Effects of incorporation of Ag into a kesterite Cu₂ZnSnS₄ thin film on its photoelectrochemical properties for water reduction

Abstract

Kesterite Cu₂ZnSnS₄ (CZTS) thin films in which the Cu site was partially replaced with Ag were prepared by spray deposition on an Mo-coated glass substrate. Successful replacement of Cu components in the CZTS lattice with Ag up to an Ag/(Cu + Ag) ratio of 0.20 was achieved. Samples with relatively low contents of Ag (Ag/(Cu + Ag) ratios of 0.05 and 0.10) showed obvious grain growth compared to that of bare CZTS, whereas samples with higher Ag contents showed an appreciable decrease in grain sizes. Photoelectrochemical properties for water reduction (H₂ production), which was examined after surface modifications with an In₂S₃/CdS double layer and Pt catalyst for H₂ evolution, depended strongly on such morphological differences; a maximum conversion efficiency, i.e., half-cell solar to hydrogen efficiency, of 2.4% was achieved by the photocathode based on the film with an Ag/(Cu + Ag) ratio of 0.10. Minority carrier dynamics examined by photoluminescence measurements indicated that such an active sample of PEC H₂ production had a relatively long carrier lifetime, suggesting that the suppression of carrier recombination at grain boundaries in the bulk of these kesterite films is one of the important factors for enhancing PEC functions.

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Introduction

Hydrogen (H₂) is used extensively as a raw material for many kinds of chemical processes. H₂ has also become important as one of the most attractive candidates as a fuel and as an energy carrier. Although the conventional resources of H₂ are fossil fuels, H₂ production from water decomposition is a promising method for solving various environmental and energy issues. Photoelectrochemical (PEC) decomposition of water has been studied extensively for the utilization of solar energy based on cost-effective materials and systems.^1–3 Two electrodes, a photocathode and a photoanode, were used for PEC water splitting: the photocathode induces water reduction into H₂ and the photoanode produces oxygen (O₂) by water oxidation on their surfaces.

Several chalcogenide thin films having p-type semiconductive properties have been studied for photocathode materials.^4–10 Cu-based selenides, sulfides and their mixed forms of selenosulfides crystallized in a chalcopyrite structure (Cu(In,Ga)(Se,S)₂) have been shown to be effective after modification of their surfaces for inducing water reduction.^11–23 We have shown that sulfide-based chalcopyrite thin films, including CuInS₂ and Cu(In,Ga)S₂, were useful as photocathodes for H₂ production from water when they were covered by an ultra-thin layer of an n-type CdS or In₂S₃ compound followed by deposition of platinum (Pt) nanoparticles as H₂ evolution catalysts.^24–28

Pure sulfide kesterite Cu₂ZnSnS₄ (CZTS) has also been studied as a candidate for photocathode materials because of its appropriate bandgap of approximately 1.5 eV for efficient sunlight absorption as well as its environmental and resource advantages compared to those of the above-mentioned chalcopyrite compounds.^29–36 After the first report of PEC water reduction using a CZTS-based photocathode by Yokoyama et al.,²⁹ several groups investigated the use of CZTS thin films with various surface modifications for realizing highly active and stable H₂ production. We have reported the achievement of bias-free water splitting under simulated sunlight (AM1.5G) radiation with a maximum sunlight conversion efficiency of 0.28% by using a CZTS-based photocathode modified with an In₂S₃/CdS double layer and Pt deposits upon combination with a BiVO₄-based photoanode.³² Further improvements were also achieved in our subsequent studies by applying several surface modifications.^35,36 The current best efficiency of unbiased solar to hydrogen conversion using a tandem cell based on a CZTS-based photocathode and a BiVO₄ photoanode is 3.17%: notable features of the tandem device are its successful increase in size up to 5 × 5 cm² and superior long-term stability over 60 h.³⁶ Although the efficiency value is close to the current best PEC water reduction photocathode based on a chalcopyrite compound thin film,¹⁹ further improvements are required to reach a necessary level for predicting its practical use.

Apart from the surface modification approach, substitutions of constituent elements in the CZTS crystalline lattice were studied as another approach to improve PEC properties for water reduction as well as further functionalization. Tay et al. used Cd as a partial substituent of Zn in a CZTS film with a compositional formula of Cu₂Cd_0.4Zn_0.6SnS₄: a half-cell solar to hydrogen conversion efficiency (HC-STH) of more than 4% was achieved by the resulting photocathode.³⁷ Regarding the durability issue, Wen et al. reported that a significant improvement in durability was achieved by alloying Ge at the Sn site.³⁸ We also found that Ge replacement was effective for inducing an upward shift of the bottom energy of the conduction band; the photocathode based on a film with complete replacement of Ge (Cu₂ZnGeS₄) showed CO₂ reduction ability.³⁹ Furthermore, Zhou et al. found that a Cu₂BaSn(S,Se)₄ thin film in which the Zn site of CZTS is fully replaced with Zn and the S component is partially substituted with Se was applicable for solar water reduction with high durability.⁴⁰ Recently, partial replacement of Cu with Ag in a CZTS film was found to be effective not only for PEC water reduction^41,42 but also for photovoltaic application.^43–45 Due to the much higher formation energy of Ag on the Zn antisite defect (Ag_Zn) than that of Cu on the Zn antisite defect (Cu_Zn),^44,46 Ag incorporation into the Cu site of CZTS leads to a reduction in the Cu_Zn content. Although Cu_Zn is a kind of p-type defect in CZTS, its energy level is deeper than the favourable Cu vacancy (V_Cu).⁴⁷ Thus, Ag incorporation should induce reduction of the Cu_Zn contents, resulting in enhancement of fluctuation and band tailing⁴⁸ and reduction of band bending (i.e., built-in potential).⁴⁹ The best device for PEC water reduction reported recently achieved a photocurrent of 17.7 mA cm⁻² at 0 V (vs. reversible hydrogen electrode (RHE)) and an onset potential of 0.85 V (vs. RHE).⁴² Since the PEC activity achieved is one of the highest values reported for a CZTS-based photocathode, it is expected that further functionalization of PEC properties can be achieved by using a photocathode based on an Ag-incorporated CZTS film. In this study, therefore, we examined PEC properties of water reduction over Ag-incorporated CZTS films with various Ag contents with focus on correlations with their structural/morphological and carrier recombination properties.

Experimental

Fabrication of CZTS and Ag-incorporated CZTS thin films

8 mL of an aqueous solution containing 9.5 mM Cu(NO₃)₂, 9 mM Zn(NO₃)₂, 12.5 mM Sn(CH₃SO₃)₂ and 60 mM of SC(NH₂)₂ with pH adjusted to 1.5 by adding a few drops of conc. HNO₃ was sprayed on an Mo-coated glass substrate (Mo/glass) by using a FUSO SEIKI Lumina STS-10SK atomizer. The temperature of the Mo/glass during the spray deposition was fixed at 380 °C by using a hotplate. The thus-obtained precursor film deposited on Mo/glass was placed with 20 mg elemental sulfur in an evacuated Pyrex ampoule (ca. 160 cm³) and annealed at 600 °C for 30 min to facilitate crystallization into CZTS. For fabrication of Ag-incorporated CZTS, silver nitrate (AgNO₃) with appropriate concentrations was also included in the source solution; the concentration of Cu(NO₃)₂ solution was accordingly reduced. For example, a source solution composed of 9.31 mM Cu(NO₃)₂ 0.19 mM AgNO₃, 9 mM Zn(NO₃)₂, 12.5 mM Sn(CH₃SO₃)₂, and 60 mM of SC(NH₂)₂ with pH adjusted to 1.5 was used to obtain a film with an Ag/(Ag + Cu) ratio of 0.02. The thus-obtained Ag-incorporated CZTS films are labelled ACZTS(Ag/(Ag + Cu) ratio).

Deposition of CdS and In₂S₃ layers

Surface modifications of the prepared CZTS and Ag-incorporated CZTS films with CdS and/or ln₂S₃ layers were performed by the chemical bath deposition method (CBD). For the deposition of a CdS layer, CZTS and Ag-incorporated CZTS films were immersed in an aqueous solution containing 12.5 mM CdSO₄, 0.22 M SC(NH₂)₂, and 11 M NH₄OH for 7 min at 60 °C. The sample was then rinsed thoroughly with distilled water. The deposition resulted in the formation of ca. 50–80 nm-thick CdS layers on the top of CZTS and Ag-incorporated CZTS films. The thus-obtained CdS-covered samples were put into an aqueous solution containing 25 mM In₂(SO₄)₃, 0.1 M CH₃CSNH₂ and 0.1 M CH₃COOH. After soaking in the chemical bath solution at 65 °C for 15 min, ca. 15 nm-thick In₂S₃ layers were deposited on the outer part of the CdS layers of the samples. Finally, the samples were washed with distilled water for further use.

Deposition of Pt for PEC water reduction

To examine the PEC water reduction properties, Pt particles as a hydrogen evolution reaction (HER) catalyst were deposited on the samples by photoelectrodeposition. The deposition was performed by using a conventional three-electrode system consisting of the sample film as a working electrode, a Pt wire as a counter electrode, and an Ag/AgCl reference electrode. These electrodes were immersed in 0.1 M Na₂SO₄ solution containing 1 mM H₂PtCl₆ (pH 6.8) and the deposition was performed with a constant potential of −0.1 V for 10 min by using a Solartron SI1280B electrochemical measurement unit. During the deposition, the working electrode was illuminated by using a Cermax LX-300F 300 W xenon lamp.

Photoelectrochemical measurements

The same three-electrode setup as that used for the above Pt deposition was used for PEC measurements. PEC H₂ generation from the Pt-modified samples (i.e., photocathodes) was examined in 0.2 mol dm⁻³ Na₂HPO₄/NaH₂PO₄ solution (pH 6.5). Illumination of simulated sunlight (AM1.5G) was performed by using an Asahi Spectra HAL-320 compact xenon light source. Potentials referred to the Ag/AgCl electrode (E_Ag/AgCl: V (vs. Ag/AgCl)) were converted to a reversible hydrogen electrode (E_RHE: V (vs. RHE)) using the Nernst equation as follows:

E_RHE = E_Ag/AgCl + 0.059 × pH + 0.199.

Half-cell solar to hydrogen efficiency (HC-STH) plots against applied potentials were also obtained from the current density-potential response of the photocathodes by using the following equation:

HC-STH (%) = J_ph × E_RHE × 100/P₀,

where J_ph is the photocurrent density (mA cm⁻²) and P₀ is the intensity of simulated sunlight (100 mW cm⁻²).

A PEC cell connected to an Agilent 490 Micro GC gas chromatography system (MS-5A column, thermal conductivity detector, Ar carrier) was used to quantify the amounts of H₂ in the gas phase during the PEC water reduction. The PEC cell immersed in a water bath to maintain the temperature at 25 °C was irradiated with simulated sunlight (AM1.5G) by using the above-mentioned solar simulator. The potential of the photocathode during photoirradiation was fixed at 0 V, 0.25 V, or 0.40 V (E_RHE).

Characterizations of CZTS and Ag-containing CZTS films

Crystalline structures of the films were determined by X-ray diffraction (XRD) using a Rigaku Mini Flex X-ray diffractometer (CuKα, Ni filter). The morphologies of the films were examined by field emission scanning electron microscopy (FE-SEM) using a Hitachi S-5000 FEG scanning electron microscope. Steady-state photoluminescence (PL) measurements were performed using a 635 nm line of a CW diode laser (Scientex OPG-3300). Spectra were analysed by a grating monochromator and detected using an Si-CCD photodetector. Time-resolved photoluminescence measurements (TRPL) were carried out with the same excitation line of 635 nm but in a pulse-mode having pulse-width and repetition rate of 100 ps and 10 MHz, respectively. Considering the spot size of the laser beam on the sample of ca. 80 μm, the photon density per pulse was estimated to be ca. 5 × 10⁹ photons cm⁻². The decay curves were measured at the peak energy (ca. 950 nm) of the corresponding PL and detected using an InGaAs-based photomultiplier tube (Hamamatsu H10330A-75). All PL and TRPL measurements were carried out at room temperature.

Results and discussion

Fig. 1a shows the XRD patterns of CZTS and Ag-incorporated CZTS films deposited on Mo/glass. The CZTS film showed reflections consistent with those of a standard profile (PDF-01-075-4155). With an increase in the charged contents of Ag⁺ ions in the source solutions for spray deposition, these reflections were shifted to the low 2θ side, as can been seen typically for (112) reflections of these films (Fig. 1b). Inductively coupled plasma (ICP) analyses of these films reported previously indicated that the Ag/(Ag + Cu) ratios in the films were almost comparable to those of charged values.⁴³ Thus, observed shifts of reflections imply replacements of Cu in the CZTS lattice with Ag having a larger atomic (or ionic) radius than that of Cu. It is noted that a weak reflection at 2θ of 28.5°, assignable to the (111) reflection of the ZnS crystal, appeared for the ACZTS films. As reported previously,⁴³ the depth profiles of CZTS and ACZTS films obtained by secondary ion mass spectrometry (SIMS) indicated that the presence of large contents of Zn components was pronounced for all of the films at the surface region. These results indicate the presence of a small fraction of ZnS at the surface of these kesterite films. A possible explanation of such segregations of ZnS at the surface is due to the evaporation of the Sn component during high-temperature heat treatment.

Fig. 1 XRD patterns (a and b) of CZTS and Ag-incorporated thin films deposited on Mo/glass. (c) Lattice parameter variations obtained from these patterns (circles) and standard profiles of CZTS (PDF-01-075-4122) and Ag₂ZnSnS₄ (PDF-01-082-5006).

As calculated from these reflections, the length of the a-axis of the kesterite unit cell increased linearly with an increase in the Ag content, whereas the length of the c-axis was almost constant regardless of the Ag content (Fig. 1c). Moreover, extrapolations of these lattice parameters to the Ag/(Ag + Cu) ratio of 1.0 showed good agreement with those of pure Cu-free kesterite Ag₂ZnSnS₄ calculated from a standard profile (PDF-01-082-5006). Thus, all of the Ag components in the Ag-incorporated films should exist mainly in their corresponding Cu sites of kesterite lattices.

Fig. 2 shows surface FE-SEM images of CZTS and Ag-incorporated films. Corresponding size distributions of grains, which were obtained by applying a normal distribution curve to the average minor diameter (D_avr) and the standard deviation (SD) calculated from more than 50 grains for each sample, are also shown in this figure. The CZTS film shows densely packed well-grown grains of ca. 1 μm in size. Similar morphologies were also observed for the ACZTS(0.02) film. Further inclusion of Ag up to the Ag/(Ag + Cu) ratio of 0.10 (ACZTS(0.05) and ACZTS(0.10) films) induced obvious grain growth. Size distribution analyses suggest ca. 1.5-times increments of their grain sizes compared to those in the bare CZTS film. On the other hand, Ag-containing films including larger amounts of Ag (ACZTS(0.15) and ACZTS(0.20) films) showed appreciable reductions in their grain sizes by nearly one order of magnitude. Empirically, partial Ag replacement is known to be effective for grain growth in several chalcopyrite films^50–52 as well as the present Ag-incorporated CZTS⁴¹ at least for an Ag/(Ag + Cu) ratio of less than 0.20 regardless of their fabrication processes. However, there is no report showing a detrimental effect of Ag incorporation on grain growth. Although detailed analyses should be performed, the observed suppression of grain growth is likely to be characteristic for the spray deposition method.

Fig. 2 Surface FE-SEM images of CZTS and Ag-incorporated thin films deposited on Mo/glass. Size distributions of grains calculated from average minor diameters (D_avgs) and standard deviations (SDs) obtained by measuring more than 50 grains are also shown on the left of each SEM image.

It is known that surface modifications with both an n-type ultrathin layer (such as CdS and In₂S₃) and an HER catalyst (such as Pt metal nanoparticles) are indispensable for inducing efficient photocurrents.^11,24,25 The present thin films also required these modifiers: regarding the n-type layer, specifically, the use of an In₂S₃–CdS double layer showed better HER ability than that of a simple CdS (or In₂S₃) single layer (Fig. S1, ESI†). As reported previously,³² the CdS layer should form the best p–n heterojunction for the CZTS-based photoabsorber; the main contribution of the In₂S₃ overlayer would be protection of direct contact of the CdS surface with the aqueous solution to avoid rapid oxidative degradation of CdS. Therefore, we used samples modified with the In₂S₃–CdS double layer and a Pt catalyst as photocathodes. Fig. 3a shows the current density-potential (J–V) curves obtained by linear sweep voltammetry (LSV) analyses of photocathodes based on CZTS and Ag-incorporated CZTS films under chopped sunlight (AM 1.5G) illumination. The photocathode based on the CZTS film showed a photocurrent density of 9 mA cm⁻² at E_RHE = 0 V. The maximum HC-STH (HC-STH_max) of the photocathode estimated from the J–V curve reached 1.6% (Fig. 3b); the photocurrent onset of the sample, defined as a potential (E_RHE) showing 0.1% of HC-STH, was estimated to be 0.72 V. Although the photocathode based on an Ag-incorporated CZTS film with low Ag content (ACZTS(0.02)) showed almost the same J–V curve as that of the bare CZTS-based photocathode, obvious increases in PEC characteristics were observed for photocathodes based on the ACZTS(0.05) and ACZTS(0.10) films, whereas appreciable drops in PEC properties appeared for the photocathodes based on the ACZTS(0.15) and ACZTS(0.20) films. For a quantitative comparison of these PEC properties, typical PEC parameters, i.e., HC-STH_max and photocurrent onset, were extracted and they were plotted against the Ag/(Ag + Cu) ratio (Fig. 3c). There were appreciable improvements in HC-STH_max values, which reached more than 2% when photocathodes based on the ACZTS(0.05) and ACZTS(0.10) films were used, though the photocurrent onset was not affected by the Ag incorporation (see below) probably due to similarities of interface properties of p–n heterojunctions in these photocathodes. The highest HC-STH_max value of 2.4% was obtained by the ACZTS(0.10)-based photocathode sample. On the other hand, appreciable drops in PEC properties appeared for photocathodes based on ACZTS(0.15) and ACZTS(0.20) films. HC-STH_max values of these photocathodes were reduced to almost one-third of the HC-STH_max value of a bare CZTS-based photocathode.

Fig. 3 (a) Current density–potential (J–V) curves of CZTS and Ag-incorporated thin films modified with a chemical bath-deposited CdS layer and Pt deposits in an Na₂HPO₄/NaH₂PO₄ solution (pH 6.5) under chopped solar simulated AM 1.5G light irradiation. (b) Corresponding HC-STH plots extracted from these J–V curves for each sample. (c) Plots of HC-STH_max and photocurrent onset E_RHEs of these photocathodes against Ag/(Ag + Cu) ratios of CZTS and Ag-incorporated thin films.

To characterize the effect of Ag incorporation into the CZTS lattice of the film on photoexcited charge recombination occurring inside the film, PL and TRPL measurements were performed for CdS-covered CZTS and Ag-incorporated CZTS films. It is known that bare kesterite and chalcopyrite films are degraded by exposure to open air and the degradation has an impact on minority carrier lifetime.^53–55 To examine the bulk properties of the films in this study, therefore, CdS-covered films, i.e., CZTS and Ag-incorporated films passivated by a CdS layer, were chosen for both PL and TRPL measurements. For PL emission properties, the normalized PL spectra of all samples showed similar results in terms of peak shape and peak position, as shown in Fig. 4a: as analysed by applying the conventional Gaussian equation, all of the measured samples showed peak-top positions of ca. 940 nm and peak widths of ca. 70 nm (Table 1). The PL peak position value corresponded to 1.32 eV in the energy scale; the value is much smaller than that of the band gap energy of CZTS (ca. 1.50 eV). In accordance with the previous PL study on selenosulfide Cu-based kesterites (Cu₂ZnSn(Se,S)₄), the observed PL emission is assignable to the band–impurity recombination.⁵³ Photoabsorption properties of Ag-incorporated CZTS crystals examined by the powder system proved that band gap energies of Ag-incorporated CZTS crystals in the present Ag content (i.e., Ag/(Ag + Cu) ratio up to 0.2) are not significantly changed by the Ag incorporation.^56,57 Moreover, band edge positions, i.e., the maximum energies of valence band (VBMs) and minimum energies of conduction band (CBMs), of the CZTS and ACZTS films were suggested to be comparable to at least the ACZTS with an Ag/(Ag + Cu) ratio up to 0.1.⁴³ Therefore, the optical absorption properties of the present Ag-incorporated CZTS crystals are assumed to be almost the same as those of the Ag-free CZTS film. Based on these considerations, the observed PL emissions for the Ag-containing samples measured in the present study are also assignable to the same band–impurity recombination path. Thus, in the following discussion, we assumed that the Ag incorporation of the present sample did not induce critical changes in the defect structure(s)-affected radiative recombination.

Fig. 4 (a) Steady-state PL and (b) TRPL spectra of CZTS and Ag-incorporated thin films covered with a chemical bath-deposited CdS layer. (c) Dependence of τ₂ and dependence of HC-STH_max obtained by TRPL measurement (for τ₂) and J–V curves shown in Fig. 3 (for HC-STH_max) on the contents of Ag (Ag/(Ag + Cu)) in CZTS and Ag-incorporated thin films.

Table 1 Parameters obtained by analyzing the PL and TRPL spectra shown in Fig. 4

Sample	λ 0 /nm	w /nm	τ 1 /ns	τ 2 /ns	C 1/C2^b
a Peak position (λ0) and width (w) determined by fitting each PL spectra with the conventional Gaussian equation: I(λ) = exp{−(λ − λ0)/w}^2. b Parameters obtained by fitting TRPL decay curves with the double-exponential function given in the main text.
CZTS	941	72.2	1.00	4.01	1.75
ACZTS(0.02)	934	71.6	0.95	4.30	1.80
ACZTS(0.05)	934	74.3	1.04	5.18	1.53
ACZTS(0.10)	940	74.0	0.98	5.42	1.19
ACZTS(0.20)	940	76.2	0.56	2.96	0.91

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As shown in Fig. 4b, the room temperature TRPL signal indicates a gradual increase in PL lifetime with an increase in the Ag content. The decay curves could be fitted by the following double-exponential function:

I(t) = C₁ exp(−t/τ₁) + C₂ exp(−t/τ₂),

where t is the time after a laser pulse excitation, C₁ and C₂ are the PL intensities of the corresponding PL components, and τ₁ and τ₂ are the fast and slow decay lifetimes, respectively. These parameters are summarized in Table 1. The fast decay component, τ₁, should be attributed to surface recombination that might be affected by other surface processes including charge separation and the laser onset, whereas the slow decay component τ₂ might be related to the bulk recombination.^54,55,58,59 For the sake of simplicity, therefore, we will focus our discussion on bulk recombination; thus, only τ₂ is considered. In direct gap semiconductors, ideal minority-carrier lifetimes are known to be in the order of μs.⁵³ However, all the observed τ₂ values are far behind the ideal one (Table 1), implying that photoexcited carriers predominately recombine non-radiatively via deep level defects localized within the bandgap, a process called Shockley–Read–Hall (SRH) recombination. The PL lifetime (τ_PL) can be expressed as a function of radiative (τ_rad) and nonradiative (τ_nr) carrier lifetimes as

1/τ_PL = 1/τ_rad + 1/τ_nr.

For τ_nr ≪ τ_rad, τ_PL almost matched with τ_nr (τ_PL ≒ τ_nr). For the bulk recombination, moreover, τ_PL ≒ τ₂ can be assumed, and thereby τ₂ = τ_nr. Based on these analyses, carrier recombination in the CZTS and Ag-incorporated films in their bulk predominately occurred via SRH recombination.

As shown in Fig. 4c, there is a clear trend for an increase in minority carrier lifetime with an increase in D_avr. The HC-STH_max values are also increased with an increase in the grain size in the kesterite films. The integrated PL intensity of the corresponding samples also roughly follow a similar tendency (data not shown). These results suggest that a deep defect(s) that existed at grain boundaries in the kesterite films would be a probable center for inducing SRH recombination. Although the origin(s) of defect structure(s) at grain boundaries has not yet been clarified, suppression of the effective area of SRH recombination would be achieved by an increase in grain size, resulting in improvements of HC-STH_max. Apart from the effect of grain boundaries, acceptor densities (N_As) calculated from capacitance–voltage (C–V) analyses of photovoltaic devices indicated that there was no appreciable decrease in N_A observed for the ACZTS film with a moderate Ag content (ACZTS(0.10)).⁴³ As mentioned above, a theoretical study suggested that the formation energy of the Cu_Zn antisite defect is much lower than other point defects such as V_Cu,⁴⁷ whereas the formation energy of the Ag_Zn antisite defect is relatively high since there are significant differences in the ionic radii between Ag and Zn ions.⁴⁶ Assuming that the Ag incorporation reduces the number of Cu_Zn antisite defects, comparable values of N_As between the ACZTS(0.10) film and the CZTS film suggest achievement of a successful combination of suppression of the formation of unfavourable Cu_Zn and increment of favourable V_Cu in the kesterite lattice of the ACZTS(0.10) film. On the other hand, significant reductions in grain sizes observed for the photocathode based on Ag-incorporated CZTS films with a relatively large Ag content mainly appeared to have detrimental effects of an increase in the effective area of the grain boundaries, leading to a drop of their PEC performances.

As discussed in a previous report on the solar cell properties of Ag-incorporated films obtained by spray deposition, the best parameters were obtained by the cell based on ACZTS(0.02).⁴³ Based on the fact that ACZTS films with relatively large grains (ACZTS(0.05) and ACZTS(0.10)) had many pinholes over the entire surface as observed at a relatively low magnification using a laser microscope,⁴³ the inconsistency with the present PEC results as well as minority carrier lifetimes is likely to be due to the occurrence of shunts in the solar cell device induced by such pinholes, whereas the present PEC device is free from the unfavourable effect of pinholes, and thus there are many opportunities for further functionalization and/or survey of materials for PEC applications without paying much attention to pinhole formation in a newly developed semiconductor film. This would be a significant characteristic of the PEC device that is superior to a solar cell.

Fig. 5a shows a time course curve of H₂ gas liberation over the photocathode based on a CZTS film at E_RHE = 0 V under illumination from simulated sunlight (AM 1.5G). A current density profile during hydrogen production and the sum of half of the electrons passing through the outer circuit (e⁻/2) during H₂ liberation at a certain interval are also plotted in this figure. The amount of H₂ produced in the gas phase increased proportionally at a rate (per 1 cm²) of 157 μmol h⁻¹ cm⁻². The corresponding e⁻/2 plot (slope: 161 μmol h⁻¹ cm⁻²) was consistent with the H₂ liberation, i.e., the faradaic efficiency, defined as the ratio in percent of the rate of H₂ evolution to that of e⁻/2 during 3 h photoirradiation, was estimated to be almost unity (ca. 98%). In addition, the observed constant photocurrent flow during the reaction indicated almost no degradation of the photocathode. As shown in Fig. 5b and c, a similar monotonous increase in H₂ accumulation was also observed when the reaction was performed at applied potentials (E_RHEs) of 0.25 V and 0.40 V. From the slopes of time course curves, the rates of H₂ evolution (per 1 cm²) were calculated to be 36 μmol h⁻¹ cm⁻² at E_RHE = 0.25 V and 3.8 μmol h⁻¹ cm⁻² at E_RHE = 0.4 V. However, in these power generation potential regions, gradual drops in the rates of H₂ liberation and photocurrent density profiles were also observed, suggesting the occurrence of degradation. Our recent studies have shown that significant improvements can be achieved by surface loadings of different n-type modifiers instead of the present In₂S₃/CdS, such as an HfO₂/CdS double layer³⁵ and an HfO₂/CdS/HfO₂ triple layer.³⁶ Although the other factors affecting durability of the present photocathode, such as stability of the incorporated Ag component, should also be clarified, applications of Ag incorporation to those stabilized devices are now in progress.

Fig. 5 Time profiles of H₂ evolution, half of the electrons passing through the outer circuit (e⁻/2), and photocurrent density over the photocathode composed of an ACZTS(0.10) film measured at different applied potentials (E_RHEs). The active area of the used photocathode was 0.33 cm².

Conclusions

The effects of the incorporation of Ag into the crystalline lattice of a CZTS film on its morphological, PEC, and minority carrier dynamic properties were investigated. Obvious morphological changes were observed among the samples with different Ag contents. The photocathodes derived from the Ag-incorporated CZTS films having relatively large grains showed better PCE properties for H₂ production than those of photocathodes based on bare CZTS and Ag-incorporated CZTS films having relatively small grains. Minority carrier lifetimes also followed these trends: relatively long lifetimes were obtained from samples that were highly active for PEC water reduction. Although the photocathodes did not have sufficient durability under operations in power-generating potential regions, the observed simple correlations among morphological, PEC, and carrier lifetime properties should be a useful guide for designing active photocathodes not only for H₂ production but also for other photosynthetic reactions.

Author contributions

S. Ikeda: conceptualization, methodology, supervision, project administration, writing – original draft, writing – review and editing; T. H. Nguyen: investigation, visualization; R. Okamoto: investigation; I. Abdellaoui: investigation, visualization; M. M. Islam: formal analysis, writing – review and editing; T. Harada: investigation, formal analysis; R. Abe: methodology; T. Sakurai: formal analysis, writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS Grants-in-Aid for Scientific Research (KAKENHI), awards no. 19H02656, 19H02822, and 20H05120.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: LSV plot of CZTS-based photocathodes with several surface modifications. See DOI: 10.1039/d1cp04075h

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