Copper kesterite Cu2ZnSnS4 is a promising photoabsorber material for solar cells and photoelectrochemical (PEC) water splitting. In this article, we will first review the crystallographic/energetic structures of Cu2ZnSnS4 in view of its applications to sunlight conversion devices. Then, historical progress in photovoltaic properties of Cu2ZnSnS4-based solar cells is introduced. Finally, studies on PEC H2 evolution over Cu2ZnSnS4-based photocathodes are reviewed in detail. For realizing efficient PEC H2 evolution, surface modifications with an n-type buffer layer (such as CdS) and a catalytic site (such as Pt nanoparticles) were found to be indispensable. Since these surface-modified photocathodes had poor resistances under an operating bias due to the occurrence of oxidative photocorrosion of the CdS layer and elimination of the Pt catalysts, coverage with a protection layer was required to improve the long-term durability. Moreover, partial or complete substitution of the constituent cations with some cations was proved to be effective for improving PEC properties. Although recent studies showed a rapid increase in PEC properties, there is room for further development of PEC properties by using effective combinations among surface protection(s), defect engineering(s), and band engineering(s).
The energy problem is one of the most important technological problems faced by the world. Current world power consumption is estimated to be ca. 19 TW, more than 80% of which is generated from fossil fuels . Since mankind’s demand for energy is already causing climate changes that might lead to serious long-term consequences for the earth, transfer of the global energy supply system to a renewable energy source is an urgent issue. Among the various kinds of renewable energy resources, sunlight has attracted much attention due to its abundancy and permanency. Based on the solar constant (1366.1 W·m−2), the total solar flux reaching the earth’s surface is estimated to be ca. 180,000 TW. When absorption and scattering are considered (i.e., assumption of 50% loss), the actual value would be 90,000 TW, indicating that ca. 0.02% of solar energy is equivalent to the energy demand by human society. Hence, part of the solution to the energy problem must come from increased exploitation of solar energy. Current practical systems of widespread deployments are based on solar cells; several energy conversion facilities with a scale of several hundred GW have been installed throughout the world . However, these sunlight-based systems require several costly devices not only for sunlight-to-electricity conversion (solar cells) but also for storage and transportation.
A solution to avoid the cost issue is solar-to-fuel conversion, in which light energy in sunlight is converted into chemical energy in the form of gas or liquid fuels . Instead of an expensive battery, fuels can be stored in a simple tank or pool without a natural discharge phenomenon. Moreover, fuels are transportable by using conventional tankers and pipelines. Among the many kinds of fuels, hydrogen (H2) is an attractive fuel because of its relatively high energy density per weight (120 MJ·kg−1) and the possibility of construction of a carbon-free substance circulation system . To realize solar-to-hydrogen conversion, one of the most probable methods consisting of a combination of current conventional technologies is water electrolysis into H2 and oxygen (O2) in conjunction with the electricity generated by solar cells. However, the process cost of the wiring over each cell as well as the necessity of a water electrolyzer including costly electrocatalysts makes this type of system expensive compared to its energy conversion efficiency .
Use of powdered photocatalysts dispersed in a solution or embedded onto a monolithic sheet is expected to become the most scalable and cost-effective device because of their simplicity [6,7]. The photocatalytic reaction is directly driven on the surface of the semiconductor by electrons and holes generated by a photoexcitation process. Since the surfaces of semiconductor powders are often inert to induce water splitting reactions, loading(s) of co-catalyst(s) for H2 and/or O2 evolution are introduced on their surfaces. The first demonstration of direct photocatalytic overall water splitting was reported in 1980s by using a SrTiO3 powder-based photocatalyst: SrTiO3 powder modified with NiO x , a catalyst for H2 evolution . However, since the photocatalyst could only utilize ultraviolet light with low photocatalytic activity, the development of visible-light-responsive photocatalyst with high activity has been investigated in order to achieve meaningful solar to hydrogen efficiencies (STHs). As a result, several photocatalyst materials including GaN:ZnO , LaMg1/3Ta2/3O2N , and Y2Ti2O5S2  have been found to be active for overall water splitting reaction under visible light irradiation. A photocatalytic reaction system using a two-step photoexcitation process is often called a Z-scheme photocatalytic system [12,13,14,15]. The system consists of two photocatalysts: one photocatalyst induces water reduction into H2 and the other produces O2 by water oxidation on its surface. Many kinds of photocatalysts have been shown to be applicable for the system. As a recent example, a photocatalyst sheet in which SrTiO3:La,Rh and BiVO4:Mo powders are embedded showed a high STH value (1.2%) . Since both the single photocatalyst system and Z-scheme system generate a mixture of H2 and O2 gases with a compositional ratio of 2:1, so-called oxyhydrogen detonation gas, installation of an additional separation system such as a separation membrane is required . This is one of the most critical problems of these particulate photocatalyst-based systems for their practical use.
Another promising system using the two-step photoexcitation process is a photoelectrochemical (PEC) cell in which two kinds of semiconductor electrodes, photocathode and photoanode for H2 and O2 evolutions, respectively, are used (Figure 1) [18,19,20]. In this system, reaction sites for H2 and O2 evolutions are located far away from each other as in a water electrolyzer. Thus, the system does not require a separation process and apparatus for generated H2 and O2. Moreover, surface modifications can be applied to each photoelectrode to improve its performances such as charge separation efficiency and stability. For photocathode materials for H2 evolution, Cu-based selenides and sulfides and their mixed forms of selenosulfides crystallized in a chalcopyrite structure (Cu(In,Ga)(Se,S)2) were shown to be effective upon modifications on their surfaces for inducing water reduction [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]. Since pioneering works in the early 2010s [21,22], appreciable improvements have been achieved: the current record STH value for unbiased overall water splitting of 3.7% upon combination with a BiVO4-based photoanode for O2 evolution was achieved by using a chalcopyrite Cu(In,Ga)Se2-based photocathode . However, due to the low abundance of constituent elements (i.e., In, Ga, and Se), these chalcopyrite compounds are unlikely to be sustainable in the long term for large-scale developments. A promising candidate for a sustainable photocathode material is copper zinc tin sulfide (Cu2ZnSnS4, commonly known as CZTS) since it has electric properties similar to those of Cu(In,Ga)Se2, but contains cheap and plentiful less-toxic elements. In this review, therefore, we focus on CZTS and related compounds for their applications as photocathodes for PEC H2 evolution under sunlight irradiation. Following the description of several fundamental aspects of these compounds as well as the operating principle of PEC water splitting, the history of development of CZTS-based photocathodes is overviewed with focus on the surface modifications and substitutions of constituent elements to improve PEC H2 evolution properties.
2 Crystallographic and energetic structures of CZTS
The CZTS crystal is derived from the zinc-blende structure of II–VI semiconductors such as ZnS. By substituting half of the group II element with a group I element (e.g., Cu) and half with a group III element (e.g., In), a I–III–VI2 compound semiconductor crystalized in a chalcopyrite structure (i.e., CuInS2) can be formed. Further substitution of the group III element of In with half of a group II element (e.g., Zn) and half of a group IV element (e.g., Sn) will result in a I2–II–IV–VI4 compound of CZTS. In general, CZTS has two principal structures known as kesterite and stannite structures, as shown in Figure 2 . Both of them are tetragonal structures that consist of a cubic close packing array of sulfur anions and half of tetrahedral voids of cations. The difference of these two structures is the arrangement of Cu and Zn atoms: the kesterite structure is an ordered array of (Cu,Sn), Se, (Cu,Zn), Se,… layers along the c-axis, whereas the stannite structure is an ordered array of (Zn,Sn), Se, Cu, Se,… layers along the c-axis. Since Cu+ and Zn2+ cations have identical number of electrons, it is difficult to determine their positions in the unit cell by conventional X-ray diffraction. However, the CZTS crystal has been considered to appear in the kesterite phase because of its relatively high levels of thermodynamical stability compared to that of the stannite phase . A structural investigation of a powdered CZTS sample using neutron diffraction also indicated crystallization of the kesterite structure .
The kesterite CZTS has a band gap (E g) of 1.5 eV. The origin of the conduction band minimum (CBM) is mainly attributed to the s orbital of Sn (Sn-5s) and s, p orbitals of S (S-3s, S-3p), whereas the valance band maximum (VBM) is derived from the d orbital of Cu (Cu-3d) and the p orbital of S (S-3p) [39,42]. These electron energy structures of CZTS suggest possible engineering of band edge potentials by partial and/or complete substitutions of constituent elements. In most of the works on the solar cell application, partial substitution of S with Se to form a sulfoselenide (CZTSSe) has been studied [43,44,45,46,47,48]. With higher Se content (i.e., lower S/(S + Se) ratios), the CBM shifts downward, while the VBM shifts upward, resulting in possible controls of band gap energies (E gs) ranging from 1.0 eV (pure selenide CZTSe) to 1.5 eV (pure sulfide CZTS) by changing the S/(S + Se) ratio, as shown in Figure 3 [49,50]. Both the experimental and theoretical investigations indicated that these copper kesterites have a large band-edge absorption coefficient >104 cm−1 as similar to that of copper chalcopyrite compounds [49,51]. Moreover, as is well known for copper chalcopyrite compounds , replacements of cation components are also effective for modifying E g, VBM, and CBM energies. Examples for these cation substitutions for band engineering of CZTS are discussed below.
Conversion of elemental precursors into the CZTS quaternary compound takes place in competition with the formation of other secondary phases such as Cu2S, SnS2, Cu2SnS3, and ZnS. A comprehensive analysis of the Cu2S–ZnS–SnS2 pseudoternary system at 400°C carried out by Olekseyuk et al. indicated that a CZTS single phase is present only within a small region . Theoretical analysis of the formation window of CZTS in a Cu2S–ZnS–SnS2 pseudoternary system also suggested the similar trend, as shown in Figure 4 . There are four two-phase fields in which one secondary phase, i.e., Cu2S, Cu2SnS3, SnS2, or ZnS, will be observed in addition to CZTS. Between them, four phase fields, where the secondary phase from the bordering fields will be formed together with CZTS, are present. From a study in the photovoltaic field, the Cu2S compound is known to be highly doped p-type, leading to the generation of a shunt current flow in a solar cell , Cu2SnS3 is p-type having moderate E g (1–1.35 eV) but is inefficient as a photoabsorber [56,57]. Since SnS2 is an n-type compound, inclusion of this phase would form a secondary diode inside the CZTS film, resulting in enhancement of carrier recombination [55,58]. ZnS is insulated with E g of 3.5 eV: it is normally silent in the solar cell compared to the other abovementioned secondary phases, though a high content of ZnS should induce reduction in the active area and increase in resistivity. An empirically optimum composition of the elemental precursor of CZTS as a photoabsorber in a solar cell is a Zn-rich and Cu-poor composition to optimize suppression of the abovementioned harmful secondary phases except for the silent ZnS phase and formation of Cu vacancy (VCu), a favorable p-type point defect.
Due to the quaternary nature, there are many types of intrinsic point defects in CZTS such as vacancies, antisites, and interstitials. Table 1 summarizes the probable point defects formed in CZTS and their calculated formation energy; corresponding energy levels are shown in Figure 5 [59,60]. To realize efficient utilization of photoexcited carriers for energy of a useful form such as electricity or fuel, it is critically important to obtain a systematic understanding of the characteristics of these defect properties that determine the level of self-doping as well as the rate of nonradiative recombination of photoexcited carriers. In chalcopyrite CuInSe2, it is well known that p-type self-doping is due to the Cu vacancy (VCu) [61,62]. For the CZTS compound, a theoretical calculation indicated that the Cu on Zn antisite (CuZn) and VCu are dominant acceptor point defects that lead to p-type conductivity [59,60]. CuZn has lower formation energy than that of VCu and an acceptor energy level of 0.12 eV above the VBM. Since VCu has relatively low acceptor energy level (0.02 eV above the VBM), VCu is the most preferable point defect in CZTS. Nagaoka et al. reported that electrical properties examined by using a high quality CZTS single crystal obtained by traveling heater method, a solution growth technique, with a Zn-rich and Cu-poor composition  revealed that native acceptors in CZTS have a larger activation energy than that of a copper chalcopyrite (Cu(In,Ga)Se2) single crystal, indicating that relatively deep level, i.e., CuZn is the main exiting point defect in the CZTS single crystal . Although, the CZTS thin films used for solar cells and PEC water splitting were obtained in a non-equilibrium state, these theoretical and experimental results suggest that CuZn exits in the CZTS films at a higher concentration than that of VCu. Due to the deeper energy level of CuZn than that of VCu, the existence of a high concentration of CuZn leads to enhancement of fluctuation and band tailing [65,66] and reduction in band bending (built-in potential) . Therefore, a successful combination of suppression of the formation of CuZn and increment in VCu in the CZTS crystal should be promising to improve efficiencies of CZTS-based devices for photon energy conversion. Doping of a secondary element for substituting constituents is a possible option to realize such a point defect engineering. Several applications of this concept for a PEC H2 evolution device will be described later.
|Type||Symbol||Formation energy1 (eV)||Electrical property|
Formation energies were extracted from the data given in ref.  for the case of a stable Cu-poor region of pure CZTS crystal.
3 CZTS thin-film solar cells
The potential of the CZTS compound for use in solar cells was first reported by Ito and Nakazawa in 1988 . They prepared CZTS thin films by atom beam sputtering and made a device with a cadmium tin oxide transparent conductive layer. The thus-obtained device showed an open circuit voltage (V OC) of 165 mV under simulated sunlight (AM 1.5G) illumination. After their findings, many efforts were made to develop thin-film solar cells using CZTS as an absorber layer. In 1996, Katagiri et al. reported successful fabrication of CZTS thin films by sulfurization of electron beam (EB)-evaporated precursor . They constructed a solar cell with a structure of Al/ZnO/CdS/CZTS/Mo/glass and achieved an appreciable sunlight conversion efficiency (η) of 0.66% for the first time. In 1997, Friedlmeier et al. fabricated CZTS thin films using thermal evaporation of the elements and binary chalcogenides in high vacuum: a solar cell based on the thus-obtained CZTS film with a structure of CZTS/CdS/ZnO showed η of 2.3% . In 1999, Katagiri et al. produced a CZTS solar cell with η of 2.63% using CZTS thin films, which were fabricated from a Cu–Sn–ZnS stack precursor layer deposited by using EB evaporation followed by sulfurization in an N2-diluted H2S atmosphere . That group also improved η to 5.45% in 2003 and to 6.7% in 2008 by optimization of the sulfurization process . Rapid improvements in solar cell properties were demonstrated in 2010–2014 by applying a sulfoselenide CZTSSe as a photoabsorber instead of CZTS [43,44,45]. Initially, Todorov et al. reported a conversion efficiency of solar cells of 9.6% by using a CZTSSe absorber layer deposited by spin coating using a hydrazine-based precursor solution containing metal binary chalcogenides followed by sulfurization or selenization . The conversion efficiency was then improved up to 12.6% in 2014 . Recently, Gong et al. achieved a record update of the sulfoselenide-based device to η of 13.0% . Regarding a pure sulfide CZTS-based device, Yan et al. broke the record: η of 11.5% was achieved by applying partial replacement of the Zn component in CZTS with Cd . The concept of substitution of constituent cations in CZTS was also applied for a PEC device, the details of which are discussed in Section 6.
4 Operation of PEC water splitting
Semiconductor–redox electrolyte contact is characterized by equilibration of the semiconductor Fermi level with the redox potential of the solution, resulting in the formation of a rectifying contact that is the same as that of a Schottky junction for a semiconductor–metal junction (Figure 1). Photogenerated electrons in a photocathode induce reduction of water into H2 under cathodic polarization, whereas photogenerated positive holes in a photoanode induce oxidation of water into O2 under anodic polarization. In the case of a PEC cell composed of the combination of a photocathode and a photoanode, the working current, J op (mA·cm−2), is obtained as an intersection point of current–potential curves of the photocathode and photoanode, as shown in Figure 6 . The corresponding potential at the crossing point is called a working potential. Therefore, to achieve a high current density, i.e., a high STH, the photocurrent at the crossing point should be improved. For a PEC cell composed of the combination of photoelectrodes, STH is defined by equation (1):
where P is the energy of incident sunlight, the value of which is usually 100 mW·cm−2 corresponding to the ASTM International established standard of sunlight spectra (AM 1.5G). It is clear for the photocathode side that realizing a large photocurrent at a relatively positive potential is directly linked to the achievement of high STH.
Deposition of an additional n-type layer over a p-type photocathode increases the charge separation efficiency through formation of a hetero-type p–n junction. Although the lattice mismatch between the two layers should be considered to suppress formation of interfacial defects, a critical carrier recombination center, this leads to an increased photocurrent and onset potential. Metal sulfide materials such as CdS with a thickness in a scale of several tens of nanometers deposited by the chemical bath deposition (CBD) method have been reported as good surface modifiers for various p-type photocathodes. Although the CBD method limits the choice of available materials, the process has a merit in terms of film conformality, process simplicity, easiness of thickness control, etching effect onto impurity phases, and possible operation in mild conditions. A CdS-based heterojunctin obtained by the CBD method was originally studied for its application in solar cells. An important parameter in a heterojunction is the band alignment of CBMs between the p-type photoabsorber and n-type buffer layer. Device simulations of solar cells composed of chalcopyrite Cu(In,Ga)Se2 absorbers with different In/(In + Ga) ratios (having different CBM energies) and n-type buffer layers with various CBM energies revealed that a spike-type alignment, i.e., the Cu(In,Ga)Se2 absorber, with its offset energy (ΔE C) of 0–0.4 eV is favorable, while a cliff-type alignment with ΔE C < 0 is unfavorable because it induces an increase in the probability of carrier recombination at the p–n heterointerface, leading to reduction in V OC (Figure 7) [70,71,72,73]. For PEC H2 evolution, the energetic property of p–n heterointerfaces should affect the photocurrent onset. Therefore, as described in the next paragraph, the optimum choice of an n-type buffer material is an important issue for achieving high PEC H2 evolution properties.
As the first example for the use of a CBD-derived CdS layer in a PEC H2 evolution system, Yokoyama et al. reported a chalcopyrite Cu(In,Ga)Se2-based photocathode . In their study, the surface of Cu(In,Ga)Se2 was modified with a CdS layer followed by deposition of Pt nanoparticles on the surface of the CdS layer (Pt/CdS/Cu(In,Ga)Se2). As shown in Figure 8, a current density–potential (J–V) plot of the Pt/CdS/Cu(In,Ga)Se2 photocathode measured in an aqueous solution of 0.1 M Na2SO4 with pH adjusted to 9.5 indicated 3 times larger photocurrent with a 0.3 V higher onset potential than that obtained by platinized Cu(In,Ga)Se2 without having a CdS buffer layer (Pt/Cu(In,Ga)Se2). In that same period, we investigated PEC H2 evolution by using a chalcopyrite CuInS2 thin film modified with Pt nanoparticles and we achieved a CdS buffer layer (Pt/CdS/CuInS2) and proved a significant enhancement of PEC H2 evolution properties by loading of a CBD-derived CdS layer . Moriya et al. performed a semi-quantitative calculation of the built-in electric field of electrolyte-chalcopyrite CuGaSe2 thin film interfaces without and with a CdS buffer layer by using Poisson’s equations . They found that the presence of CdS led to an increase in depletion layer width at a certain potential, indicating an enhancement of carrier separation at the interface region of the CuGaSe2 thin film (Figure 9). As another example of an n-type buffer layer, we have demonstrated the effectiveness of a CBD-deposited In2S3 buffer layer for a chalcopyrite CuInS2-based photocathode instead of CdS . Spectroscopic evaluation of the CBM offsets of In2S3/CuInS2 and CdS/CuInS2 revealed a favorable spike-type positive CBM offset with ΔE C of 0.27 eV in the In2S3/CuInS2 heterojunction, whereas the CdS/CuInS2 junction had an unfavorable cliff-type negative offset of ΔE C of –0.47 eV (Figure 10), leading to appreciable enhancement of the interface carrier recombination .
5 CZTS photocathode for PEC H2 evolution
Table 2 summarizes the existing reports on PEC properties of copper-kesterite-based photocathodes. The first application of CZTS as a photocathode was in 2010 by Yokoyama et al., and a photocurrent density of 2.3 mA·cm−2 at 0 VRHE was achieved with a CZTS thin film obtained by sulfurization of an EB-deposited Cu/Sn/ZnS precursor layer modified with CdS and Pt nanoparticles . They also reported the effect of loading of a TiO2 layer between Pt and CdS: the Pt/TiO2/CdS/CZTS stacked photocathode showed a maximum photocurrent density of 9 mA·cm−2 at 0 VRHE and a photocurrent onset of 0.6 VRHE. Although details were not discussed, surface-loaded TiO2 might act as a mediator for electron transfer from the CdS layer to the Pt catalyst. To evaluate a single photocathode under application of external bias voltage, half-cell solar to hydrogen efficiency (HC-STH) was defined by equation (2):
where E RHE is the working potential referring to a reversible hydrogen electrode (VRHE) . The Pt/TiO2/CdS/CZTS stacked photocathode achieved the maximum HC-STH of 1.2% at 0.22 VRHE. Improvements in photocurrent density of ca. 12–13 mA·cm−2 at 0 VRHE and photocurrent onset of 0.85 VRHE were reported by Yang et al. using a Pt/TiO2/CdS/CZTS stacked photocathode obtained by using a solution-processed CZTS thin film and a TiO2 layer deposited by an atomic layer deposition (ALD) technique .
|Year||Photocathode||pH||J 0 (mA·cm−1)||Onset (VRHE)||HC-STHmax (%)||STH (%)||Ref.|
A 300 W xenon lamp with a UV light cut off filter (less than 420 nm) was used as a light source.
The value was obtained after 10 h photoirradiation.
For the use of a CZTS-based photocathode in a Z-scheme or tandem device, a critical problem of the photocathode modified with a CdS layer and Pt nanoparticles is its poor durability, specifically under applied biases of more than 0 VRHE, i.e., the power generating region. When PEC H2 evolution is performed at, for example, 0.6 VRHE, the Pt/CdS/CZTS photocathode stack is subjected to oxidative corrosion of CdS layers  and delamination of the Pt catalyst . To improve the stability, we found that deposition of a CBD-derived In2S3 layer used for the above-described chalcopyrite CuInS2-based photocathode over the surface of the CdS/CZTS stack significantly enhanced its PEC stability . After the deposition of Pt nanoparticles, the photocathode composed of the Pt/In2S3/CdS/CZTS stack showed higher durability than that of the photocathode without the In2S3 interlayer (Pt/CdS/CZTS). Although the Pt/In2S3/CdS/CZTS photocathode was gradually degraded when it used a power generating region, we demonstrated for the first time a successful bias-free overall water splitting by using a CZTS-based photocathode upon combination with BiVO4 (Figure 11). The STH value of the tandem cell was 0.28%.
Further improvements in PEC H2 evolution properties were also achieved in our subsequent studies by focusing on protection of the CdS layer to suppress its oxidative photocorrosion. In 2018, an HfO2 ultrathin layer (ca. 6 nm) deposited over the CdS/CZTS stack by ALD was found to be effective as a protection layer . An unbiased solar water splitting device based on the Pt/HfO2/CdS/Cu2ZnSnS4 photocathode in tandem with a BiVO4 photoanode not only exhibited an STH of more than 1% but also showed high long-term durability of over 10 h (Figure 12). In terms of the application of another CDB-derived n-type layer, ZnS was also found to be effective as a protection layer. Although the photocathode was not proved to have long-term stability, it showed the maximum HC-STH of 2.1% . In our recent study, more than 7% efficient solar water splitting with long-term stability of 24 h was achieved by using an HfO2/CdS/HfO2 sandwiched buffer layer . The effect of the deposited HfO2 ultrathin layer (0.5 nm) by ALD at the CdS/CZTS interface would be suppression of carrier recombination inside the photocathode, whereas the HfO2 layer (6 nm) between CdS and Pt or the electrolyte was likely to facilitate electron transfer without being trapped. Notable features of the tandem device were its successful increase in size up to 5 × 5 cm2: the large device exhibited STH of 3.17% with superior long-term stability over 60 h.
6 Substitution of constituent cations in CZTS to improve PEC performance
As discussed above, the p-type semiconducting properties of CZTS predicted by theoretical studies suggested a critical limitation induced by the presence of CuZn antisite point defects at a high concentration. In the photovoltaic field, substitution of Zn with other cations of larger ionic size is hypothesized to increase the formation energy of CuZn. Among the variety of divalent cations, Cd2+ proved to be a positive substituent of Zn2+: partial substitution of Zn2+ in the crystalline lattice of the CZTS film with Cd2+ (CCZTS) induced reduction in CuZn (thus leading to reduction in band tailing) as well as increase in grain size, resulting in enhancement of solar cell properties including short circuit current density (J SC), fill factor (FF), and η . Tay et al. applied a CCZTS film fabricated through a solution-processed spin coating to PEC H2 evolution. As a result, the CCZTS film with a Cd/Zn ratio of 0.67 (i.e., 40% of Zn2+ in CZTS being substituted with Cd2+) showed the best performance: photocurrent density of the CCZTS-based photocathode at 0 VRHE reached 17.5 mA·cm−2 with the maximum HC-STH of 4%, whereas that of a bare CZTS-based photocathode was 5 mA·cm−2 at 0 VRHE . As another substituent, Ba2+ was also studied not only for PEC H2 evolution [81,82,83,84] but also for photovoltaic application [85,86]. Since Ba adopts much larger coordination due to the large ionic size and its electropositive nature, the formation of detrimental cationic disorders such as CuZn would be suppressed. For the PEC system, for example, substitution of Zn2+ in a sulfoselenide CZTSSe thin film with Ba2+ (CBTSSe) led to relatively high photocurrent density of more than 12 mA·cm−2 at 0 VRHE after applying modifications of its surface with TiO2/CdS bilayers and Pt nanoparticles .
As a similar theory to Cd2+ substitution, recent theoretical results reported by Chen et al. predicted that Ag-based kesterite (i.e., Ag2ZnSnS4 [AZTS]) has a small number of AgZn antisite point defects because significant differences in ionic radii between Ag and Zn ions lead to relatively high formation energy of the defects . Therefore, application of Ag+ substitution was also studied in photovoltaics [88,89,90] as well as PEC H2 evolution [91,92,93]. It is known that the pure AZTS tends to show an n-type semiconductive characteristic . Thus, in all of the work reported, CZTS films in which Cu2+ was partially substituted by Ag+ with an Ag/(Ag + Cu) ratio up to 0.4 were used. Regarding the PEC performance, actual improvements in the photocurrent density at 0 VRHE were proved. Specifically, a recent work by Tay et al. achieved appreciable improvement in photocurrent density at 0 VRHE (17.7 mA·cm−2) and onset potential of the photocurrent (0.85 VRHE) by using ACZTS photocathodes with an optimum Ag+ content: the maximum HC-STH reached 5.2% . As mentioned above for Cd2+ substitution, high photocurrent density was attributed to reduction in CuZn and grain growth induced by the partial substitution. A significantly positive onset of photocurrent would be derived from lowering VBM energy, leading to an increase in built-in potential.
Substitution of Sn4+ in a CZTS thin film was also studied for its PEC applications. Wen et al. demonstrated significant improvements in thin film quality, i.e., appreciable grain growth, achieved by partial substitution of Sn4+ with Ge4+ (CZGTS), resulting in improvement in photocurrent density at 0 VRHE (11.1 mA·cm−2) . We found the effectiveness of Ge4+ replacement to induce an upward shift of the energy of the CBM; the photocathode based on the film with complete replacement of Ge4+ (CZGS) resulted in showing CO2 reduction ability after applying surface modification of the ZnS layer . Although functions of the surface coverage with ZnS have not yet been fully clarified, the modification might result in modulation of band alignment, passivation of interfacial defects, and creation of efficient catalytic sites for CO2 reduction, leading to a ca. five times larger rate of CO evolution than that when using a CZGS thin film without any modification (Figure 13).
7 Further developments of photocathodic properties
Although the abovementioned recent reports show a rapid increase in PEC properties of copper kesterite compounds for water reduction, further improvements are required to reach a necessary level for predicting its practical use. Regarding the improvement of bulk properties of the kesterite thin film, extending the above-discussed substitution concept to dual substitution would be worthy to be investigated. Indeed, Hadke et al. realized a kesterite-thin-film-based solar cell with sunlight conversion efficiency of 10.1% by applying partial substitutions of Cu and Zn sites in CZTS with Ag and Cd (Cu1.8Ag0.1Cd0.3Zn0.7SnS4), respectively, the efficiency value of which was higher than that of a solar cell based on a kesterite thin film with 30% of Zn site substituted with Cd . They suggested that the Cd substitution is likely to improve bulk properties of the kesterite thin film, leading to increase in carrier mobility and lifetime, whereas Ag substitution mainly contributes to improvements in interface quality of the kesterite n-type buffer layer (CdS) heterojunction, resulting in an increase in V OC. Moreover, in view of band engineering, substitution of constituent elements in CZTS can also lead to modulate the E g of CZTS. Therefore, band gap gradings also attempted to improve PEC properties of kesterite-based photocathodes. As has been proved by copper chalcopyrite Cu(In,Ga)Se2-based solar cells , fabricating a V-shape grading of CBM would lead to improvements in saturated current density, photocurrent onset potential, and maximum HC-STH.
Another promising approach for improvements of PEC photocathodic properties is doping(s) of alkaline metal cations in the kesterite lattice. In the Cu(In,Ga)Se2 solar cell technology, sodium (Na) doping is known to be indispensable to achieve a high efficiency, though the influence of Na-doping is still a debated topic . Moreover, a potassium (K) treatment was found to be effective for further improvement of sunlight conversion efficiency of the Cu(In,Ga)Se2 solar cell . Although the impact of the K-doping is also fully understand, critical properties affected by the K-doping are changes in chemical composition of the Cu(In,Ga)Se2 layer and interface properties of the Cu(In,Ga)Se2-CdS heterojunction. Regarding the copper kesterite-based solar cell, Li et al. reported that the Na doping in co-evaporated CZTSe thin film induced increase in the hole density as well as making the acceptor energy shallower, leading to enlargement of built-in potential to improve V OC of the solar cell based on the Na-doped CZTSe thin film. The doped Na in the CZTSe thin film was also considered to reduce deep recombination centers in the kesterite, leading to further increase in V OC of the solar cell. Thus, we can expect further improvements in PEC properties by applying appropriate loadings of alkaline components.
Copper kesterite CZTS is a promising candidate for a photocathode material for PEC water splitting because of its cost effectiveness and less toxicity of constituent elements, appropriate photoabsorption properties, usability in the polycrystalline state, and chemical stability. Recent reports have shown rapid increases in the photocurrent density and onset potential of the photocurrent. One of the critical issues of these non-oxide-based PEC devices is its durability under operating conditions. Several works suggested the effectiveness of the use of a protection layer that is resistant to oxidative corrosion of surface-loaded components of the photocathode, such as CdS, leading to maximum stability up to several days for the continuous use of bias-free overall water splitting in a Z-scheme (tandem) system. The other important problem of the CZTS material is its point defect nature. To solve that problem, substitutions of constituent elements in the CZTS lattice were hypothesized: a novel defect engineering concept could be achieved by the replacement of Zn2+ with Cd2+ and Ba2+ as well as by replacement of Cu+ with Ag+. Moreover, the concept of cation substitution also led to alterations of CBM and VBM energies of CZTS, resulting in improvements of PEC H2 evolution property (especially photocurrent onset) as well as expression of CO2 reduction ability. Although the PEC properties that have so far been achieved have not reached the necessary level for practical use, there is room for further development of PEC properties by using effective combinations among surface protection(s), defect engineering(s), and band engineering(s).
This work was supported by JSPS Grants-in-Aid for Scientific Research (KAKENHI), awards no. 19H02822, 20H05120, and 21K19033.
Funding information: JSPS Grants-in-Aid for Scientific Research (KAKENHI), awards no. 19H02822. JSPS Grants-in-Aid for Scientific Research (KAKENHI), awards no. 20H05120. JSPS Grants-in-Aid for Scientific Research (KAKENHI), awards no. 21K19033.
Author contributions: Shigeru Ikeda: conceptualization, writing – original draft, and writing – review and editing.
Conflict of interest: There is no conflict of interest.
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