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Time-Resolved Observations of Photo-Generated Charge Carrier Dynamics in Sb2Se3 Photocathodes for Photoelectrochemical Water Splitting Wooseok Yang, Seungmin Lee, Hyeok-Chan Kwon, Jeiwan Tan, Hyungsoo Lee, Jaemin Park, Yunjung Oh, Hyunyong Choi, and Jooho Moon ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05446 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018
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Time-Resolved Observations of Photo-Generated Charge Carrier Dynamics in Sb2Se3 Photocathodes for Photoelectrochemical Water Splitting Wooseok Yang1, Seungmin Lee2, Hyeok-Chan Kwon1, Jeiwan Tan1, Hyungsoo Lee1, Jaemin Park1, Yunjung Oh1, Hyunyong Choi2, and Jooho Moon*1 1
Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea 2
School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea
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ABSTRACT
Solar energy conversion by photoelectrochemical (PEC) devices is driven by separation and transfer of photo-generated charge carriers. Thus, understanding carrier dynamics in a PEC device is essential to realize efficient solar energy conversion. Here, we investigate time-resolved carrier dynamics in emerging low-cost Sb2Se3 nanostructure photocathodes for PEC water splitting. Using terahertz spectroscopy, we observed an initial mobility loss within tens of picoseconds due to carrier localization and attributed the origin of carrier localization to the rich surface of Sb2Se3 nanostructures. In addition, a possible recombination at the interface between Sb2Se3 and the back contact is elucidated by time-resolved photoluminescence analysis. We also demonstrated the dual role of the RuOx co-catalyst in reducing surface recombination and enhancing charge transfer in full devices using intensity-modulated spectroscopy. The relatively low onset potential of the Sb2Se3 photocathode is attributed to the sluggish charge transfer at a low applied bias, rather than to fast surface recombination. We believe that our insights on carrier dynamics would be an important step towards achieving highly efficient Sb2Se3 photocathodes.
KEYWORDS: photoelectrochemical water splitting, Sb2Se3, carrier dynamics, terahertz spectroscopy, time-resolved photoluminescence, intensity-modulated spectroscopy
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Reducing energy-related CO2 emissions is one of the most important challenges facing the 2 °C scenario for regulating global warming within 2 °C by 2050.1 Photoelectrochemical (PEC) water splitting is considered an attractive approach to produce hydrogen energy carriers with a low carbon footprint from omnipresent water by harnessing solar energy.2 In PEC water-splitting devices, the equilibration of the semiconductor’s chemical potential (Fermi level) with the oxidation/reduction potential of the electrolyte can create an electric field (referred to as the built-in field) at the semiconductor-liquid junction (SCLJ). Upon absorption of a solar photon, the semiconductor transforms the absorbed photons into electrons and holes that are subsequently separated by the built-in field. The excited electronic states travel to the SCLJ and are eventually transferred to the electrolyte to drive electrochemical reactions, such as oxidation and reduction of water to generate oxygen and hydrogen gases, respectively.3, 4 In the hydrogen evolution reaction (HER), for example, the performance of a PEC is contingent on the contribution of the generated electrons to the HER by avoiding possible losses, such as initial photoconductivity decay, recombination at bulk/interfaces, and sluggish charge transfer.5-8 Therefore, understanding charge carrier dynamics is of significant importance to propose strategies to suppress undesirable losses and facilitate charge transfer. Antimony selenide (Sb2Se3) has recently emerged as a promising non-toxic and low-cost light absorber for solar energy conversion devices. After its successful application in thin-film solar cells by Jiang Tang’s group,9-11 Sb2Se3 photocathodes for PEC water splitting were also developed. Our group demonstrated hydrogen production from Sb2Se3-based photocathodes consisting of Sb2Se3 nanoneedles decorated with TiO2 and Pt.12 In addition, it was demonstrated that earth-abundant MoSx as a co-catalyst can replace precious Pt in Sb2Se3 photocathodes.13 Prabhakar et al. reported that low-temperature sulfurization of a Sb2Se3-MoSx stack induced an
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improved PEC performance, with a high photocurrent density of 16 mA cm–2 at 0 V vs. a reversible hydrogen electrode (VRHE) in strongly acidic media.14 An anti-photocorrosive CdS/TiO2 bilayer was also deposited on Sb2Se3 photocathodes to enhance PEC performance and stability in the neutral electrolyte; it led to a photocurrent density of 8.5 mA cm–2 at 0 VRHE and stable photocurrent for over 10 h.15 Despite these notable developments, the obtained photocurrent density is still far below the theoretical maximum value (about 40 mA cm–2 assuming 1.2 eV band gap and 100% incident photon-to-current efficiency) and the onset potential (0.3–0.4 VRHE) is smaller than that of other low-cost photocathodes, such as Cu2O16 and Cu2ZnSnS4.17,
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It is noteworthy that the onset potential of Cu2O photocathode is largely
enhanced by using Ga2O3 buffer layer owing to the favourable band alignment between Cu2O and Ga2O3.16 The case of Cu2O photocathode implies that the onset potential of Sb2Se3 photocathode could also be enhanced by exploring proper buffer materials to produce enough photovoltage by p-n junction formation. In other words, this is due to the fact that the development history of Sb2Se3 photocathodes is relatively short and there are very few reports on the utilization of Sb2Se3 as photocathodes for PEC water splitting. Considering its great potential, further in-depth investigation is thus urgently needed. Here, we investigate the photo-generated charge carrier dynamics in Sb2Se3nanostructured photocathodes in a broad timescale ranging from a few picoseconds (ps) to a few milliseconds (ms). Sb2Se3 nanorod arrays are used as light absorbers protected by TiO2 while RuOx, which is relatively inexpensive and more robustly bound to TiO2 compared to Pt,19 is used as a co-catalyst for the hydrogen evolution reaction. The RuOx/TiO2/Sb2Se3/Au/fluorine-doped tin oxide (FTO) photocathode exhibits a photocurrent of ~10 mA cm–2 at 0 VRHE as well as a stable operation over 2 h. We conducted several time-resolved analyses, such as the ultrafast
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terahertz (THz) spectroscopy, time-resolved photoluminescence (TRPL) analysis, and intensitymodulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) to elucidate the unexplored carrier dynamics of Sb2Se3 photocathodes. From carrier localization at a ps timescale to charge transfer and surface recombination, within a few hundreds of ms, involved in fully working devices are investigated. By carrying out these analyses in a wide range of timescales, we aim to bridge the gap between fundamental carrier transfer properties and device operation while identifying the advantageous inherent features of Sb2Se3 as a photocathode in conjunction with addressing its limitations and providing insights for further improvement.
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RESULTS AND DISCUSSION
Figure 1. Scanning electron microscopy (SEM) images of (a) bare Sb2Se3, (b) TiO2/Sb2Se3, and (c) RuOx/TiO2/Sb2Se3 on an Au/FTO substrate (scale bar = 300 nm). (d) Linear sweep voltammogram (LSV) curves of a RuOx/TiO2/Sb2Se3/Au/FTO photocathode under intermittent one-sun illumination in a H2SO4 electrolyte (pH ~ 1). (e) IPCE of a RuOx/TiO2/Sb2Se3 photocathode biased at 0 V vs. RHE. Integration of the IPCE values with the solar AM 1.5 spectrum (black line) yields the same photocurrent value (dotted line) as that observed under one-sun illumination in (c). (f) Chronoampherometry (CA) curve of a RuOx/TiO2/Sb2Se3 photocathode biased at 0 V vs. RHE. Figure 1a shows the scanning electron microscopy (SEM) image of bare Sb2Se3 nanostructures. As reported in our previous study,20 TGA 5 (i.e., molar ratio of TGA:EA = 5:95) Sb-Se solutionderived Sb2Se3 reveals the morphology of 1-dimensional (1D) nanorod arrays. The thickness of the nanorods increases uniformly upon the atomic layer deposition of TiO2 (Figure 1b), which is
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indicative of the conformal coating of TiO2 on Sb2Se3 nanorods. The roles of TiO2 and band alignment between Sb2Se3 and TiO2 were already described in our previous work.12, 13 The RuOx co-catalyst was deposited by a galvanostatic method as described in the Experimental Section (Figure 1c). The PEC performance of the RuOx/TiO2/Sb2Se3/Au/FTO photocathode was measured with respect to a sample area of 0.6 cm2 in argon-purged 0.1 M aqueous H2SO4 (pH ~ 1). As shown in Figure 1d, the photocurrent of the RuOx/TiO2/Sb2Se3/Au/FTO photocathode increases upon repeating the linear sweep voltammogram (LSV) measurements due to the initial electrochemical reduction of RuOx (the so-called activation of RuOx).19 The photocurrent reaches a value as high as 10 mA cm–2 at 0 VRHE, which is consistent with the IPCE, as shown in Figure 1e, revealing the light-harvesting capability of our Sb2Se3 photocathodes up to a photon wavelength of 1000 nm. Despite the promising photocurrent density owing to wide-range light harvesting, the maximum IPCE value is ~34%, indicating that there is room for further improvement. In the chronoamperometry (CA) measurements, photocurrent gradually increased until 30 min, implying that the activation of RuOx occurred for the first 30 min (Figure 1f). The brief drop to zero current resulting from blocking the illumination shows that the measured current indeed originated from photo-generated electrons. The photocurrent retains 100% of its initial value for 2 h, after which it slightly decreases. A comparison of photocurrent and stability with currently reported Sb2Se3 photocathodes are shown in Table S1.
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Figure 2. (a) Temporal decay of the pump-induced change in THz transmission upon excitation of Sb2Se3. (b) Diffusion length and effective mobility of Sb2Se3 nanostructure as a function of fluences calculated from the photoconductivity and time constant values.
The carrier dynamics involved in our Sb2Se3 nanostructures at an early stage (
