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Letter

Cu-doped NiOx as Effective Hole-Selective Layer for High-Performance Sb2Se3 Photocathode for Photoelectrochemical Water Splitting Hyungsoo Lee, Wooseok Yang, Jeiwan Tan, Yunjung Oh, Jaemin Park, and Jooho Moon ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00414 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 6, 2019

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ACS Energy Letters

Cu-doped NiOx as Effective Hole Selective Layer for High Performance Sb2Se3 Photocathode for Photoelectrochemical Water Splitting Hyungsoo Lee†, Wooseok Yang†, Jeiwan Tan, Yunjung Oh, Jaemin Park, and Jooho Moon* Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea Corresponding Author *E-mail: [email protected]

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ABSTRACT

Although antimony triselenide (Sb2Se3) has been intensively investigated as a low-cost p-type semiconductor for photoelectrochemical (PEC) water splitting, most previous studies focused on only the top interface of Sb2Se3 photocathodes. Herein, a solution-processed Cu-doped NiOx (Cu:NiOx) thin film is proposed as an effective bottom contact layer for the Sb2Se3 photocathode. The photocurrent density of the Sb2Se3 photocathode is improved to a record-high level of 17.5 mA cm–2 upon the insertion of Cu:NiOx capable of blocking the recombination at the back interface, while facilitating hole extraction. Electrochemical impedance spectroscopy and intensity-modulated photocurrent spectroscopy, in conjunction with other observations, indicate that the enhanced photocurrent is due to the improved quality of the bottom contact without a noticeable change in the top interface. This study not only provides new insight into the role of the bottom contact layer in photocathodes, but also is an important step toward efficient PEC H2 production via a solution-processable Earth-abundant photoelectrode.


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TOC GRAPHICS

0 2

Current density (mA cm )

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-5 -10 -15 -20 -25 -0.1

FTO/Sb2Se3/TiO2/Pt FTO/Cu:NiOx/Sb2Se3/TiO2/Pt

0.0

0.1

0.2

0.3

0.4

Potential (V vs. RHE)


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To address the drastically increasing demand for clean energy and the abnormal acceleration of global warming, ecofriendly energy production has received considerable research attention.1 Photoelectrochemical (PEC) water splitting is a promising technology for converting solar energy into a C-free H2 fuel suitable for transportation, storage, and utilization.2,3 Solar-to-H2 PEC conversion primarily involves light absorption by a semiconductor, resulting in the generation of excited electron–hole pairs. These oppositely charged carriers should be effectively separated to avoid recombination and eventually transported to the electrolyte side to drive redox reactions.4,5 In a photocathode comprising a p-type semiconductor, for example, the photoexcited electrons should drift to the semiconductor–electrolyte interface to drive the hydrogen evolution reaction (HER), while the holes are transported to a counter electrode via the back contact and external circuit, followed by the oxygen evolution reaction.6 To achieve a high PEC performance, suppressing the charge recombination and facilitating the extraction of the photoexcited charges is necessary. One of the widely used strategies to enhance the charge extraction in the photocathode for PEC water splitting is to form a p-n junction by depositing an n-type semiconductor on the surface of a p-type semiconductor.7 Upon the formation of the p-n junction, the equilibration of the Fermi level leads to a built-in potential, promoting charge separation. Even after the separation of the carriers, the electrons accumulated at the surface of the semiconductor can recombine owing to the sluggish kinetics of the HER; therefore, cocatalysts having high HER activity are generally deposited on the surface of semiconductors.3,8,9 The slow injection of the photoexcited holes to the back contact is likely to induce charge accumulation at the interface between the back contact and the semiconductor, resulting in recombination at the back interface.10 Thus, interface engineering at both the top and bottom


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ACS Energy Letters

junctions of the photocathode should be properly performed to improve the performance of the PEC device. Antimony triselenide (Sb2Se3) has emerged as a nearly ideal p-type semiconductor material for PEC photocathodes because of its desirable characteristics, i.e., its narrow bandgap, high absorption coefficient, phase stability, and easy processability, as well as the abundance of the constituent elements.7,11-13 Recently, an Sb2Se3-based photocathode exhibited remarkable performance for H2 production through the deposition of an n-type overlayer and a co-catalyst. The first reported Sb2Se3 photocathode employed n-type TiO2 and a Pt co-catalyst.7 In addition, Earth-abundant MoSx was employed on top of the Sb2Se3/TiO2 junction as a co-catalyst.14 On the other hand, Prabhakar et al. deposited MoSx directly onto Sb2Se3 without TiO2, where the MoSx presumably functioned as a n-type overlayer and a co-catalyst simultaneously.15 After the sulfurization of the Sb2Se3-MoSx stack, the photocurrent density reached 16 mA cm–2 at 0 V versus a reversible hydrogen electrode (RHE, VRHE). Moreover, n-type CdS was reported as a buffer layer between n-type TiO2 and Sb2Se3 for enhancing the photovoltage in a neutral electrolyte.16,17 Recently, C60 was introduced as a charge-transfer promoter between TiO2 and Pt, significantly enhancing the stability for up to 10 h.18 All the aforementioned studies involved interfacial engineering only on the top of the Sb2Se3 absorbers. However, a recent study on the carrier dynamics of a Sb2Se3 photocathode clearly addressed the possibility of severe charge recombination at the back interface,19 because the conduction-band minimum (CBM) of Sb2Se3 is located close to the work function of the back contact electrode, such as fluorine-doped tin oxide (FTO) or Mo.20,21 To fully utilize the photogenerated carriers, interfacial engineering at the junction between the Sb2Se3 absorber and the back contact for hampering the recombination at the bottom interface should be investigated. In this regard, a hole-selective layer (HSL), which is


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capable of transporting holes only while preventing electron backflow, is required for improving the interface quality of Sb2Se3 and the back contact.22-25 P-type semiconducting materials such as NiOx,26-29 V2O5,30,31 CuSCN,30,32,33 and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)27 are generally used as a hole-selective contact in photovoltaic applications owing to their sufficient hole conductivity and solution processability.34 However, for application in a hole-selective interfacial layer of a Sb2Se3 photocathode, p-type materials should satisfy stringent requirements. First, the CBM of the HSL should be higher than that of Sb2Se3 to block back electron transfer. Second, the valence-band maximum (VBM) of the HSL should be located close to the work function of the back contact and the VBM of Sb2Se3 for rapid hole transfer. Third, the HSL must be thermally and chemically stable, without dissolution or decomposition, during the fabrication of Sb2Se3. Considering these requirements, NiOx is a promising candidate as an HSL for Sb2Se3 photocathodes. A high-quality NiOx thin film that maintains high chemical and thermal stabilities can be easily obtained via a sol-gel process.26,27 Additionally, NiOx—having a wide bandgap—has a more negative CBM than Sb2Se3, allowing the blockage of electron backflow. Moreover, the VBM of NiOx is in the range of −5.4 to −5 eV;29,32,35 thus, NiOx can render proper band alignment with respect to Sb2Se3. Recently, Zhang et al. investigated alternative back contacts for Sb2Se3 solar cells, demonstrating that Au contacts can be replaced by low-cost NiOx/Ni back contact owing to its low contact barrier.36 Their findings suggested the necessity of further studying for reducing the high resistance of NiOx thin film for further improvement. In this study, we employed NiO-based HSLs for a high-efficiency Sb2Se3 photocathode. High-quality NiO-based HSLs were deposited on an FTO-coated glass substrate via a low-cost sol-gel process, and an Sb2Se3 absorber was deposited on the HSLs via a previously reported


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ACS Energy Letters

solution process.7,11,14 The PEC performance of the Sb2Se3 photocathode was strongly influenced by the properties of the HSLs, and an appropriate amount of Cu doping into NiOx was crucial for achieving high PEC performance. With the optimized Cu-doped NiOx (denoted as Cu:NiOx), our Sb2Se3 photocathode exhibited a remarkable photocurrent of ~17.5 mA cm–2 at 0 VRHE. We demonstrated that the enhanced PEC performance was due to the improved charge separation at the bottom interface of the Sb2Se3 photocathode by eliminating all other possible enhancement factors, such as the light absorption, the microstructure, and the charge-transfer kinetics at the electrolyte interface. Our results elucidate the importance of the bottom contact—which reduces the photocurrent loss with the aid of an HSL—for the photocathode efficiency, which has rarely been studied thus far.


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Scheme 1. Schematic showing the overall fabrication process of the multilayered photocathode in a configuration of FTO glass/NiOx (or Cu:NiOx)/Sb2Se3/TiO2/Pt. a) Ni precursor solution was obtained by dissolving Ni(II) nitrate in a mixture of 2-methoxyethanol (2ME) and acetylacetone, whereas Cu(II) nitrate was added to prepare the Cu-doped Ni precursor solution. Undoped and Cu-doped NiOx films were produced by spin-coating the precursor solutions, followed by annealing at 350°C. b) Schematic showing the successive fabrication process of the multilayered photocathodes in a configuration of Sb2Se3/TiO2/Pt on top of either FTO/NiOx or FTO/Cu:NiOx.

Figure 1. Top-view SEM images showing the surface structures of a) bare FTO, b) NiOx-HSL, and c) Cu:NiOx-HSL (inset reveals the cross-sectional microstructure). d) GIXRD patterns of NiOx and Cu:NiOx on a Si wafer. Thick HSLs (~200 nm) are used for GIXRD measurement to obtain clear XRD pattern. e) Cross-sectional SEM image of the Cu:NiOx-HSL cell (i.e., FTO/Cu:NiOx/Sb2Se3/TiO2/Pt). The inset shows the cross-sectional structure of the fabricated device. f) Linear sweep voltammograms obtained under simulated solar illumination (AM 1.5 G) of Sb2Se3/TiO2/Pt photocathodes on three different electrodes—bare FTO, FTO/NiOx, and FTO/Cu:NiOx—in an Ar-purged H2SO4 electrolyte (pH ≈ 1).


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ACS Energy Letters

The fabrication process for the Sb2Se3 photocathodes with two different inserted HSLs (undoped NiOx and Cu-doped NiOx) is illustrated in Scheme 1a. The HSLs were prepared by spin-coating either NiOx or 3 mol% Cu-doped NiOx (Cu:NiOx) precursor solutions on an FTO substrate (Scheme 1a), followed by the deposition of an Sb2Se3 absorber layer and surface modifications with TiO2 and Pt (Scheme 1b), as previously reported.7,11,14 The microstructure of HSLs deposited on the FTO substrate was observed via scanning electron microscopy (SEM) (Figure 1a-c). Both the NiOx and Cu:NiOx layers exhibited similar morphologies with a thickness of ~30 nm (inset of Figure 1c), resembling the surface features of the FTO substrate. The phase identification of the NiOx and Cu:NiOx interlayers was performed via grazing-incidence X-ray diffraction (GIXRD) (Figure 1d). Note that we used thicker HSLs (~ 200 nm, 7 spin-coating iterations) on Si wafer for GIXRD measurement to achieve clear peak intensities and to avoid overlapping with the (200) peak of FTO substrate at the vicinity of 37°. Both samples exhibited diffraction peaks at 37°, 43.46°, and 63.56°, corresponding to the (111), (200), and (220) crystal planes of NiO, respectively.28 After the Cu doping, there were no Cu-metal or Cu-oxide peaks, and the NiO (200) peak shifted slightly toward a lower angle, similar to a previously reported result for Cu:NiOx.37 It can be understood that Cu was sufficiently doped into the NiOx lattice without segregation, which slightly increased the lattice constant because the ionic radius of Cu (~82 pm) is larger than that of Ni (~78 pm).38 Energy-dispersive X-ray spectroscopy (EDS) mapping revealed a uniform elemental distribution for both Ni and Cu (Figures S1a-c). The surface chemical information for both NiOx and Cu:NiOx films on FTO substrate was further investigated by X-ray photoelectron spectroscopy (XPS) analysis (Figure S2). Both Cu:NiOx and NiOx thin films reveal nearly identical Ni 2p spectrum revealing Ni2+ peak as well as Ni3+,


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similar to the previous solution-processed NiOx thin film.28 On the other hand, Cu 2p spectrum is only observed in the Cu-doped sample. Despite the weak intensity due to the small amount of dopant (~3 mol%), the successful and uniform doping of Cu was confirmed by XPS analysis in conjunction with EDS mapping. Prior to the PEC device fabrication, we investigated the crystal structure, microstructure, and optical properties of Sb2Se3 on different substrates. X-ray diffraction (XRD) revealed that the solution-derived Sb2Se3 layers on three different substrates—FTO, FTO/NiOx, and FTO/Cu:NiOx—had the same orthorhombic structure (Figure S3a), demonstrating that the phase evolution and crystallinity of Sb2Se3 were unaffected by the type of the underlying substrate. In addition, there is no indication of the XRD peaks at 33° where possible NiSbSe (PDF#0080107), NiSe (PDF#003-2005) or NiSe2 (PDF#041-1495) phases are located, as shown in the enlarged XRD pattern in Figure S3b. Thus, it is believed that no secondary phases related to NiSe exist in our Sb2Se3 photocathode. SEM images indicated that the Sb2Se3 deposited on the three different substrates had the same microstructure, exhibiting nanorod-like morphologies with similar diameters and lengths (Figure S4). The ultraviolet–visible (UV–vis) absorbance spectra for Sb2Se3 deposited on the three different substrates are shown in Figure S5, suggesting nearly identical absorption edges and a negligible difference in the absorbance within the range of 300–1100 nm. Tauc plots allowed us to determine the optical bandgaps for the three different Sb2Se3 structures; similar bandgaps in the range of 1.14 to 1.15 eV were obtained (Figure S6). All these results indicate that the substrate type and the presence of HSLs have no influence on the quality of the Sb2Se3 absorber and are thus irrelevant to the PEC performance. Three different multilayered photocathodes were fabricated with the configurations of FTO/undoped NiOx/Sb2Se3/TiO2/Pt, FTO/Cu:NiOx/Sb2Se3/TiO2/Pt, and FTO/Sb2Se3/TiO2/Pt,


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which are denoted as NiOx-HSL cell, Cu:NiOx-HSL cell, and Without-HSL cell, respectively (see the Experimental procedure for details). A representative cross-sectional SEM image of the Cu:NiOx-HSL cell is shown in Figure 1a. A thin and uniform Cu:NiOx film with a thickness of ~30 nm on rough FTO glass was observed, whereas a nanorod-shaped Sb2Se3 layer having a thickness of ~400 nm was observed on Cu:NiOx-HSL. A TiO2 layer as well as a scattered Pt cocatalyst conformally covered the Sb2Se3 layer, resulting in a complete multilayered photocathode device for water splitting. This observation confirmed that the thin Cu:NiOx-HSL maintained its thermal and chemical stabilities during the solution deposition of the upper Sb2Se3 absorber layer and the annealing at 350 ℃. Additional SEM analysis revealed that the other two photocathodes, i.e., the NiOx-HSL cell and Without-HSL cell, had similar microstructural features to the Cu:NiOx-HSL cell (the results are not shown here), because all the devices were fabricated with identical materials and conditions for Sb2Se3, TiO2, and Pt, except for the HSLs. Figure 1b shows the PEC performances of the three different photocathodes, which were measured via linear sweep voltammetry (LSV) in an H2SO4 electrolyte (pH 1) under 1-sun illumination. The Without-HSL cell exhibited a photocurrent density of ~10 mA cm–2 at 0 VRHE, and the insertion of thin HSLs improved it to 12.5 mA cm–2 (NiOx-HSL cell) and 17.5 mA cm–2 (Cu:NiOx-HSL cell). The Cu:NiOx-HSL cell also exhibited an increase in the onset potential of 0.34 VRHE compared with the Without-HSL cell and NiOx-HSL cell (~ 0.3 VRHE). Figure S7 shows the statistical results for 10 samples of each device type, confirming the excellent reproducibility of our photocathode devices: the photocurrent density at 0 VRHE was 10.5 ± 2 mA cm–2 (WithoutHSL cell), 12.5 ± 1.5 mA cm–2 (NiOx-HSL cell), and 17.5 ± 1.5 mA cm–2 (Cu:NiOx-HSL cell). The PEC performances of previously reported Sb2Se3-based photocathodes are presented in Table S1. Our Cu:NiOx-HSL cell exhibited a record-high photocurrent exceeding those of


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previous photocathodes in which either Au or Mo was used as a bottom contact material. Considering that a metallic bottom contact (Au or Mo) can serve as a reflector for additional light harvesting, which is advantageous for the photocurrent, we can provisionally conclude that our Cu:NiOx-HSL has a superior hole-selective ability to a metallic contact (Au and Mo). To clarify the behavior of the reflected light in the CuNiOx-HSL cell, the optical properties of our NiO-based HSLs were measured and compared with those FTO/Au substrates. As shown in Figure S8, the transmittance and reflectance of FTO/NiOx and FTO/Cu:NiOx were very similar to that of the FTO substrate. On the other hand, the FTO/Au substrate reflected a large amount of photons above 500 nm, while the NiO-based HSLs showed negligible reflectance over the entire wavelength range of incident photons. In addition, the Sb2Se3 on the FTO substrate transmitted approximately 5%–20% of the photons having a wavelength in the range of 800 to 1000 nm (Figure S9). These observations suggest that our NiO-based HSLs are highly transparent, allowing for light transmission without back reflection, and thus have no influence on the PEC performance, unlike the Au layer. FTO/Au/Sb2Se3/TiO2/Pt exhibited a photocurrent of 13 mA cm–2 mA at 0 VRHE (Figure S10), which was slightly higher than that of the Without-HSL cell but significantly lower than that of the Cu:NiOx-HSL cell, despite the possible reflection of light. Thus, as supported by the nearly identical absorption spectra of the Sb2Se3 photocathode with and without the HSL layer (Figure S5), we can reasonably exclude the optical enhancement as the origin of the enhanced photocurrent of the Cu:NiOx-HSL cell. It is also noteworthy that the efficiency of Sb2Se3 thin film solar cell improved via grain boundary inversion due to the Cu doping in the grain boundary of Sb2Se3 thin film.39 However, unlike the previous study in which Sb2Se3 thin film was intentionally immersed into CuCl2 solution, the amount of Cu in our HSL is significantly low (3 mol%) so that the effect of Cu diffusion to Sb2Se3 is expected to be


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ACS Energy Letters

negligible. Furthermore, as our Sb2Se3 absorber consists of single crystalline Sb2Se3,11 the grain boundary inversion via Cu doping can be excluded from the origin of the performance enhancement. The stability of the Without-HSL cell and Cu:NiOx-HSL cell was confirmed in an H2SO4 electrolyte to validate the device stability under simulated solar illumination at 0 VRHE (Figure S11a, b), showing nearly 25% and 75% retention of the initial photocurrent density after operation for 4 h, respectively. The enhanced stability of the Cu:NiOx-HSL cell indicates that the efficient hole extraction at the bottom interface by the HSLs can possibly influence the stability of PEC devices. In our previous study on the stability of the TiO2-protected Sb2Se3 photocathode,18 we demonstrated that the instability of the TiO2-protected Sb2Se3 photocathode is mainly caused by the photo-corrosion of TiO2 occurred by the accumulated electrons when the charge transfer rate is sluggish. In addition, it was reported that the photo-excited electrons can be accumulated at the surface of photoelectrodes when the opposite charges (holes) are accumulated at the bottom.10 Thus, it can be speculated that the efficient hole extraction via the Cu:NiOx-HSL is also advantageous for the stability by reducing the accumulation of the electrons in the TiO2 layer. Gas chromatography was used to confirm that the photocurrent measured during the stability test corresponded to the HER without any side reactions, revealing nearly 100% Faradaic efficiency (Figure S11c).


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Figure 2. a) IPCE spectra measured at 0 VRHE and a Bode plot of the IMPS measured at 0.1 VRHE using monochromatic light of b) 627 nm and c) 447 nm for the three different Sb2Se3 photocathodes (arrows are guides to the eye).

Figure 2a shows the results of incident photon-to-current conversion efficiency (IPCE) measurements performed at 0 VRHE under irradiation with monochromatic light, indicating that


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all the Sb2Se3 photocathodes utilized near-infrared light up to ~1000 nm owing to their small bandgap of 1.15 eV. The integrated photocurrent density for the Without-HSL cell, NiOx-HSL cell, and Cu:NiOx-HSL cell obtained from the IPCE curves was 10, 12, and 17 mA cm–2, respectively. The integrated photocurrent density well matched the photocurrent density determined via LSV, indicating the reliability of our LSV and IPCE measurements. While the Cu:NiOx-HSL cell exhibited higher IPCE values over the entire spectral region compared with the other cell configurations, the IPCE spectra normalized by the IPCE value at 370 nm exhibited a more remarkable IPCE enhancement in the long-wavelength (λ > 500 nm) region (Figure S12) than in the short-wavelength region. Because lower-energy photons (i.e., longer-wavelength photons) generally penetrate deeper into the absorber layer, the notable IPCE enhancement for the long-wavelength photons indicates the improved quality of the bottom interface, as previously discussed for a Cu2O photocathode with Cu-Ni mixed oxide as a bottom contact.24 Therefore, we speculate that the insertion of the HSLs had a larger influence on the extraction of the excited charge carriers from the photons at the bottom interface than on the extraction of the excited carriers in the near-surface region. To further investigate the wavelength-dependent photocurrent enhancement and surface kinetics of the Sb2Se3 photocathodes, we performed intensity-modulated photocurrent spectroscopy (IMPS), as shown in Figures 2b and c. Measurements of the current-signal changes during the frequency sweep of the intensity-modulated light indicated whether the enhancement at each wavelength arose from the improved surface charge-transfer kinetics.19 The frequency at the maximum point of the imaginary part of the current in the IMPS measurement is the chargetransfer frequency (fct), which is proportional to the inverse of the charge-transfer time (τct). Figures 2b and c show Bode plots of the IMPS measurement for the three different Sb2Se3


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photocathodes using monochromatic light of two different wavelengths: 627 and 447 nm. Interestingly, all the samples exhibited nearly identical charge-transfer frequencies (as indicated by the dashed lines), indicating that the change in the charge-transfer kinetics was negligible regardless of the wavelength and the presence of the HSL layers. The charge-transfer rate can be expressed as τct = 1 / (2πfct)40,41 and was calculated to be in the range of 19 to 25 μs (Table S2), which is similar to previously reported values.41 The only noticeable difference in the IMPS spectra is the current values, particularly for the Cu:NiOx-HSL cell at a wavelength of 627 nm, which is consistent with the IPCE results showing significantly enhanced values at a long wavelength for the Cu:NiOx-HSL cell. The IMPS results, in conjunction with the IPCE data, clearly suggest that the surface charge-transfer kinetics were not influenced by the HSL layers, despite the significantly enhanced photocurrent. These results indicate that the HSL layers only affected the bottom interface, without significantly altering the top catalysis interface, motivating us to further elucidate the quality of the bottom interface.


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Figure 3. EIS spectra for three different Sb2Se3-based photocathodes—Without-HSL cell (orange), NiOx-HSL cell (blue), and Cu:NiOx-HSL cell (green)—under simulated solar illumination (AM 1.5 G). The inset shows the simple equivalent circuit model used for data fitting. The scatter points represent the original experimental data, and the solid lines are the fitted curves.


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Table 1. Area-specific resistance values and CPE obtained from the deconvolution of the EIS spectra.

Photocathode

Without-HSL cell

RS (Ω·cm2)

9.41

RP (Ω·cm2)

9.25

CPE (F sn–1 cm–2) 1.601 × 105 (n = 0.88)

NiOx-HSL cell

10.85

7.09

3.27 × 105 (n = 0.76)

Cu:NiOx-HSL cell

5.87

6.85

1.56 × 105 (n = 0.86)

To elucidate the charge-carrier transport mechanism at the bottom interface in the Sb2Se3 photocathode upon introduction of the HSLs, we performed electrochemical impedance spectroscopy (EIS), which is an effective technique for identifying the resistive components in photoelectrodes via application of a sinusoidal alternating-current (AC) signal. The EIS was performed at 0.1 VRHE with 1-sun illumination in the range of 300 kHz to 0.1 Hz with an AC voltage of 10 mV. Figure 3 shows a Nyquist plot for the Without-HSL cell, NiOx-HSL cell, and Cu:NiOx-HSL cell. The equivalent circuit model (inset of Figure 3), which is widely used in PEC water splitting, was employed for fitting.42,43 The equivalent circuit consists of one series resistance (RS) and a parallel combination of a polarization resistance (RP) and a constant-phase element (CPE), which represent the high-frequency intercept and a well-defined semicircle,


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respectively. Notably, the resistance related to the charge-transfer reaction (i.e., HER), which is generally observed at a low frequency (~1 Hz), is undetectable here because we used a highly active Pt co-catalyst, similar to previous results.9 The Bode plot in Figure S13 more clearly shows the absence of the low-frequency arc. The RP, which is associated with the total resistance of charge transport and recombination in the photoelectrode, decreased gradually with the insertion of NiOx and Cu:NiOx HSLs, as shown in Table 1. This indicates that the insertions of NiOx and Cu:NiOx HSLs were beneficial for hampering the charge recombination and ensuring efficient charge transport. On the contrary, the RS, which represents the sum of the resistances related to the substrate, wire connection, and electrolyte, increased significantly when undoped NiOx-HSL was inserted. The high RS for the NiOx-HSL cell indicates that the photoexcited holes possibly recombined at the NiOx layer owing to its high electrical resistance. On the other hand, the Cu:NiOx-HSL cell exhibited the smallest values of both RS and RP, indicating that the Cu:NiOx-HSL played a beneficial role in both charge transport and hole extraction, which could explain the highest PEC performance of the Cu:NiOx-HSL cell. These results confirm that the bottom-interface engineering through the introduction of the HSL was the major factor causing the photocurrent improvement in the Sb2Se3 photocathode.


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Figure 4. c-AFM mapping for a) NiOx and b) Cu:NiOx on an FTO substrate. c) I–V plot measured in the dark for calculating the electrical conductivity of NiOx (blue) and Cu:NiOx (green).


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To investigate the reduction of the resistivity of NiOx, which is related to the RS in the EIS spectra, with the addition of Cu, conductive atomic force microscopy (c-AFM) was performed. As shown in Figures 4a and b, a significant difference in the current distribution between the NiOx and Cu:NiOx thin films was observed when a bias of 2 V was applied. In the Cu:NiOx thin film, conductive red and green colors were dominant over the entire area, and only a small portion of the area was occupied by the resistive blue color. In contrast, the NiOx thin film exhibited a small conductive area, and the blue color was dominant, which could have resulted in the recombination of the photoexcited carriers. The enhancement of the electrical conductivity of NiOx by Cu doping was confirmed by the current–voltage (I–V) curves for an FTO/NiOx/Au dot electrode structure, as shown in Figure 4c. The electrical conductivity of Cu:NiOx (based on the slope of the I–V curve) was determined to be 8.71 × 10−3 S cm–1, which is two-fold higher than that of NiOx (4.61 × 10−3 S cm–1). According to the previous study on Cu doping of an NiOx thin film,44,45 the enhanced electrical conductivity is due to the generation of Ni vacancies and holes with the Cu doping:46 2NiO

CuO

CuxNi + V''Ni + 2h ∙ + 3OxO

(1)

The increased hole concentration is also indicated by the Mott–Schottky plot, which is useful for determining the flat band potential (Vfb) and the acceptor concentration (NA) by measuring the capacitance formed at the semiconductor–electrolyte contact as a function of the applied potential. As shown in Figure S14, the Cu doping of the NiOx thin film resulted in the increase of the NA from 2.04 × 1019 to 2.69 × 1019 cm−3. Additionally, the Vfb shifted slightly—from approximately 1.01 to 1.025 eV—with the Cu doping, presumably owing to the negatively shifted Fermi level of NiOx. Although the correlation between the electrical conductivity of the HSLs and the PEC performance is difficult to describe quantitatively, the enhanced electrical


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conductivity of NiOx, which was manifested by the c-AFM and I–V analyses, could be the origin of the reduced RS in the EIS results and the enhanced PEC performance.

Figure 5. Ultraviolet photoelectron spectra for the three different layers—NiOx, Cu:NiOx, and Sb2Se3—obtained using He I radiation at 21.21 eV for determining a) Ecutoff and b) Eedge. c) Schematic band diagram showing the relative energy positions of VBM and EF for the three different layers with respect to the FTO before equilibrium. Band energy diagram after equilibrium: d) Without-HSL cell, e) NiOx-HSL cell, and f) Cu:NiOx-HSL cell. Schematic


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ACS Energy Letters

illustrating the effects of the HSL under the Sb2Se3, indicating the blocking of electrons and the restraining of the recombination at the bottom interface.

To further elucidate the role of the HSLs, understanding the band position of the HSLs and their alignment with respect to the Sb2Se3 semiconductor is important. The surface electronic structures of NiOx, Cu:NiOx, and Sb2Se3 obtained via ultraviolet photoelectron spectroscopy (UPS) are presented in Figure 5. The secondary-electron cutoff (Ecutoff) obtained via extrapolation to the linear part of the binding-energy edge is shown in Figure 5a. The Fermi level (EF) of each material was calculated using the following equation: EF = Ecutoff − 21.21 eV (under He I radiation)

(2)

Figure 5b shows the valence-band edge (Eedge), which represents the difference between EVBM and EF, for each material. Using the Eedge values, the definite VBM level of the samples was calculated as follows: EVBM = EF + Eedge

(3)

Notably, NiOx and Cu:NiOx have nearly identical Ecutoff levels (~16.5 eV) approximately 0.1 eV lower than that of Sb2Se3, indicating that the EF level of the both HSLs is located 0.1 eV below that of Sb2Se3. On the other hand, the Eedge of the Cu:NiOx film, which is close to that of Sb2Se3, is lower than that of NiOx. According to the UPS results, the EF and VBM of the HSLs with respect to Sb2Se3 are illustrated in Figure 5c. Upon the Cu doping of NiOx, the VBM moved toward the negative direction, accompanied by a negligible change of the EF, presumably owing to the formation of a Cu-to-Ni antisite close to the VBM of NiOx. Accordingly, the band alignment between the Sb2Se3 and the back contacts after the EF equilibration is schematically depicted in Figures 5d–f. The photoexcited electrons and holes possibly recombined at the


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FTO/Sb2Se3 interface because of the electron backflow, as shown in Figure 5d. When the NiOxHSL was added between the FTO and Sb2Se3, the electron backflow was prevented owing to the overwhelmingly positive CBM of NiOx; however, the hole extraction through NiOx may have been sluggish because of the undesirable VBM matching. On the other hand, for Cu:NiOx, the photoexcited holes were efficiently extracted through the HSL owing to the desirable band alignment and the high electrical conductivity, as shown in Figure 5f. This result is in a good agreement with literatures showing that a small difference in the band offset can result in a significant difference in the performance of semiconductor-based devices.47,48 The band structures of the different HSL-based Sb2Se3 photocathodes, in conjunction with the electrical properties, indicate that the EIS results and the PEC performance depended on the type of HSL. We inserted a Cu:NiOx-HSL into an Sb2Se3 photocathode for performance enhancement and elucidated the role of the HSL. Upon the addition of a thin Cu:NiOx layer underneath the Sb2Se3, the photocurrent density of the Sb2Se3 photocathodes was significantly enhanced (from 10 to 17.5 mA cm–2) at 0 VRHE without any noticeable changes in the morphology or optical properties of the photocathodes. IPCE and IMPS measurements indicated that the top-surface properties of the Sb2Se3 photocathode, such as the catalytic activity for the HER, were unaffected by the insertion of the HSLs. EIS and conductivity analyses revealed why the Cu doping of the NiOx thin film was crucial for the high photocurrent density: NiOx can enhance the charge transport, but highly resistive NiOx can act as a recombination center of photoexcited holes. Investigation of the band alignment between the Sb2Se3 and the back contact layers provided clear evidence for the performance enhancement due to the charge-extraction mechanism, depending on the type of HSL. Thus, an innovative strategy was presented for enhancing the


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ACS Energy Letters

PEC performance through HSLs by suppressing the electron backflow and facilitating the hole extraction.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.XXXXXX. EDS mapping, XPS, transmittance and reflectance spectra, Mott–Schottky plot of NiOx and Cu:NiOx films. XRD, SEM images, absorbance and transmittance spectra, Tauc plot of Sb2Se3 flims. Table of PEC performance of Sb2Se3-based photocathodes. Current density statistics, normalized IPCE spectra and charge-transfer time table of

FTO/HSLs/Sb2Se3/TiO2/Pt.

PEC

measurement

of

FTO/Au/Sb2Se3/TiO2/Pt.

Chronoamperometry measurement and gas chromatography of FTO/Cu:NiOx/Sb2Se3/TiO2/Pt.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]. Tel.: +82-2-2123-2855. Fax: +82-2-312-5375 Author Contributions †These authors contributed equally to this work. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Research Foundation (NRF) of Korea grant (No. 2012R1A3A2026417) funded by the Ministry of Science and ICT.


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