Solution-Processed Earth-Abundant Cu

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Solution-Processed Earth-Abundant CuBaSn(S,Se) Solar Absorber Using a Low-Toxicity Solvent

Betul Teymur, Yihao Zhou, Edgard Ngaboyamahina, Jeffrey T. Glass, and David B. Mitzi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02556 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 11, 2018

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Chemistry of Materials

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

Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, United States ‡

Department of Chemistry, Duke University, Durham, North Carolina 27708, United States Department of Electrical & Computer Engineering, Duke University, Durham, North Carolina 27708, United States

§

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 eV 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 µm-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 pre-baking 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 a ~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 solutionprocessed 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 that 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. Cu2ZnSnS4 (CZTS) or Cu2ZnSn(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 (12.6%)7, PEC photocurrent density and the halfcell solar-to-hydrogen efficiency (HC-STH) (17 mA/cm2 and ~ 4% at 0 VRHE, respectively)10 still remain substantially behind what can be achieved with CIGSSe/CdTe (>20%, 32.5 mA/cm2 and ~8.5% at 0 VRHE4,12, respectively). The relatively low performance levels for CZTSSe devices stem in part from the similar sizes of Cu+/Zn+2/Sn+4 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 by atoms of dissimilar chemistry, ionic size and coordination.15,16 As a demonstration of this concept, Cu2BaSn(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 (8fold) compared to Zn2+ (0.74 Å; 4-fold), Cu+2 ( 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 a trigonal structure with P31 space group, characterized by a corner-sharing network of Sn-S and Cu-S tetrahedra 2 ACS Paragon Plus Environment

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with 8-fold coordinated Ba atoms, while the stoichiometries x > 3 lead to a similar orthorhombic structure with Ama2 space group and larger band gap.17, 19, 21, 23, 24 Recent studies examining the fullwidth-at-half maximum and energy position of the photoluminescence (PL) peak relative to the band gap, coupled with an analysis of the cut-off 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 vacuum-deposited 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 source31-33, 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



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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 soda-lime 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 vacuumdeposited 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. Experimental Section Molecular Ink Synthesis and Absorber Layer Formation: A molecular solution was prepared using the commercially available precursors Cu(CO2CH3)2 (98%, 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 DMSO (99.99%, anhydrous, Sigma-Aldrich) and stirred overnight at room temperature until the 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


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pre-baked 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 ºC 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 ºC and then cooled to 340 ºC 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 post-deposition film sulfurization. To incorporate selenium atoms, the sulfurized film was annealed in selenium vapor at 570 ºC for 5 min and allowed to cool to 470 ºC 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.

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°C to 37°C (depending on the film preparation step) and with oxygen and water levels at < 5 ppm.

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 ) 5 ACS Paragon Plus Environment


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was deposited to improve electron charge extraction and protect devices from photo corrosion.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 1mM H2PtCl6 was used as the electrolyte. The deposition was done by polarizing the TiO2/CdS/CBTSSe electrode at -0.4V vs Ag/AgCl for 2 min. After the Pt deposition, the as-prepared Pt/TiO2/CdS/CBTSSe electrode was tested employing a 0.5M Na2SO4, 0.5M 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 was 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-300 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 Supplementary Information. 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 6 ACS Paragon Plus Environment

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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 employed an initial lowtemperature pre-baking stage of the solution-processed Cu2BaSnS4 (CBTS) film without using a cover and sulfur vapor. Within the film prepared in this initial pre-baking 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 pre-baking 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 pre-baking temperature from 340 ̊C to 540 ̊C, coupled with the availability of sulfur. The films prepared using excess amount of thiourea (30 mole %) in the solution with or without sulfur vapor at high pre-baking temperature of 540 ̊C yield the X-ray diffraction patterns shown in Figure 2a. The availability of sulfur environment during pre-baking each layer is important for controlling Ba(SO4) formation, since the excess amount of thiourea as a sulfur source is insufficient to eliminate Ba(SO4) formation. As shown in Figure 2b, increase in the pre-bake temperature (with added sulfur during the anneal) 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



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these investigations, we concluded that, for our molecular solution process, increasing S activity through a high-temperature S-rich pre-baking 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 formation of CZTSSe under a variety of experimental conditions.37 In order to remove the Cu2SnS3 secondary phase, an excess of Ba(NO3)2 (up to 20 mole%) 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/pre-baking (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.

Figure 2. a) Using a pre-baking 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 pre-baking step under sulfur vapor


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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 dash lines, respectively. XRD peaks labelled “?” 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.03065-7569.

In order 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), (009) peaks at 2θ = 13.69°, 33.04°, 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.



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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 Supplemental 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 top-view and d) cross-sectional image of the CBTSSe film (9 layers) deposited on Mo/SLG.

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 high-temperature pre-baking steps (involving sulfur vapor) for the current 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 cross-sectional scanning 10 ACS Paragon Plus Environment

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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 micron-size grains ranging from ~0.9 µm to ~4.5 µm, with only a few pin holes across the film surface (see Figure S4 for a wider SEM view of the film surface), leading to a suitable film structure for high-performance 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 keV45, 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 carried out to verify the band gap (Eg) of the solution-deposited 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 cut-off 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 11 ACS Paragon Plus Environment


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therefore of disorder and band tailing within the semiconductor. The narrow observed peak FWHM for the current film (~45 nm 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 (6 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, 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 Cu-based 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 solid 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 -0.05mA/cm2), benefits from the band alignment provided by the TiO2/CdS overlayers and also points to the high quality of the solutionprocessed 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, 12 ACS Paragon Plus Environment

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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 cut-off 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 ~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.

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 hours under illumination of simulated sunlight (AM 1.5G). The potential was kept at 0 VRHE.



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A stability test of the Pt/TiO2/CdS/CBTSSe photocathode was performed at 0 VRHE for 10 hours 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 minutes after 3 hours to check the dark current, yielding a low value of -2.5 µA/cm2, similar to that (-0.8µA/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 mA/cm2 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 interface50,51 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 V 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. 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 phase was eliminated by a pre-baking step involving a high 14 ACS Paragon Plus Environment

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temperature and sulfur-rich environment. Our film deposition process, using high temperature prebaking between each spin coated 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 solution-deposited CBTSSe PEC device, exhibiting a photocurrent of ~10mA/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 pin-hole-free 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. Supporting Information 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. Corresponding Authors *Correspondence: [email protected] *Correspondence: [email protected] Author Contributions #B.T. and Y.Z. contributed equally to this work. The solution-processed 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 Acknowledgements The Authors would like to 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 No. 1511737 and by the Duke University Energy Initiative 15 ACS Paragon Plus Environment


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Research Seed Fund. The authors would also like to 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 (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.


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Solution-Processed Earth-Abundant Cu

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