we also evaluated the lifetime of the SHJ photocathodes in different pH electrolytes. Figure 3d shows the 15161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960

results of stability tests performed on a SHJ photocathode sample with an area of 0.83 cm2 in both the 1 M acidic electrolyte (pH=0.4) at -0.0236 V vs. NHE (0 V vs. RHE) and 1 M neutral electrolyte (pH=7.25) at -0.764 V vs. NHE (0.336 V vs. RHE, where the JSA occurred). The stability of the SHJ photocathode was tested for up to 10 h of operation in a 1 M acidic electrolyte, and was excellent. The inset to Figure 3d shows the results of stability test for 1 h switching off/on of the light illumination. The current density returns back to the original value every time it is switched, revealing the good photoactivity of the SHJ photocathode after the 3 hour stability test. After 10 h of operation, the current density showed a decay of 8.37 mA/cm2, which could be attributed to the decrease in resistivity of ITO due to reduction under reductive conduction, and then the loss of Pt due to unstable ITO support.42 Operation in a neutral electrolyte showed longer stability (with a 6.38 mA/cm2 decay after 20 h of operation) than operation in an acidic electrolyte. Figure S9 in Supporting Information shows the stability of SHJ solar cell. The photocurrent is very stable after 20 h of operation, demonstrating that the light-induced degradation in a-Si can be negligible due to the usage of ultrathin a-Si:H.18 Therefore, the decrease in current density of SHJ photocathodes is mainly from electrical loss of ITO and less amount of Pt. Comparison of Si-based photocathodes. The high performance of the SHJ photocathode was highlighted by comparison with several quintessential Si-based 17 ACS Paragon Plus Environment

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photocathodes, including homojunction c-Si,12 a-Si,11 and metal-insulator-semiconductor (MIS) photocathodes.43 The SHJ photocathode exhibited a significantly greater current density compared to any other previously reported for Si photocathodes. Almost no current density was generated at 0.6 V vs. RHE for the homojunction c-Si and MIS photocathodes. To enable water splitting without an external voltage bias, Si-based photoelectrodes need to gain additional 15161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960

energy through the multijunction design.4 For a two-component illumination design, a Si photocathode can be integrated with a photoanode. The overall water splitting efficiency (i.e., solar-to-hydrogen (STH) production efficiency, ????) of the photoelectrolysis system is estimated by overlapping the individual J-V curve obtained for each photocathode and photoanode, and can be calculated using eq. 4,1,44 ????=????(?.???)???, (4) where JOP is the maximum operating current density for the integrated PEC system, which is the current density at the intersection of the two J-V curves. In an integrated PEC system for achieving current matching, the final JOP and ???? of a serial combination of photoelectrodes is limited by the photoelectrode with the smallest photocurrent. The high photocurrent of our SHJ photoelectrodes makes them a good candidate for the bottom cell in a two-component illumination design for obtaining high-efficiency water splitting without an external voltage bias. The outlook for SHJ photoelectrodes. Finally, we replaced the Pt catalyst with the cost-effective Ni (Figure 3f). The catalytic performance of Ni was optimized as shown in Figure S10 in Supporting Information. The photocathode characteristics are summarized in Table 5. The SHJ photocathode with Ni showed a high current density (>30mA/cm2) at 0 V vs. RHE under 1-sun AM 1.5G illumination. This result demonstrates the potential of SHJ photocathodes for cost-effective applications. It is 18 ACS Paragon Plus Environment

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worth noting that these SHJ photoelectrodes can still be further optimized, by including light-trapping structures for enhancing light absorption and surface reactive sites, grid contact electrodes for improving carrier collection, and protective layers for prolonged stability. When sunlight passes through a liquid, the light is easily scattered (diffused). Accordingly, omnidirectional performance of the photoelectrode is desired. Hierarchical structures have been shown to exhibit favorable omnidirectional 15161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960

broad-band light-trapping ability without causing serious recombination loss.25,37,45 Improving carrier transport by the use of grid contact electrodes can increase the FF, which could potentially lead to a steep increase in the JOP of an integrated PEC system.41 By optimizing these conditions, it should be possible to achieve an SHCE for a SHJ photocathode of higher than 15%. The low carrier recombination SHJ photoelectrochemical cell is an attractive candidate for high efficiency oxygen and hydrogen evolution. 19 ACS Paragon Plus Environment

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