Solution-Processed Earth-Abundant Cu2BaSn(S,Se)4 Solar Absorber

Article Cite This: Chem. Mater. 2018, 30, 6116−6123

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Solution-Processed Earth-Abundant Cu2BaSn(S,Se)4 Solar Absorber Using a Low-Toxicity Solvent Betul Teymur,#,† Yihao Zhou,#,† Edgard Ngaboyamahina,§ Jeffrey T. Glass,*,†,§ and David B. Mitzi*,†,‡ †

Department of Mechanical Engineering and Materials Science, ‡Department of Chemistry, and §Department of Electrical & Computer Engineering, Duke University, Durham, North Carolina 27708, United States

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S Supporting Information *

ABSTRACT: Cu2BaSn(S,Se)4 (CBTSSe) has recently gained substantial attention as an alternative absorber material for photovoltaic (PV) and photoelectrochemical (PEC) applications due to the abundance of the constituent elements, a large absorption coefficient, tunable band gap ranging from 1.5 to 2 eV, and reduced tendency for antisite disorder relative to Cu2ZnSn(S,Se)4. In this study, as an alternative to more expensive vacuum-based film-deposition processes, we report a low-toxicity solution-based process for the fabrication of high quality CBTSSe absorber layers with micrometer-scale film thickness and grain size. The facile process involves spin-coating an environmentally benign solution of highly soluble, inexpensive, and commercially available precursors, Ba(NO3)2, Cu(CO2CH3)2, and SnI2, followed by sequential sulfurization/ selenization annealing. A high-temperature prebaking step under sulfur vapor is needed for each film layer to avoid forming the difficult-to-remove impurity phase, Ba(SO4), when starting from the soluble Ba(NO3)2 reagent. The solution-based CBTSSe films have been employed in a Pt/TiO2/CdS/CBTSSe photocathode structure (e.g., for water splitting), exhibiting an ∼10 mA/ cm2 current density at 0 VRHE, comparable to that of vacuum-deposited CBTSSe PEC devices. Our approach for the fabrication of CBTSSe absorbers represents a first step in achieving low-cost and large-scale solution-processed solar devices based on this material.

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INTRODUCTION CdTe and multinary thin-film chalcogenide absorbers based on Cu(In,Ga)(S,Se)2 (CIGSSe) currently represent the fastest growing segment of the commercial thin-film photovoltaic (PV) market.1−3 These systems further provide an important opportunity for application in photoelectrochemical (PEC) devices, demonstrating photocurrents of 32.5 mA/cm2 at 0 VRHE.4 However, despite the success of CIGSSe and CdTe PV/ PEC devices, these zinc-blende-related semiconductors rely on elements that are either costly and/or rare in the earth’s crust (e.g., In, Te) or present toxicity issues (e.g., Cd), thereby limiting pervasive application. To develop a material that is truly compatible with terawatt deployment, alternatives are needed that employ less toxic and lower cost elements, while also offering low-cost manufacturing options and maintaining or improving upon the advantages of CIGSSe/CdTe absorbers with respect to direct band gap tunability and high device performance. Cu 2 ZnSnS 4 (CZTS) or Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin-film devices have recently received substantial attention due to a suitable/tunable band gap (1.0−1.6 eV) and a high absorption coefficient in the visible spectral range (∼105 cm−1),5,6 leading to performance advances in PV7,8 and PEC devices.9−11 Despite significant progress, the record PV power conversion efficiency (PCE),7 PEC photocurrent density, and the half-cell solar-to-hydrogen efficiency (HC-STH) (12.6%, 17 mA/cm2 and ∼4% at 0 VRHE, respectively)10 still remain © 2018 American Chemical Society

substantially behind what can be achieved with CIGSSe/CdTe (>20%, 32.5 mA/cm2, and ∼8.5% at 0 VRHE,4,12 respectively). The relatively low performance levels for CZTSSe devices stem in part from the similar sizes of Cu+/Zn2+/Sn4+ ions and similar tetrahedral coordinations, which facilitate antisite disordering and corresponding defect states that serve as recombination centers.13,14 To overcome antisite disordering in multinary chalcogenide semiconductors, we have recently suggested a design principle in which the constituent atoms of the multinary compounds are replaced with atoms of dissimilar chemistry, ionic size, and coordination.15,16 As a demonstration of this concept, Cu2 BaSn(S,Se) 4 (CBTSSe) has been proposed as an alternative to CZTSSe.15−21 The CBTSSe structure incorporates earth-abundant Ba2+, which has substantially different size (1.56 Å)22 and coordination (8-fold) compared to Zn2+ (0.74 Å; 4-fold), Cu2+ (0.74 Å; 4-fold), and Sn4+ (0.69 Å; 4-fold). Like CZTSSe, the quasi-direct band gap of CBTSSe can be tuned over a fairly wide range of 1.5−2.0 eV16 by varying the sulfur-to-selenium ratio, suggesting that this absorber material may be well-suited for single and multijunction PV and PEC applications. Cu2BaSn(S4−xSex) materials with 0 < x ≤ 3 adopt Received: June 17, 2018 Revised: August 9, 2018 Published: August 10, 2018 6116

DOI: 10.1021/acs.chemmater.8b02556 Chem. Mater. 2018, 30, 6116−6123

Article

Chemistry of Materials

Figure 1. (a) Schematic illustration of the absorber layer preparation steps. (b) Set point temperatures of the hot plate during the sulfurization/ selenization processes. While the dark gray color box represents the quartz cover, the lighter gray feature on top denotes an additional aluminum foil covering. All the steps were completed in a nitrogen-filled glovebox with temperature ranging between 28 and 37 °C (depending on the film preparation step) and with oxygen and water levels at 3 lead to a similar orthorhombic structure with Ama2 space group and larger band gap.17,19,21,23,24 Recent studies examining the full width at half-maximum (FWHM) and energy position of the photoluminescence (PL) peak relative to the band gap, coupled with an analysis of the cutoff in absorption at the band gap, indicate that antisite disorder is significantly reduced in CBTSSe relative to CZTSSe.15,16,18−21 To demonstrate the promising absorber characteristics of CBTSSe, initial vacuumdeposited PV and PEC devices exceeding 5% power conversion efficiency (for PV) and photocurrent density of 12 mA/cm2 at 0 VRHE with HC-STH of 1.85% (for PEC) have been demonstrated.18,19 Besides the importance of earth-abundance/low-toxicity, electronic tunability, defect resistance, and ultimately high device performance, a viable solar absorber technology should aim to reduce manufacturing costs, for which a key factor relates to the film deposition process. Solution-based approaches are particularly attractive due to lower capital costs, higher process throughput, and improved material utilization.8,25−30 Despite the promise of solution-based processing, many challenges hinder successful implementation of these approaches for the class of multinary chalcogenides, including the generally insoluble nature of covalently bonded metal chalcogenides, the issue of removing solvent and foreign components from the precursor film, and the difficulty in achieving uniform and compact films with large grain size. Additionally, heat treatment after precursor deposition (e.g., often >500°C) is essential to enable chemical and physical reactioni.e., solvent removal, precursor decomposition, target compound nucleation, and film grain growth/densification. Such high-temperature annealing, however, often leads to detrimental volatile element loss (and associated difficulty in controlling film stoichiometry), substrate−absorber reaction, and secondary phases.27,31 Despite these difficulties, among the many hydrazine-free molecular solution paths for depositing metal chalcogenide semiconductors, spin-coating from a dimethyl sulfoxide (DMSO) solution containing soluble metal salts coupled with a sulfur source,31−33 first demonstrated for CZTSSe by Ki et al.33 in 2011 and for CIGSSe by Uhl et al.34 in 2015, has provided an attractive pathway to achieve high-performance PV/PEC devices. Several challenges

have been addressed for this solution processing pathway, including the need to control the redox chemistry occurring within the solution and during reaction, how to control segregation into a bilayered film structure, and the challenges of passivating grain boundaries within the resulting films, leading to a highest PCE of 11.8% for CZTSSe PV devices.31−33 To date, the only solution-processed CBTSSe material has been recently reported by McCarthy and Brutchey35 using a thiol−amine solvent mixture. In their approach, bulk Cu2S, BaS, SnO, and Se powders were dissolved in a so-called “alkahest solvent” followed by annealing at 350 °C, yielding phase-pure CBTSSe. The thin film obtained from spin-coating two layers of the as-prepared solution on a glass substrate shows a rough structure with voids, and no device application has been demonstrated. Two challenges for solution processing CBTSSe include the generally poor solubility for Ba salts and the low electronegativity for Ba (relative to for example, Zn), which can lead to difficult to remove Ba salts as secondary phases within the final targeted CBTSSe films. In the current work, we prepare CBTSSe thin films from a solution of highly soluble, inexpensive, and commercially available precursors using the environmentally friendly low-toxicity solvent DMSO. After gaining an understanding of the chemistry involved with incorporating the highly electronegative Ba ion, deposition by spin-coating of the solution onto molybdenum-coated sodalime glass (Mo/SLG) yields single-phase, continuous (nearly pinhole-free) films. The resulting films when employed in a PEC device yield a photocurrent of 10 mA/cm2 at 0 VRHE, comparable to that obtained from vacuum-deposited CBTSSe PEC devices. This early device result indicates that our simple solution preparation approach is viable for fabricating high quality films for a broad range of CBTSSe solar devices.

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EXPERIMENTAL SECTION

Molecular Ink Synthesis and Absorber Layer Formation. A molecular solution was prepared using the commercially available precursors Cu(CO2CH3)2 (99.99%, Sigma−Aldrich), anhydrous SnI2 (99.99%, Sigma-Aldrich), Ba(NO3)2 (99.99%, Sigma-Aldrich), and thiourea (NH2CSNH2) (≥99.0%, Sigma-Aldrich). The solution stoichiometry was maintained using 2.56/1.28/1.54/5.38 mmol of Cu/Sn/Ba/thiourea (i.e., nominally Ba and S rich relative to the ideal stoichiometry of CBTSSe), respectively. All the precursors were mixed simultaneously in 3.5 mL of DMSO (99.99%, anhydrous, Sigma-Aldrich) and stirred overnight at room temperature until the 6117



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precursors completely dissolved. The dark brown CBTS precursor solution was filtered using a 2 μm PTFE filter prior to coating to remove any possible extraneous particles (e.g., dust) from the solution. Mo/SLG substrates were cleaned using the following steps: sonication for 3 min each in acetone/DI water/dilute (1%) ammonium hydroxide in consecutive order, followed by argon plasma cleaning for 10 min. The solution was spin-coated onto the clean Mo/ SLG at a spin speed of 2500 rpm/min and then immediately prebaked on a hot plate for 2 min (using a quartz cover to maintain an elemental sulfur vapor atmosphere, provided by the addition of some elemental S under the cover). Exploration of various prebaking temperatures shows that higher temperature is the most effective at producing single phase films (as described in the Results and Discussion section), and 540 °C provides the best results. The film prepared after repeating this process (either 6 or 9 times, as noted in the text) was again sulfurized for 10 min at 540 °C and then cooled to 340 °C in 5 min before removing from the heating cycle. During this stage, an aluminum foil covering was added to the quartz cover to provide better temperature uniformity during the postdeposition film sulfurization. To incorporate selenium atoms, the sulfurized film was annealed in selenium vapor at 570 °C for 5 min and allowed to cool to 470 °C over 10 min, before removal from the process. A schematic illustration of these sulfurization and selenization steps is shown in Figure 1. All solution preparation, spin-coating, and annealing were conducted in a N2-filled glovebox. PEC Device Fabrication. To form a p−n junction, a CdS layer (∼50 nm) was grown on the CBTSSe film via a chemical bath deposition method (CBD).16 Subsequently, an atomic-layer-deposited (ALD) TiO2 film (∼30 nm) was deposited to improve electron charge extraction and protect devices from photocorrosion.18 One edge of the as-prepared TiO2/CdS/CBTSSe film was scratched to expose the Mo substrate, and this edge was electrically connected to a copper wire through In/Ga metal paste. A glass tube was used to protect the copper wire from electrochemical reactions (see the scheme in Figure S7). Side surfaces were covered by an insulating resin (Loctite, EA 9462). Pt nanoparticles were electrochemically deposited using a three-electrode setup. Pt mesh and Ag/AgCl in saturated KCl solution were used as the counter electrode and reference electrode, respectively. 0.1 M Na2SO4 with 1 mM H2PtCl6 was used as the electrolyte. The deposition was done by polarizing the TiO2/CdS/CBTSSe electrode at −0.4 V vs Ag/AgCl for 2 min. After the Pt deposition, the as-prepared Pt/TiO2/CdS/CBTSSe electrode was tested employing a 0.5 M Na2SO4, 0.5 M KH2PO4 electrolyte (pH = 4.3). Characterization Methods. The X-ray diffraction (XRD) data were collected using a PANalytical Empyrean powder X-ray diffractometer under ambient conditions using Cu Kα radiation to determine the phase purity and composition of the solution-deposited CBTSSe films. The oriented CBTSSe diffraction pattern was simulated using CrystalMaker software (version 10.0.5). XRD pattern profile fitting was performed using the HighScore Plus v3 software package. The film microstructure and overall morphology were investigated using a FEI XL30 scanning electron microscope (SEM). Energy dispersive X-ray spectroscopy (EDX) was performed using a Bruker XFlash 4010 and an acceleration voltage of 20 kV to quantify the film composition. To determine the photoluminescence characteristics of the film, measurements were performed at room temperature (442 nm laser excitation) using a Horiba Jobin Yvon LabRAM ARAMIS system. Electrochemical measurements were performed at room temperature in a standard three-electrode cell using a Bio-Logic SP-200 electrochemical interface. A 150 W Xe lamp with AM 1.5G filter was used as the light source. Incident photon to current efficiency was measured by a customized Newport-Oriel system powered by a 300 W Xe (ozone-free) lamp. Further details of the electrochemical characterization are available in the Supporting Information.

Article

RESULTS AND DISCUSSION

Despite having a composition similar to CZTSSe, the substitution of Ba for Zn introduces a lower electronegativity atom (0.89 for Ba and 1.65 for Zn on the Pauling scale) and a different preferred coordination for this atom (i.e., likely leading to significant differences in the phase diagrams and chemistries). In addition to low electronegativity, the characteristically low solubility for Ba salts hinders the targeting of a molecular solution approach. In our study, a molecular solution of CBTS precursors was successfully prepared by simultaneously dissolving copper(II) acetate, tin(II) iodide, barium nitrate (a soluble Ba source), and thiourea into an environmentally benign DMSO solvent, as described in the Experimental Section. After all precursors are completely dissolved, the solution color gradually transforms to a dark brown color (Figure 1). In contrast to the precursor redox equilibration described by Xin et al.31 for CZTSSe, the alternative solution preparation process (i.e., the process that involves separate dissolution of the metal precursors rather than simultaneous) did not change the final brown color of the solution, as shown in Figure S1, suggesting that the precursor redox equilibration in our experiment is independent of solution preparation process. Unlike the optimized process illustrated in Figure 1, we had first tried an initial low-temperature prebaking stage of the solution-processed Cu2BaSnS4 (CBTS) film without using a cover and sulfur vapor. Within the film prepared in this initial prebaking stage, besides the binary and ternary chalcogenides, Ba(SO4) was frequently observed as a secondary phase and was difficult to remove with subsequent annealing steps (i.e., sulfurization and selenization). Ba(NO3)2 decomposes according to the following reaction pathway:36 2Ba(NO3)2 + heat → 2BaO + 4NO2 + O2

This decomposition produces BaO, which in turn can react with a sulfur source under low-temperature conditions to form Ba(SO4), as shown in Figure S2a using a prebaking temperature of 340 °C for the CBTS films.36 To circumvent Ba(SO4) formation, we explored sulfur reactivity/kinetics during the prebaking step, by varying the prebaking temperature from 340 to 540 °C, coupled with the availability of sulfur. The films prepared using excess amount of thiourea (30 mol %) in the solution with or without sulfur vapor at high prebaking temperature of 540 °C yield the X-ray diffraction patterns shown in Figure 2a. From these data, the availability of excess sulfur in the environment during prebaking each layer appears to be important for controlling Ba(SO4) formation, with the sulfur-added condition leading to less of the impurity phase. As shown in Figure 2b, increase in the prebake temperature (with added sulfur during the anneal) also significantly decreases the intensity of the Ba(SO4) XRD peaks. When temperatures higher than 490 °C are employed (Figure S2b), Ba(SO4) essentially disappears from the XRD scans of the resulting films, contributing to the final choice of 540 °C for this step. As a result of these investigations, we concluded that, for our molecular solution process, increasing S activity through a high-temperature S-rich prebaking step appears to be necessary to avoid Ba(SO4) formation (as in Figure 1). While the modified process yields a film with no Ba(SO4), when using a stoichiometric (with respect to the various metals) solution, impurities of Cu2SnS3 remain within the film, a commonly observed secondary phase during the 6118


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13.69°, 33.04°, and 50.47°, respectively. The intensity of these peaks is considerably higher than that for the analogous simulated powder diffraction pattern for a randomly oriented material, as shown in Figure S3, indicating preferred orientation along the c-axis direction. Solution-processed CZTSSe absorbers formed from a DMSO solution have often yielded a multilayer morphology containing a fine grained bottom layer rich in impurities (e.g., carbon) and large grain upper layer.38−43 The multiple hightemperature prebaking steps (involving sulfur vapor) for this study and subsequent sulfurization/selenization of the film enable large CBTSSe grains to form as essentially a single layer across the film thickness (1 μm), as shown in the crosssectional scanning electron microscopy (SEM) image in Figure 3d. The evolution of the film morphology from prebaking step to sulfurization step is also shown in Figure S5. The top (Figure 3c) and cross-sectional SEM images reveal that the solution-processed selenized film offers micrometer-size grains ranging from ∼0.9 to ∼4.5 μm, with only a few pinholes across the film surface (see Figure S4 for a wider SEM view of the film surface), leading to a suitable film structure for highperformance PEC or PV devices. Besides CBTSSe grain structure, the SEM cross-section image also indicates the presence of a Mo(S,Se)2 layer with a thickness of about 360 nm forming at the interface between the CBTSSe and Mo. While a thin Mo(S,Se)2 layer has been shown to enhance film adhesion and provide an ohmic contact for CZTSSe films, a thicker layer may increase series resistance and lower device performance.44 The absorber film stoichiometry has also been characterized using energy dispersive X-ray spectroscopy (EDX) (Figure S6). Considering the possible inhomogeneity across the sample, we measured several spots and report an average of the results. The standard deviation of the composition of each element was found to be less than 1.2 at. %. The compositional analysis of the film yields the elemental ratios Cu/(Ba + Sn) = 1.00 and Ba/Sn = 1.00, which indicate good agreement with the expected elemental ratios for CBTSSe. The value of the Se/(S) and Cu/Se ratios are 0.89 and 0.96, respectively. Therefore, the average final CBTSSe film composition is approximately Cu2.0Ba1.0Sn1.0(S2.3Se2.1) from the EDX data (corresponding to x ≈ 2). When analyzing the EDX data, substantial overlap exists between the Mo−L and S−K X-ray emission lines, as can be seen in Figure S6 at 2.29 and 2.31 keV,45 leading to uncertainty regarding the sulfur quantification. The higher than expected sulfur in the final composition of the Cu2.0Ba1.0Sn1.0(S2.3Se2.1) film could be due to this uncertainty, along with the additional issue of sulfur/selenium associated with the Mo(S,Se)2 interfacial layer. Photoluminescence (PL) measurements (Figure 3b) were performed to verify the band gap (Eg) of the solutiondeposited CBTSSe film. Shin et al.16 have shown that for CBTSSe the PL peak falls within 10 meV of the band gap value (as determined from either the absorption data Tauc plot or the long wavelength cutoff in the PV/PEC device quantum efficiency curves) and that therefore the PL peak position represents a good estimate for the band gap value. In the current sample, the relatively sharp PL peak at 737.8 nm corresponds to a band gap of 1.68 eV. Emission below the band gap value, as determined from absorption data, provides a mapping of emissive states within the band gap and therefore of disorder and band tailing within the semiconductor. The narrow observed peak FWHM for the current film (∼45 nm

Figure 2. (a) Using a prebaking temperature of 540 °C, Ba(SO4) still exists if layers are prepared without sulfur vapor during the anneal. (b) XRD patterns demonstrating the effectiveness of using a higher temperature prebaking step under sulfur vapor to eliminate Ba(SO4) (PDF no. 01-072-1378) formation. The Mo peak at 2θ = 40.5° and more distinguishable Cu2SnS3 (PDF no. 01-089-4714) and Ba(SO4) peaks are shown with ∗, #, and dashed lines, respectively. XRD peaks labeled “?” at 2θ = 22.3817 and 29.5295 cannot be indexed to any known phases. A reference XRD pattern for CBTS is given from PDF no. 03-065-7569.

formation of CZTSSe under a variety of experimental conditions.37 To remove the Cu2SnS3 secondary phase, an excess of Ba(NO3)2 (up to 20 mol %) was added to the precursor solution (as described in the Experimental Section). The reason behind the importance of this excess Ba is still under investigation. Ultimately, the goal for the spin-coating/ prebaking (repeated several times to build up sufficient thickness) and subsequent sulfurization steps is to achieve a single phase or nearly single-phase film of pure CBTS, as a starting point for the selenization process. To tune the band gap of the CBTS film, a selenization process was performed at 570 °C. XRD analysis of the resulting films (Figure 3a; also see Figure S3 for a wider 2θ range) demonstrates the single-phase nature of the resulting films. The peak positions and sharpness confirm the phase purity and high degree of film/absorber crystallinity. The refined (Pawley fitting parameters are discussed in more detail in the Supporting Information) lattice constants of the selenized film are a = 6.5229(5) Å and c = 16.249(2) Å, corresponding to a unit cell volume of 599.55(12) Å3, which is smaller than that for the x = 3 sample reported for a similarly annealed sputtered film and therefore suggesting a lower x value.18−21 In addition, from indexing the XRD pattern, the films prepared under this condition tend to exhibit preferred orientation along the (00l) direction, as can be seen in Figure 3a from the intensity of (003), (006), and (009) peaks at 2θ = 6119


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Figure 3. (a) X-ray diffraction (XRD) pattern for the solution-processed CBTSSe film (black) on Mo/SLG. Pawley fitting (red dots) to a reference XRD pattern, Cu2BaSnS4−xSex (P31) (x = 2) as presented by Shin et al.,16 shows the single-phase nature and yields the lattice constants given in the text. The intensity difference between observed and calculated peaks (red line) is also shown (for more detailed discussion of the Pawley fitting, see the Supporting Information). Miller indices are noted for higher intensity peaks, and the Mo film peak is centered at 2θ = 40.5°. The simulated powder diffraction pattern of Cu2BaSnS2Se2 (P31 space group) accounting for preferred orientation is presented with the blue line. The XRD pattern over a wider 2θ range of 10°−70° is given in Figure S3, along with a comparison of the randomly and c-axis-oriented simulated XRD patterns. (b) The room temperature normalized photoluminescence (PL) spectrum was obtained using a 442 nm laser excitation. (c) SEM topview and (d) cross-sectional image of the CBTSSe film (nine layers) deposited on Mo/SLG.

width), similar to that obtained for sputtered films, reflects the reduced effects of disorder (i.e., less cation antisite disorder) relative to CZTSSe.16,18−20 Shin et al.16 examined the Cu2BaSnS4−xSex (0 ≤ x ≤ 3) unit cell parameters as a function of band gap with increasing Se content, from which we can find that Se content x = 1.7 corresponds to the band gap value of Eg = 1.68 eV (as observed from PL measurement). This x value is in reasonable agreement with the x obtained from EDX (x ≈ 2), especially given the complicating influence of the Mo(S,Se)2 interfacial layer to the EDS analysis. To further demonstrate film quality/characteristics, solar water splitting photocathodes based on the solution-processed CBTSSe films (six layers with the total thickness of around 600 nm) were fabricated with the electrode scheme shown in Figure 4a and Figure S7. The electrolyte consisting of 0.5 M Na2SO4 and 0.5 M KH2PO4 (pH = 4.3) was used for all the experiments. TiO2 and CdS overlayers were employed to enhance the charge separation and transport between CBTSSe and the electrolyte. It has been reported that a p−n junction structure can significantly improve the performance of Cubased chalcogenide photocathodes.18,46,47 A Pt catalyst was applied to further enhance the charge transfer kinetics. The photopotential of the Pt/TiO2/CdS/CBTSSe electrode was first measured under open circuit voltage conditions (red curve in Figure S8) and steady AM 1.5G illumination, yielding 0.22 V as a result of the quasi-Fermi level splitting.48 Large recombination spikes appear after initiating the “light on” condition, indicating slow kinetics at the electrode/electrolyte

interface and the likely presence of surface states. After switching the light off, the potential drops quickly and then slowly rises to a more anodic value. This peculiar behavior is possibly due to the diffusion and drift of trapped electrons (holes) during the illumination to the electrode/electrolyte interface (back counter electrode).49 Linear sweep voltammetry (LSV) of the Pt/TiO2/CdS/ CBTSSe photocathode (Figure 4b) yields a photocurrent density of 10 mA/cm2 at 0 VRHE. The relatively high photocurrent (comparable to vacuum-processed CBTSSe photocathodes)18 indicates the excellent optoelectronic properties of the solution-processed CBTSSe films. The onset potential of the Pt/TiO2/CdS/CBTSSe structure, 0.56 VRHE (arbitrarily set as the potential where the photocurrent reaches −0.05 mA/cm2), benefits from the band alignment provided by the TiO2/CdS overlayers and also points to the high quality of the solution-processed device. The half-cell solar-to-hydrogen efficiency (HC-STH), calculated from the continuous current density−potential curve (Figure S9), yields a maximum efficiency of 0.89% at 0.175 VRHE, which is at a similar level compared to the vacuum-based CBTSSe photocathode.18 Incident photon to current efficiency (IPCE) was measured on the Pt/TiO2/CdS/CBTSSe photocathode at 0 VRHE (Figure 4c). Fitting to the long wavelength cutoff of the IPCE data, the bandgap of CBTSSe is determined to be 1.64 eV (Figure S10), in agreement with the value obtained from the PL measurement. The photocurrent calculated by integrating the IPCE data under an AM 1.5G spectrum is 6120


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Figure 4. (a) Schematic of the Pt/CdS/TiO2/CBTSSe photocathode. (b) Linear sweep voltammetry of the Pt/CdS/TiO2/CBTSSe photocathode before and after the stability test. The electrode was tested under illumination of simulated sunlight (AM 1.5G). The scan rate is 10 mV/s from anodic potential to cathodic potential. The HC-STH curves derived from the linear sweep voltammetry are shown in Figure S9. (c) Incident photon to current efficiency (IPCE) of the Pt/CdS/TiO2/CBTSSe photocathode at 0 VRHE. (d) Stability test of the Pt/CdS/TiO2/CBTSSe structure for 10 h under illumination of simulated sunlight (AM 1.5G). The potential was kept at 0 VRHE.

∼11 mA/cm2, also in good agreement with the LSV results. The sharp absorption edge in the long wavelength range suggests less band tailing in the solution-processed CBTSSe films relative to CZTSSe. The decreases in IPCE in the wavelength range of 530 nm (∼2.34 eV) and 400 nm (∼3.1 eV) are possibly due to the absorption of CdS and TiO2 overlayers, respectively. A stability test of the Pt/TiO2/CdS/CBTSSe photocathode was performed at 0 VRHE for 10 h under simulated AM 1.5G sunlight (Figure 4d). No photocurrent decrease was observed during the test, implying good device protection was obtained from the TiO2/CdS overlayers. In the stability test, the light was switched off for 10 min after 3 h to check the dark current, yielding a low value of −2.5 μA/cm2, similar to that (−0.8 μA/ cm2) before the test. The consistent dark current indicates that the electrode surface remains unchanged after the relatively long duration of the test. It is worth noting that the absolute photocurrent density increases from ∼10 to ∼12 mA/cm2 during the stability test. Similar photocurrent increase was also observed for vacuum-processed CBTSSe photocathodes.18 The photocurrent increase has been attributed to trap state filling at the CBTSSe/CdS interface,50,51 and the passivation effect is found to be long-lived, as shown in the red LSV curves after the stability test in Figure 4b. Multiple LSV scans in light were performed to check the stability of the increased photocurrent (Figure S11), and photopotential measurements were performed to investigate the charge separation property of the photocathode after the stability test (Figure S8). In Figure

S11, the photocurrent ratio at 0 VRHE after relative to before the stability test is plotted versus the number of LSV cycles. The photocurrent ratio decreases slightly after each LSV cycle but remains larger than 1.2 even after the photopotential measurement. This result deviates from previous literature reports50,51 for which the photocurrent quickly restores to its initial value after anodic potentials are applied to empty the filled trap states. The photopotential also significantly improves from 0.22 to 0.41 V, with smaller transient peaks, as seen in Figure S8. Under steady illumination, the photopotential is created by the balance of electron−hole pair generation, separation, and recombination. Therefore, the photopotential improvement implies less recombination, enhanced charge separation inside the photocathode, or possibly higher light absorption. As a result, the HC-STH increases from 0.89% at 0.175 VRHE to 1.75% at 0.239 VRHE after the stability test (Figure S9). After the electrochemical test, cross-section SEM images were taken and shown in Figure S12. It can be seen that the TiO2/CdS/CBTSSe layered structure was well preserved (Figure S12b) even after extended electrochemical measurements, indicating the excellent stability of the Pt/TiO2/CdS/ CBTSSe photocathode.

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CONCLUSIONS

In summary, we have demonstrated a facile solution processing method to fabricate CBTSSe thin films, using commercially available precursors that can be solubilized in a low-toxicity solvent, DMSO. The difficult-to-remove Ba(SO4) impurity 6121


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(RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI). All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of the NSF.

phase was eliminated by a prebaking step involving a hightemperature and sulfur-rich environment. Our film deposition process, using high-temperature prebaking between each spincoated layer, followed by sulfurization and selenization steps, forms a dense, 1 μm thick, single-phase CBTSSe absorber layer with large grains (0.9−4.5 μm) and a band gap of 1.68 eV. Additionally, we demonstrated the first prototype solutiondeposited CBTSSe PEC device, exhibiting a photocurrent of ∼10 mA/cm2 at 0 VRHE (increasing to ∼12 mA/cm2 during the stability test), comparable with analogous devices based on vacuum-processed CBTSSe films. Further optimization of the solution deposition approach, as well as the conditions for sulfurization and selenization, should lead to continued improvement in the film qualities (i.e., creating a pinholefree uniform films with reduced Mo(S,Se)2 layer formation and tailored band gap), ultimately providing a pathway to an environmentally responsible route for earth abundant element, air-stable, and low-cost thin-film PEC/PV cells.

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(1) Green, M. A.; Hishikawa, Y.; Dunlop, E. D.; Levi, D. H.; HohlEbinger, J.; Ho-Baillie, A. W. Y. Solar cell efficiency tables (version 51). Prog. Photovoltaics 2018, 26, 3−12. (2) First Solar Press Release, First Solar Achieves yet Another Cell Conversion Efficiency World Record; 24 February 2016. (3) First Solar Press Release, First Solar Builds the Highest Efficiency Thin Film PV Cell on Record; 5 August 2014. (4) Mali, M. G.; Yoon, H.; Joshi, B. N.; Park, H.; Al-Deyab, S. S.; Lim, D. C.; Ahn, S.; Nervi, C.; Yoon, S. S. Enhanced Photoelectrochemical Solar Water Splitting Using a Platinum-Decorated CIGS/CdS/ZnO Photocathode. ACS Appl. Mater. Interfaces 2015, 7, 21619−21625. (5) Paier, J.; Asahi, R.; Nagoya, A.; Kresse, G. Cu2ZnSnS4 as a potential photovoltaic material: A hybrid Hartree-Fock density functional theory study. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 115126. (6) Chen, S.; Gong, X. G.; Walsh, A.; Wei, S.-H. Electronic structure and stability of quaternary chalcogenide semiconductors derived from cation cross-substitution of II-VI andI-III-VI2 compounds. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 165211. (7) Wang, W.; Winkler, M. T.; Gunawan, O.; Gokmen, T.; Todorov, T. K.; Zhu, Y.; Mitzi, D. B. Device Characteristics of CZTSSe ThinFilm Solar Cells with 12.6% Efficiency. Adv. Energy Mater. 2014, 4, 1301465. (8) Todorov, T. K.; Reuter, K. B.; Mitzi, D. B. High-efficiency Solar Cell with Earth-abundant Liquid-Processed Absorber. Adv. Mater. 2010, 22, E156−E159. (9) Yokoyama, D.; Minegishi, T.; Jimbo, K.; Hisatomi, T.; Ma, G.; Katayama, M.; Kubota, J.; Katagiri, H.; Domen, K. H2 Evolution from Water on Modified Cu2ZnSnS4 Photoelectrode under Solar Light. Appl. Phys. Express 2010, 3, 101202. (10) Tay, Y. F.; Kaneko, H.; Chiam, S. Y.; Lie, S.; Zheng, Q.; Wu, B.; Hadke, S. S.; Su, Z.; Bassi, P. S.; Bishop, D.; Sum, T. C.; Minegishi, T.; Barber, J.; Domen, K.; Wong, L. H. Solution-Processed CdSubstituted CZTS Photocathode for Efficient Solar Hydrogen Evolution from Neutral Water. Joule 2018, 2, 537−548. (11) Yang, W.; Oh, Y.; Kim, J.; Jeong, M. J.; Park, J. H.; Moon, J. Molecular Chemistry-Controlled Hybrid Ink-Derived Efficient Cu2ZnSnS4 Photocathodes for Photoelectrochemical Water Splitting. ACS Energy Lett. 2016, 1, 1127−1136. (12) Kumagai, H.; Minegishi, T.; Sato, N.; Yamada, T.; Kubota, J.; Domen, K. Efficient solar hydrogen production from neutral electrolytes using surface-modified Cu(In,Ga)Se2 photocathodes. J. Mater. Chem. A 2015, 3, 8300−8307. (13) Gokmen, T.; Gunawan, O.; Todorov, T. K.; Mitzi, D. B. Band tailing and efficiency limitation in kesterite solar cells. Appl. Phys. Lett. 2013, 103, 103506. (14) Chen, S.; Walsh, A.; Gong, X. G.; Wei, S. H. Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 EarthAbundant Solar Cell Absorbers. Adv. Mater. 2013, 25, 1522−1539. (15) Zhu, T.; Huhn, W. P.; Wessler, G. C.; Shin, D.; Saparov, B.; Mitzi, D. B.; Blum, V. I2−II−IV−VI4 (I = Cu, Ag; II = Sr, Ba; IV = Ge, Sn; VI = S, Se): Chalcogenides for Thin-Film Photovoltaics. Chem. Mater. 2017, 29, 7868−7879. (16) Shin, D.; Saparov, B.; Zhu, T.; Huhn, W. P.; Blum, V.; Mitzi, D. B. BaCu2Sn(S,Se)4: Earth-Abundant Chalcogenides for Thin-Film Photovoltaics. Chem. Mater. 2016, 28, 4771−4780. (17) Xiao, Z.; Meng, W.; Li, J. V.; Yan, Y. Distant-Atom Mutation for Better Earth-Abundant Light Absorbers: A Case Study of Cu2BaSnSe4. ACS Energy Lett. 2017, 2, 29−35.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02556. Electrochemical measurement details; solution preparation details; XRD data relating to additional annealing temperature variations; Pawley fitting details and simulated XRD data for the CBTSSe films; large-scale SEM top image of the film; EDX spectrum data; photopotential and HC-STH of the Pt/TiO2/CdS/ CBTSSe electrode before and after the stability test; IPCE data; LSV of the Pt/TiO2/CdS/CBTSSe photocathode; cross-section SEM of PEC device (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (D.B.M.). *E-mail jeff[email protected] (J.T.G.). ORCID

David B. Mitzi: 0000-0001-5189-4612 Author Contributions #

B.T. and Y.Z. contributed equally to this work. The solutionprocessed films were prepared and characterized by B.T., whereas Y.Z. prepared and measured the photoelectrochemical devices. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. Charles B. Parker for providing scientific guidance. Garrett Wessler and Jon-Paul Sun are thanked for their support in revising the manuscript and guidance using the CrystalMaker software to create simulated XRD data. This material is based upon work supported by the National Science Foundation under Grant 1511737 and by the Duke University Energy Initiative Research Seed Fund. The authors also acknowledge the support of the National Science Foundation under Grant ECCS-1344745. This work was performed in part at the Duke University Shared Materials Instrumentation Facility (SMIF), a member of the North Carolina Research Triangle Nanotechnology Network 6122


Article


(38) Xiao, Z. Y.; Yao, B.; Li, Y. F.; Ding, Z. H.; Gao, Z. M.; Zhao, H. F.; Zhang, L. G.; Zhang, Z. Z.; Sui, Y. R.; Wang, G. Influencing Mechanism of the Selenization Temperature and Time on the Power Conversion Efficiency of Cu2ZnSn(S,Se)4-Based Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 17334−17342. (39) Werner, M.; Sutter-Fella, C. M.; Romanyuk, Y. E.; Tiwari, A. N. 8.3% efficient Cu2ZnSn(S,Se)4 solar cells processed from sodiumcontaining solution precursors in a closed reactor. Thin Solid Films 2015, 582, 308−312. (40) Schnabel, T.; Ahlswede, E. On the interface between kesterite absorber and Mo back contact and its impact on solution-processed thin-film solar cells. Sol. Energy Mater. Sol. Cells 2017, 159, 290−295. (41) Neuwirth, M.; Zhou, H.; Schnabel, T.; Ahlswede, E.; Kalt, H.; Hetterich, M. A multiple-selenization process for enhanced reproducibility of Cu2ZnSn(S,Se)4 solar cells. Appl. Phys. Lett. 2016, 109, 233903. (42) Haass, S. G.; Diethelm, M.; Werner, M.; Bissig, B.; Romanyuk, Y. E.; Tiwari, A. N. 11.2% Efficient Solution Processed Kesterite Solar Cell with a Low Voltage Deficit. Adv. Energy Mater. 2015, 5, 1500712. (43) Clark, J. A.; Uhl, A. R.; Martin, T. R.; Hillhouse, H. W. Evolution of Morphology and Composition during Annealing and Selenization in Solution-Processed Cu2ZnSn(S,Se)4. Chem. Mater. 2017, 29, 9328−9339. (44) Zeng, F.; Sun, K.; Gong, L.; Jiang, L.; Liu, F.; Lai, Y.; Li, J. Back contact-absorber interface modification by inserting carbon intermediate layer and conversion efficiency improvement in Cu2ZnSn(S,Se)4 solar cell. Phys. Status Solidi RRL 2015, 9, 687−691. (45) Albert, C.; David, T.; Eric, M. X-ray data booklet. Center for Xray Optics and Advanced Light Source, Lawrence Berkeley National Laboratory, 2009; p 10-01. (46) Yokoyama, D.; Minegishi, T.; Maeda, K.; Katayama, M.; Kubota, J.; Yamada, A.; Konagai, M.; Domen, K. Photoelectrochemical water splitting using a Cu(In,Ga)Se2 thin film. Electrochem. Commun. 2010, 12, 851−853. (47) Moriya, M.; Minegishi, T.; Kumagai, H.; Katayama, M.; Kubota, J.; Domen, K. Stable hydrogen evolution from CdS-modified CuGaSe2 photoelectrode under visible-light irradiation. J. Am. Chem. Soc. 2013, 135, 3733−3735. (48) Yu, X.; Sivula, K. Photogenerated Charge Harvesting and Recombination in Photocathodes of Solvent-Exfoliated WSe2. Chem. Mater. 2017, 29, 6863−6875. (49) Lin, Y.; Xu, Y.; Mayer, M. T.; Simpson, Z. I.; McMahon, G.; Zhou, S.; Wang, D. Growth of p-type hematite by atomic layer deposition and its utilization for improved solar water splitting. J. Am. Chem. Soc. 2012, 134, 5508−5511. (50) Guijarro, N.; Prévot, M. S.; Yu, X.; Jeanbourquin, X. A.; Bornoz, P.; Bourée, W.; Johnson, M.; Le Formal, F.; Sivula, K. A Bottom-Up Approach toward All-Solution-Processed High-Efficiency Cu(In,Ga)S2 Photocathodes for Solar Water Splitting. Adv. Energy Mater. 2016, 6, 1501949. (51) Guijarro, N.; Prévot, M. S.; Johnson, M.; Yu, X.; Bourée, W. S.; Jeanbourquin, X. A.; Bornoz, P.; Le Formal, F.; Sivula, K. CuInGaS2 photocathodes treated with SbX3(X = Cl, I): the effect of the halide on solar water splitting performance. J. Phys. D: Appl. Phys. 2017, 50, 044003.

(18) Zhou, Y.; Shin, D.; Ngaboyamahina, E.; Han, Q.; Parker, C. B.; Mitzi, D. B.; Glass, J. T. Efficient and Stable Pt/TiO2/CdS/ Cu2BaSn(S,Se)4 Photocathode for Water Electrolysis Applications. ACS Energy Lett. 2018, 3, 177−183. (19) Shin, D.; Zhu, T.; Huang, X.; Gunawan, O.; Blum, V.; Mitzi, D. B. Earth-Abundant Chalcogenide Photovoltaic Devices with over 5% Efficiency Based on a Cu2BaSn(S,Se)4 Absorber. Adv. Mater. 2017, 29, 1606945. (20) Shin, D.; Saparov, B.; Mitzi, D. B. Defect Engineering in Multinary Earth-Abundant Chalcogenide Photovoltaic Materials. Adv. Energy Mater. 2017, 7, 1602366. (21) Shin, D.; Ngaboyamahina, E.; Zhou, Y.; Glass, J. T.; Mitzi, D. B. Synthesis and Characterization of an Earth-Abundant Cu2BaSn(S,Se)4 Chalcogenide for Photoelectrochemical Cell Application. J. Phys. Chem. Lett. 2016, 7, 4554−4561. (22) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (23) Ge, J.; Yu, Y.; Yan, Y. Earth-abundant trigonal BaCu2Sn(SexS1−x)4(x = 0−0.55) thin films with tunable band gaps for solar water splitting. J. Mater. Chem. A 2016, 4, 18885−18891. (24) Ge, J.; Yu, Y.; Yan, Y. Earth-Abundant Orthorhombic BaCu2Sn(SexS1−x)4 (x ≈ 0.83) Thin Film for Solar Energy Conversion. ACS Energy Lett. 2016, 1, 583−588. (25) Todorov, T.; Mitzi, D. B. Direct Liquid Coating of Chalcopyrite Light-Absorbing Layers for Photovoltaic Devices. Eur. J. Inorg. Chem. 2010, 2010, 17−28. (26) Tanaka, K.; Fukui, Y.; Moritake, N.; Uchiki, H. Chemical composition dependence of morphological and optical properties of Cu2ZnSnS4 thin films deposited by sol−gel sulfurization and Cu2ZnSnS4 thin film solar cell efficiency. Sol. Energy Mater. Sol. Cells 2011, 95, 838−842. (27) Mitzi, D. B.; Gunawan, O.; Todorov, T. K.; Wang, K.; Guha, S. The path towards a high-performance solution-processed kesterite solar cell. Sol. Energy Mater. Sol. Cells 2011, 95, 1421−1436. (28) Li, W.; Tan, J. M. R.; Leow, S. W.; Lie, S.; Magdassi, S.; Wong, L. H. Recent Progress in Solution-Processed Copper-Chalcogenide Thin-Film Solar Cells. Energy Technol. 2018, 6, 46−59. (29) Hsu, W.-C.; Bob, B.; Yang, W.; Chung, C.-H.; Yang, Y. Reaction pathways for the formation of Cu2ZnSn(Se,S)4 absorber materials from liquid-phase hydrazine-based precursor inks. Energy Environ. Sci. 2012, 5, 8564−8571. (30) Hibberd, C. J.; Chassaing, E.; Liu, W.; Mitzi, D. B.; Lincot, D.; Tiwari, A. N. Non-vacuum methods for formation of Cu(In, Ga)(Se, S)2 thin film photovoltaic absorbers. Prog. Photovoltaics 2010, 18, 434−452. (31) Xin, H.; Katahara, J. K.; Braly, I. L.; Hillhouse, H. W. 8% Efficient Cu2ZnSn(S,Se)4 Solar Cells from Redox Equilibrated Simple Precursors in DMSO. Adv. Energy Mater. 2014, 4, 1301823. (32) Xin, H.; Vorpahl, S. M.; Collord, A. D.; Braly, I. L.; Uhl, A. R.; Krueger, B. W.; Ginger, D. S.; Hillhouse, H. W. Lithium-doping inverts the nanoscale electric field at the grain boundaries in Cu2ZnSn(S,Se)4 and increases photovoltaic efficiency. Phys. Chem. Chem. Phys. 2015, 17, 23859−23866. (33) Ki, W.; Hillhouse, H. W. Earth-Abundant Element Photovoltaics Directly from Soluble Precursors with High Yield Using a Non-Toxic Solvent. Adv. Energy Mater. 2011, 1, 732−735. (34) Uhl, A. R.; Katahara, J. K.; Hillhouse, H. W. Molecular-ink route to 13.0% efficient low-bandgap CuIn(S,Se)2 and 14.7% efficient Cu(In,Ga)(S,Se)2 solar cells. Energy Environ. Sci. 2016, 9, 130−134. (35) McCarthy, C. L.; Brutchey, R. L. Solution Deposited Cu2BaSnS4−xSex from a Thiol−Amine Solvent Mixture. Chem. Mater. 2018, 30, 304−308. (36) Ropp, R. C. Encyclopedia of the Alkaline Earth Compounds; Elseveir: Oxford, UK, 2013. (37) Walsh, A.; Chen, S.; Wei, S.-H.; Gong, X.-G. Kesterite ThinFilm Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4. Adv. Energy Mater. 2012, 2, 400−409. 6123


Solution-Processed Earth-Abundant Cu2BaSn(S,Se)4 Solar Absorber

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