BaCu2Sn(S,Se)4: Earth-Abundant Chalcogenides for Thin-Film

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BaCuSn(S,Se) – Earth-Abundant Chalcogenides for Thin-Film Photovoltaics Donghyeop Shin, Bayrammurad Saparov, Tong Zhu, William P. Huhn, Volker Blum, and David B. Mitzi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01832 • Publication Date (Web): 14 Jun 2016 Downloaded from http://pubs.acs.org on June 19, 2016

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Chemistry of Materials

BaCu2Sn(S,Se)4 – Earth-Abundant Chalcogenides for Thin-Film Photovoltaics Donghyeop Shin,1,2# Bayrammurad Saparov,1,2# Tong Zhu,1 William P. Huhn,1 Volker Blum,1,2* David B. Mitzi1,2* 1

Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC

27708, USA 2

Department of Chemistry, Duke University, Durham, NC 27708, USA

#

Both authors equally contributed to this work.

ABSTRACT Chalcogenides such as CdTe, Cu(In,Ga)(S,Se)2 (CIGSSe), and Cu2ZnSn(S,Se)4 (CZTSSe) have enabled remarkable advances in thin-film photovoltaic performance, but concerns remain regarding (i) the toxicity (CdTe) and (ii) scarcity (CIGSSe/CdTe) of the constituent elements, and (iii) the unavoidable anti-site disordering that limits further efficiency improvement (CZTSSe). In this work, we show that a different materials class, the BaCu2SnSexS4-x (BCTSSe) system, offers a prospective path to circumvent difficulties (i)-(iii) and to target new environmentally-friendly and earth-abundant absorbers. Anti-site disordering and associated band tailing are discouraged in BCTSSe due to the distinct coordination environment of the large Ba2+ cation. Indeed, an abrupt absorption edge and sharp associated photoluminescence emission demonstrate a reduced impact of band tailing in BCTSSe relative to CZTSSe. Our combined experimental and computational studies of BCTSSe reveal that the compositions 0≤x≤4 exhibit a tunable nearly-direct or direct bandgap in the 1.6–2eV range, spanning relevant values for singleor multiple-junction photovoltaic applications. For the first time, a prototype BaCu2SnS4-based thin-film solar cell has been successfully demonstrated, yielding a power conversion efficiency of 1.6% (0.42 cm2 total area). The systematic experimental and theoretical investigations, combined with proof-of-principle device results suggest promise for BaCu2SnSexS4-x as a thinfilm solar cell absorber.

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■ INTRODUCTION Conventional thin-film absorbers such as CdTe and Cu(In,Ga)(S,Se)2 (CIGSSe) have demonstrated record power conversion efficiencies (PCE) of 21.5%1 and 21.7%2, respectively, through decades-long device optimization efforts, yet CdTe relies on the toxic heavy-metal Cd and rising cost and scarcity of In, Ga, and Te may limit the scalability of the associated technologies. The emergence of the kesterite-based solar cell absorber, Cu2ZnSnS4 (CZTS), and more generally Cu2ZnSn(S,Se)4 (CZTSSe), has been an important step forward in the design of earth-abundant photovoltaics (PV), with the ultimate promise of broad scalability and low cost.3-7 The first CZTS device report of 0.66% PCE spurred active research on CZTSSe-based solar cells, which after extensive optimization effort has led to PCE values of up to 12.6%.8, 9 While there has been substantial progress in CZTSSe device performance, the open-circuit voltage, Voc, currently represents the key performance-limiting factor,10-13 with the field now converging on the notion that Voc is primarily limited by band tailing associated with anti-site disorder and resulting potential fluctuations.13-15 Two major sources of anti-site disorder are copper-on-zinc (denoted as CuZn) and zinc-on-copper (ZnCu) anti-site defects.16-18 Neutron and X-ray analysis of bulk and thin film materials directly detects this disordering at levels of >10%,19, 20 which when combined with the expected relatively shallow position of these defects (~100 meV from band edges) can readily account for band tailing in the CZTSSe absorber layers. Additionally, although the majority of Sn is in the tetravalent state in Cu2ZnSn(S,Se)4, divalent Sn2+ is not unusual in chalcogenides—for example, both SnS and SnSe are known compounds in a Sn2+ formal oxidation state. Indeed, Sn multivalency could create deep levels within the band gap (and associated non-radiative recombination) when Sn2+ occupies a Zn2+ site.17,

18, 21, 22

Therefore, the low Voc of CZTSSe PV, stemming at least in part from the Cu-Zn and Sn-Zn antisite disorder, remains a major obstacle for further PCE improvement for kesterite-related devices. In the CZTSSe structure, all cations have a tetrahedral coordination of chalcogen anions and each metal has a similar ionic size, which discourages a highly segregated cation distribution in the crystal structure. In an attempt to tackle the problem of anti-site disorder in CZTSSe, and in particular disorder involving the Zn site, additional chemical (e.g., size, electronegativity) and structural (e.g., coordination environment) differentiation may therefore be beneficial within the crystal lattice. Several studies have been done to address anti-site disordering in CZTSSe23, 24 2 ACS Paragon Plus Environment

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through, for example, atomic substitution of Ag on the Cu site. However, Ag-alloyed CZTSSe shows n-type semiconductor behavior for higher substitution levels, which is undesirable if one would like to maintain the conventional CZTSSe/CIGS device architecture. Given the PVfriendly electronic characteristics of the three-dimensional (3-D) network of tetrahedrallycoordinated Cu and Sn within the CZTSSe structure, one plausible way of targeting betterdefined site selectivity is by examining the family of compounds in which alkaline-earth metals substitute for Zn within the Cu2AESn(S,Se)4 (AE = Sr, Ba) compounds, which offer 1.5–2.0 eV bandgaps.25,

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These materials are structurally distinct from the kesterite CZTSSe and other

purely zinc-blende-type compounds, with BaCu2SnS4 (i.e., Cu2BaSnS4) and SrCu2SnS4 (i.e., Cu2SrSnS4) adopting a trigonal SrCu2SnS4-type structure with space group P31 (see Figure 1). The SrCu2SnS4-type structure features CuS4 and SnS4 tetrahedra that share common corners, forming a 3-D network analogous to that for kesterite. However, unlike the kesterite structure, the large electropositive cations Sr2+/Ba2+ sit inside a S8 square antiprism. The size and electronegativity disparity, and very different coordination environments around Cu+, Sr2+/Ba2+ and Sn4+ likely can discourage formation of Cu-Sr/Ba and Sn-Sr/Ba anti-site disorder. In short, the proposed strategy in the current work is to chemically eliminate the equivalent of the Zn site in CZTS, i.e., the site which acts as a center of undesirable Cu-on-Zn and Sn-on-Zn anti-site disordering in kesterite-based solar cells, thereby exploring structural motifs beyond the traditional zinc-blende-related frameworks of CdTe, CIGSSe, and CZTSSe. Here we report solid state synthesis of BaCu2SnSexS4-x (0 ≤ x ≤ 4) and sputtering-based thinfilm deposition of BaCu2SnS4, as well as detailed structural/optical characterization and a proofof-principle PV device demonstration for this material. BaCu2SnSexS4-x compositions with 0 < x ≤ 3 are found to be isostructural with the parent BaCu2SnS4; however, the fully-substituted Seanalog, BaCu2SnSe4, obtained from a solid state reaction, adopts the SrCu2GeSe4-type structure with space group Ama2. Diffuse reflectance, UV-Vis absorption and photoluminescence data reveal that the BaCu2SnSexS4-x (also denoted as “BCTSSe”) compositions exhibit a tunable band gap in the 1.6–2 eV range. Based on the experimentally determined structure, HSE0627, 28 hybrid density-functional theory (DFT) calculations corroborate the observed phase stability, the nearly direct or direct fundamental band gaps, and also provide band structures over the full range of S:Se compositions. Finally, in order to demonstrate the potential of the BCTSSe series for PV application, we report the results of our first solar cell device study using sputtered BaCu2SnS4 as 3 ACS Paragon Plus Environment


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the active solar cell absorber layer. An abrupt transition in the device quantum efficiency (spectral response) and sharp photoluminescence emission near the absorption edge demonstrate that the impact of band tailing and non-radiative recombination is indeed reduced in BCTSSe, relative to CZTSSe.

■ EXPERIMENTAL AND COMPUTATIONAL SECTION Bulk synthesis and thin film deposition. Solid-state reactions were employed to prepare bulk samples with the targeted compositions, BaCu2SnSexS4-x (x = 0, 1, 2, 3 and 4). Stoichiometric mixtures of BaS (Materion, 99.9%), SnS (Alfa Aesar, 99.5%), CuS (Sigma-Aldrich, 99%), BaSe (Materion, 99.5%), SnSe (Alfa Aesar, 99.999%) and CuSe (Alfa Aesar, 99.5%) were carefully ground/homogenized and cold-pressed into pellets inside a nitrogen-filled glovebox. The pellets were placed inside quartz tubes, and the quartz tubes were flame-sealed under dynamic vacuum (~7×10-7 Torr). The reaction mixtures were then heated to 650 °C during 5-6 hours inside a box furnace, and kept at this temperature for 10-15 hours. After this step, the reactions were cooled to room temperature by switching off the furnaces. BaCu2SnS4 thin films were prepared using a two-step method by means of sulfurization of sputtered Cu-Ba-Sn-S precursor layers. The precursor layers were co-deposited using Cu, Sn, and BaS targets on Mo-coated (for PV device fabrication) and bare glass (for optical characterization) substrates. The sulfurization process took place in a nitrogen-filled dry box. The precursors were annealed at 570 °C for 10 min in an atmosphere with excess sulfur. The final thickness of the BaCu2SnS4 film was approximately 800-1000 nm. Since the primary goal of this work is to demonstrate the properties of BCTSSe as a materials class, for device demonstration, the following well-known device structure used for CIGSSe and CZTSSe solar cells was adopted for the BaCu2SnS4 absorber: ITO/i-ZnO/CdS/Absorber/Mo/Glass. To fabricate BaCu2SnS4 devices, a 50 nm-thick CdS buffer layer was deposited using chemical bath deposition, followed by the deposition of a 50 nm-thick i-ZnO and 160 nm-thick In2O3-SnO2 (ITO) window/transparent top contact layer using radio-frequency (RF) sputtering.29-31 Finally, a patterned Al (500 nm)/Ni (50 nm) front contact was deposited on the top of the device using a thermal-evaporator, to enhance collection of the photogenerated carriers. The device area (0.425 cm2 total area) was defined by mechanical scribing. 4 ACS Paragon Plus Environment

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Characterization methods. The phase compositions and crystal structures for resulting products were studied using a PANalytical Empyrean powder X-ray diffractometer under ambient conditions using Cu Kα radiation. CrystalMaker software (version 9.2.5) was used to create crystal structure images. To determine the band gap of bulk samples, diffuse reflectance measurements were performed on a QE-R Quantum Efficiency/Reflectivity measurement system from Enlitech. Optical absorption measurements were performed on a Shimadzu UV-3600 spectrophotometer. Photoluminescence measurements were performed at room temperature (442 nm laser excitation) using a Horiba Jobin Yvon LabRAM ARAMIS system. Scanning Electron Microscope (SEM) images were taken using a FEI XL30 SEM system. The current densityvoltage (J-V) characteristics of the devices were measured using a Keithley 2400 source meter. The illumination source was a Newport Oriel 92192 solar simulator with an AM 1.5G filter, operating at 100 mW/cm2. The device external quantum efficiencies (EQE) were measured using an Enlitech QE-R system. A calibrated Si solar cell was used as a reference for the J-V and EQE measurements. Computational approach. Hybrid DFT calculations were performed using the FHI-aims32-34 all-electron code, a high-accuracy35 implementation of electronic structure theory based on numeric atom-centered orbital basis sets, implementing a linear-scaling approach to hybrid functionals.36,

37

The default numerical settings, referred to as “really_tight” in FHI-aims

(specifying basis sets, integration grids, and Hartree potential), were used with a k-point grid of 8x8x4 to sample the Brillouin zone that corresponds to the unit cell shown in Figures 1(a) and 2(a). Basis sets used for Cu, Ba, Sn, S, Se are given in the Table S1 (Supporting Information (SI)). Spin-orbit coupling was incorporated into calculated band structures and densities of states (DOS) using a second-variational, first-order perturbation approach,38 recently implemented in FHI-aims. Local minimum-energy geometries of the Born-Oppenheimer surface were obtained with residual total energy gradients below 5x10-3 eV/Å for atomic positions and 5x10-3 eV/Å3 for stress tensor based unit cell relaxation39. Unless otherwise noted, all calculations presented use the short-range screened hybrid exchange-correlation functional HSE0627, 28 with fixed screening parameter ω = 0.2 A−1 and exchange mixing parameter α = 0.25. The HSE06 functional with these parameters has been shown to yield qualitatively reliable band parameters, e.g., for III-V semiconductors.40 The value α = 0.25 is kept fixed throughout this work; however, the experimentally determined fundamental band gaps could be recovered by setting α = 0.274 (α = 5 ACS Paragon Plus Environment


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0.268) for the compounds Ba2CuSnS4 (and analogous CZTS material) at their experimental unit cell parameters, with internal atomic coordinates determined by the HSE06 functional (Figure S1, SI). Carrier effective masses m* were estimated from calculated band structures by numerical fitting to expressions of the form E(k) = E0 + ħ2/(2m*)·(k-k0)2 at selected maxima (minima) k0 of the valence (conduction) bands.

■ RESULTS AND DISCUSSION Crystal structures, bulk synthesis and properties. BaCu2SnS4 crystallizes in the trigonal SrCu2SnS4-type structure41, 42 (Figure 1) with noncentrosymmetric space group P31. Similar to the structures of zinc-blende-type semiconductors such as CZTS, the crystal structure of BaCu2SnS4 features a three-dimensional (3-D) polyanionic [Cu2SnS4]2- network made of corner-sharing CuS4 and SnS4 tetrahedra. However, unlike zinc-blende-derived structures, the polyanionic framework exhibits channels in which the large Ba2+ cations reside. The resultant coordination environment for the Ba2+ cation site is a distorted square antiprism formed by eight S atoms. Coupled with the difference in electronegativity, the size difference and coordination environment difference between the cations in (Ba2+)(Cu+)2(Sn4+)(S2-)4 should discourage formation of the undesirable anti-site defects and recent computational analysis confirms that, indeed, CuBa and BaCu defects are much less likely to form compared to the analogous Zn anti-site defects in CZTSSe.26 In contrast, the selenide analog, BaCu2SnSe4, has been reported to crystallize in the orthorhombic SrCu2GeSe4-type structure (Figures 2 and S2, SI) with noncentrosymmetric space group Ama2.43, 44 Further deviating from the zinc-blende structure, the 3-D polyanionic [Cu2SnSe4]2- network in BaCu2SnSe4 is built upon corner- and edge-shared CuSe4 and SnSe4 tetrahedra—i.e., edge-sharing Cu2Se6 double tetrahedra are interconnected by corner-sharing SnSe4 tetrahedral units. The polyanionic structure in BaCu2SnSe4 exhibits similar voids to that in the sulfide analog, giving identical Ba2+ cation coordination environment. Therefore, based on crystal structure analysis, reduced cationic antisite defect formation is also expected for BaCu2SnSe4.


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Figure 1. (a) Polyhedral view of the crystal structure of BaCu2SnS4 (SrCu2SnS4-type).41, 42 (b) A close-up view of the polyanionic [Cu2SnS4]2- fragment, emphasizing the corner-shared CuS4 and SnS4 tetrahedra. (c) Coordination polyhedron of the Ba2+ cation in BaCu2SnS4 is formed by eight S atoms in a distorted square antiprismatic geometry. The Ba, Cu, Sn and S atoms and their corresponding coordination polyhedra are shown in orange, blue, green and red, respectively.

Figure 2. (a) Polyhedral view of the orthorhombic structure of BaCu2SnSe4 (SrCu2GeSe4type).43, 44 (b) A close-up view of the polyanionic [Cu2SnSe4]2- fragment emphasizing the cornershared SnSe4, and corner- and edge-shared CuSe4 tetrahedra. (c) Coordination polyhedron of the Ba2+ cation in BaCu2SnSe4 is formed by eight Se atoms in a distorted square antiprismatic geometry. The Ba, Cu, Sn and Se atoms and their corresponding coordination polyhedra are shown in orange, blue, green and red, respectively. 7 ACS Paragon Plus Environment


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Our bulk synthetic efforts targeting BaCu2SnS4 and BaCu2SnSe4 yielded the anticipated phases, as noted in the powder X-ray diffraction (PXRD) patterns (Figures 3 and 4). The compounds adopt the above-described SrCu2SnS4- and SrCu2GeSe4-type structures, respectively. Note that in literature, there have been suggestions26, 45 that BaCu2SnSe4 is isostructural with BaCu2SnS4, or alternatively that it adopts the SrCu2SnS4-type structure. However, our solid-state reactions targeting BaCu2SnSe4 result in a phase with the orthorhombic SrCu2GeSe4-type structure. In order to determine the solubility limit of Se in the BaCu2SnS4-type structure, several reactions targeting BaCu2SnSexS4-x (x = 1, 2 and 3) were carried out. Based on PXRD data (Figure 4), BaCu2SnSexS4-x (x = 1, 2, and 3) members are isostructural with the sulfide parent BaCu2SnS4. The lattice parameters and space groups of BaCu2SnSexS4-x phases are summarized in Table 1.

Figure 3. Powder X-ray diffraction (PXRD) pattern for a bulk powder sample of BaCu2SnS4 (black). Pawley fit (red) to the trigonal SrCu2SnS4-type structure41, 42 (P31) gives a = 6.3662(1) Å and c = 15.8287(2) Å with reliability factors of Rp = 4.45 %, wRp = 5.76 % and goodness-of-fit = 1.49; the difference plot is shown in blue. Miller indices for selected peaks are displayed; Miller indices for lower intensity and overlapping peaks are omitted for clarity.


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Figure 4. A comparison of PXRD patterns for BaCu2SnSexS4-x (x = 0, 1, 2, 3, and 4) phases. Upon incorporation of the larger element Se into the structure of the trigonal parent BaCu2SnS4 (red, x = 0), the unit cell parameters increase, resulting in a systematic shift of peaks to lower angles. The selenide analog, BaCu2SnSe4 (black, x = 4), shows a distinctly different PXRD pattern, confirming the fact that it adopts the orthorhombic SrCu2GeSe4-type structure.43, 44 The lattice parameters of BaCu2SnSe4 were refined using a Pawley fit (see Figure S2, SI) to a = 11.1105(2) Å, b = 11.2275(2) Å and c = 6.7436(1) Å in the space group Ama2, corroborating the literature reported values.44

Table 1. Summary of lattice parameters (from PXRD data) and band gaps (from direct and indirect fits of the diffuse reflectance data) for BaCu2SnSexS4-x. x

Space group

0

P31

Lattice parameters, Å a

b

6.3662(1)

6.3662(1)

c 15.8287(2)

Band gap, eV Direct fit

Indirect fit

1.95

1.88



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1

P31

6.4294(3)

6.4294(3)

16.0021(6)

1.80

1.73

2

P31

6.5076(1)

6.5076(1)

16.2018(3)

1.63

1.61

3

P31

6.5699(1)

6.5699(1)

16.3681(2)

1.55

1.52

4

Ama2

11.1105(2) 11.2275(2) 6.7436(1)

1.72

1.64

The evolution of lattice parameters in the BaCu2SnSexS4-x (0 ≤ x ≤ 3) solid solution approximately follows Vegard’s law (Figure 5), i.e., the increase in the unit cell parameters with increasing Se-concentration (x indicates the as-loaded Se content) is nearly linear. Band gaps from our diffuse reflectance measurements on powdered bulk BaCu2SnSexS4-x samples are presented in Table 1. Since the exact nature of the band gap is questioned in literature,26, 45 we present band gaps from both direct and indirect Tauc fits of the diffuse reflectance data (Figure S3, SI). It is clear that the variation of band gaps in the BaCu2SnSexS4-x (0 ≤ x ≤ 3) solid solution is linear, as also predicted by theory (Figure 6 and previous DFT results26). However, the observed 1.72 eV direct band gap and 1.64 eV indirect band gap obtained for BaCu2SnSe4 is in a stark contrast with the 1.28 eV value predicted by DFT results in earlier work, which assumed the trigonal SrCu2SnS4-type structure.26 The sharply different band gap of BaCu2SnSe4 is, however, consistent with the different crystal structure of this compound compared to that for the BaCu2SnSexS4-x (0 ≤ x ≤ 3) compositions, as discussed below.

Figure 5. Evolution of unit cell parameters with increasing Se-content in the BaCu2SnSexS4-x (0 ≤ x ≤ 3) solid solution. 10 ACS Paragon Plus Environment

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Figure 6. Evolution of band gap values, obtained using diffuse reflectance data on powder (bulk) samples, with increasing Se-content in the BaCu2SnSexS4-x (0 ≤ x ≤ 4) solid solution (green and blue lines marked with triangles for direct and indirect band gap fits, respectively) and calculated fundamental gaps (HSE06 functional with spin-orbit coupling), using the experimental lattice parameters with internal atomic positions optimized using the HSE06 functional (red line, circles). BaCu2SnSexS4-x (x = 3) belongs to the P31 space group.

Table 2. Fundamental band gaps (eV) for the BaCu2SnSexS4-x system, calculated by HSE06+SOC based on different configurations (unit cell lattice parameters and/or internal atomic coordinates) obtained by geometry optimization using the PBE functional, the HSE06 functional, and the HSE06 functional but with the lattice parameters constrained to the experimental values. Relaxation Method

Full PBE geometry

HSE06+SOC band gap for x = 0

1

2

3

4

1.39

-

-

-

-



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Full HSE06 geometry

1.71

1.57

1.43

1.27

1.48

Exp. lattice parameters

1.74

1.62

1.47

1.33

1.50

+ HSE06 atomic positions

Electronic structure calculations. The calculated band structure of zinc-blende-derived structures depends significantly on the atomic coordinates used. Compared to semilocal DFT (PBE46 functional), atomic coordinates derived from the HSE06 functional are known47 to substantially improve the quality of band gap predictions. Specifically, the electronic properties are sensitive to Cu-S and Sn-S bond lengths, which are not well described by the PBE functional. In addition, the overall lattice parameter has a similar effect. For BaCu2SnSexS4-x (0 ≤ x ≤ 4), Table 2 shows HSE06-calculated fundamental band gaps for unit cell parameters and atomic coordinates obtained from three different approaches to geometry relaxation: (i) full relaxation using the PBE functional (i.e., both lattice parameters and atomic coordinates optimized by DFTPBE), (ii) full relaxation using HSE06, and (iii) a relaxation approach that fixes the lattice parameters at their (room temperature) experimental values, but leaves the atomic coordinates in the unit cell to be determined by HSE06. The lattice parameters used are given in Table S2 (SI). Band structure comparisons for geometries (i)-(iii) indicate clear differences between all three cases, seen for full PBE vs. full HSE06 relaxation of BaCu2SnS4 in Figure S4 (SI), and for the experimental unit cell with HSE06-relaxed internal coordinates in Figure 7. In the present work, we are in the fortunate position to have experimentally determined unit cell parameters available from X-ray powder diffraction for all compounds of interest. Since (iii) shows closest agreement between the calculated and experimental fundamental band gaps (i.e., compare with experimental band gaps in Table 1), we employ experimental unit cell parameters with HSE06relaxed internal coordinates for the remainder of this work (calculated gaps in Figure 6 are in excellent qualitative agreement with experiment). For the two space groups P31 (S-rich structures) and Ama2 (fully Se-substituted structure), we also calculated the formation energies of BaCu2SnSexS4-x (0 ≤ x ≤ 4) using the HSE06 functional. For mixed S-Se stoichiometries, we included all possible S-Se configurations within a single conventional unit cell (see Figure S5(a), SI) for both space groups (P31 and Ama2). For the two


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pure compounds (x = 0 and x = 4), the calculated difference in formation energy is just under 10 meV/atom, while the difference for the intermediate compositions is much smaller, indicating close competition and an eventual trend towards Ama2 for high x. Band structures show very little variation when varying the S-Se configuration at fixed composition (Figure S5(b), SI). For the remainder of this work, we adopt the lowest formation energy structures for both space groups in a single conventional unit cell at a given composition x. The development of band structures and electronic densities of states (DOS) with x is shown in Figure 7. For BaCu2SnSexS4-x (0 ≤ x ≤ 3), the structures have a slightly indirect (or alternatively “nearly direct”) band gap. The valence bands are relatively flat near the valence band maximum (VBM), with only a 17 meV difference between the highest band at Γ and the actual VBM (off-Γ) for x = 0 (similar to Ref. 23) and a 12 meV difference for x = 3. In contrast, for BaCu2SnSe4, a direct band gap is observed. The DOS for valence bands and conduction bands are similar for BaCu2SnSexS4-x (0 ≤ x ≤ 3), although there are slight shape changes in the DOS of the conduction band as the concentration of Se increases. For BaCu2SnSe4, the DOS shows clearer changes, e.g., the opening of an additional gap in the conduction band. Effective masses derived from the valence and conduction bands of BaCu2SnS4 (x = 0) and BaCu2SnSe4 (x = 4) are summarized in Table S3 (SI). For electrons, effective masses at the conduction band minimum (Γ) of BaCu2SnS4 are almost isotropic (mc = 0.22–0.23 in units of the free electron mass m0) and closely comparable with HSE06-derived values mc ≈ 0.18–0.19 in the literature for CZTS48. Similar, somewhat less isotropic values are also found for BaCu2SnSe4 (mc = 0.16–0.35). In contrast, the expected hole effective masses at the VBM parallel to the c axes of BaCu2SnS4, mh|| = 1.43 (off-Γ), and BaCu2SnSe4, mh|| = 1.64 (at Γ), are significantly larger than either the electron effective masses in these materials or the corresponding longitudinal mh|| in CZTS.48 This implies a somewhat reduced longitudinal hole mobility in the Ba compounds, compared to CZTS; on the other hand, the reduced band curvature also implies a higher density of states at the VBM, which could enhance the absorption of the material at the band edges. The transverse hole effective masses (estimated at Γ for both compounds), on the other hand, show intermediate values (mh⊥(Γ) = 0.83–0.87 for BaCu2SnS4, mh⊥(Γ) = 0.51–0.67 for BaCu2SnSe4), which approximately fall within the same ranges as values reported earlier for CZTS48 and CIGS49. Finally, we note that, for BaCu2SnS4, the VBM is within room temperature kT of a 13 ACS Paragon Plus Environment


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much more strongly dispersive band V3 at the Γ point (Figure S6, SI), which should also contribute to the material’s transport properties from a semiclassical point of view.

Figure 7. (a) Crystal structures and calculated band structures and densities of state (DOS) (HSE06 functional with spin-orbit coupling using the experimental lattice parameters with internal atomic positions optimized using the HSE06 functional) for (a) BaCu2SnS4 (x = 0), (b) BaCu2SnSe3S (x = 3), and (c) BaCu2SnSe4 (x = 4). The band structures on the right side (top and bottom) are zoomed versions of the red rectangular boxes indicated in the band structures for x = 0 and x = 4. The Ba, Cu, Sn, S and Se atoms are shown in green (big sphere), blue, grey, yellow, and light green (small sphere), respectively. The total DOS are shown as black lines, while partial DOS from Ba, Cu, Sn, S, and Se are shown as blue, red, purple, grey, and green lines, respectively. (d) Brillouin zones and selected k-space paths for BaCu2SnSexS4-x (0 ≤ x ≤ 4).


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Thin-film deposition and properties. To further explore materials properties, BaCu2SnS4 films were deposited onto Mo-coated and bare glass substrates using a two-step process, involving radio-frequency (RF) sputtering followed by sulfurization in a nitrogen-filled glovebox (see Experimental Section). To determine the structural quality of the BaCu2SnS4 films, Xray diffraction measurements were performed (Figure 8(a)), confirming the single-phase and well-crystallized nature of the films. All peaks in the X-ray diffraction pattern are assigned to the BaCu2SnS4 phase, consistent with solid-state reaction data as seen in Figure 3. The weak broad features at 2θ= 34o and 58o correspond to a MoS2 phase between the Mo and BaCu2SnS4 layers (noted with asterisks in the X-ray diffraction pattern). Such an interfacial reaction layer has also been widely observed in Cu(In,Ga)(S,Se)2 and Cu2(Zn,Sn)(S,Se)4 thin-film devices.8,

11, 29

Scanning electron microscopy (SEM) images of a BaCu2SnS4 film top-view and cross-section indicate that the film is continuous, without pin-hole defects and with cross-sectional grains extending a significant fraction of the film thickness (with, however, some voids being observed), thereby facilitating effective photogenerated carrier collection.

Figure 8. X-ray diffraction pattern (a) and SEM top-view and cross-section images (b) for a sputtered BaCu2SnS4 film on a Mo-coated glass substrate. In (a), a peak arising from the Mo underlayer is noted and the interfacial MoS2 layer peaks are also marked with asterisks.



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Optical absorption (UV-vis) measurements were also carried out for the BaCu2SnS4 films, with the estimated band gap from the Tauc plot (Figure 9(a)) yielding 2.02 eV, assuming a direct band gap. In comparison, the plot of (αhν)1/2 vs hν, assuming an indirect band gap, yielded a similar 1.99 eV (Figure S7, SI). The small difference between these two values may be reflective of the nearly direct band gap, as indicated from the computational results. In addition to UV-vis absorption, photoluminescence (PL) measurement (Figure 9(b)) was performed to examine radiative recombination and confirm the BaCu2SnS4 film optical band gap. A relatively sharp PL peak emitted at 610 nm (corresponding to 2.03 eV) is consistent with the band gap values derived from the UV-vis spectroscopy for the film and diffuse reflectance on the analogous bulk sample.

Figure 9. (a) Direct band gap fit of the Tauc plot obtained from the absorbance data for the BaCu2SnS4 thin film and (b) photoluminescence spectrum for the BaCu2SnS4 film obtained with 442 nm laser excitation at room temperature.

Thin-film prototype devices. As proof-of-principle of BaCu2SnS4 for the PV absorber application (as opposed to the production of a fully optimized device, a process expected to take much longer based on experiences with more mature PV materials), the two-step-deposited BaCu2SnS4 films on Mo-coated glass substrate were incorporated into the device structure described in the Experimental section. Illuminated current density-voltage (J-V) data for a BaCu2SnS4 solar cell are shown in Figure 10(a), yielding an open-circuit voltage (Voc), shortcircuit current density (Jsc), fill factor (FF), and total area power conversion efficiency (PCE) of 16 ACS Paragon Plus Environment

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713 mV, 4.11 mA/cm2, 55.32, and 1.62%, respectively, representing the first demonstration of a BaCu2SnS4 thin-film PV device. Average photovoltaic parameters for the four PV devices shown in the inset of Figure 10(a) are Voc= 699 ± 11 mV, Jsc= 4.13 ± 0.06 mA/cm2, FF= 53.49 ± 1.33, and PCE= 1.54 ± 0.08 % (Table S4, SI). Regardless of bias scanning directions, the device efficiencies are identical (Figure S8, SI), demonstrating the lack of electrical hysteresis. As a point of comparison, and as mentioned above, the first reported CZTSSe efficiencies amounted to PCE=0.66 %.9 In addition to the J-V curve for the BaCu2SnS4 device, the external quantum efficiency (EQE) was also measured, as shown in Figure 10(b). The photocurrent density calculated from the integrated EQE spectrum is 4.26 mA/cm2, which is in good agreement with the measured Jsc under one-sun condition. As expected, an abrupt decrease in EQE values (between 600 and 650 nm wavelengths) corresponds to the band gap cut-off for the material and the band gap derived from the EQE data (2.04 eV; see Figure S9, SI) agrees with the value determined from the optical spectroscopy measurements. The initial device reported here are encouraging and further improvements can be readily envisioned through optimization of chemical composition and band gap of the absorber layer, as well as improvements of interfaces and contacts. The relatively sharp cut-off in the EQE (also directly noted in the absorbance data of Figure 9(a)) suggests that band tailing may not be a substantial issue in this material.13 For BaCu2SnS4, the red shift of the PL peak (2.03 eV) relative to the band gap (2.04 eV), as determined from the EQE inflection point, is only approximately 10 meV (Figure S9, SI). Given that the PL peak offset values for Cu(In,Ga)(S,Se)2 and Cu2(Zn,Sn)(S,Se)4 are 30 and 100 meV, respectively,13 this small offset value provides further evidence of very low levels of band tailing in the BaCu2SnS4 films. Additionally, the full width at half maximum (FWHM) of the PL peak for BaCu2SnS4 (≈ 40 nm) is much smaller, compared to the FWHM for Cu(In,Ga)(S,Se)2 (≈ 100 nm) and Cu2(Zn,Sn)(S,Se)4 (≈ 190 nm) reported in the literature.13 Presumably, the very different coordination environments around Cu+, Ba2+ and Sn4+ are discouraging the formation of Cu-Ba and Sn-Ba anti-site disordering, leading to the observed suppression of band tailing. According to a previous computational study on defect properties of this material system,26 the formation energy of Cu-on-Ba (denoted as CuBa) and Ba-on-Cu (denoted as BaCu) anti-site defects is 1.0 eV higher than that of Cu vacancy (VCu), implying that this cationic disorder and associated band



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tailing in BaCu2SnS4 materials may effectively be avoided, in good agreement with the experimental observations from the current study. Despite the sharp band edge exhibited in the BaCu2SnS4 materials, EQE values in the wavelength range between 400 to 600 nm are ~25-40%, indicating that the photogenerated carrier collection efficiency requires improvement. Higher device efficiencies are expected as a result of continued optimization of the chemical composition and enhancement of the film microstructure, i.e., minimizing recombination mechanisms. Additionally, as shown above, the introduction of Se into the film should reduce the band gap into a range that is more suitable for a single junction PV device under an AM1.5 spectrum. For fully Se-substituted BaCu2SnSe4 with a direct band gap, it could also be utilized as a potential earth-abundant metal PV material.

Figure 10. (a) Light (solid line) and dark (dashed line) current density-voltage (J-V) characteristics (inset shows a set of four PV devices on a typical substrate) and (b) external quantum efficiency (EQE) spectrum of the same BaCu2SnS4 thin-film solar cell (black line) and integrated photocurrent density that is expected under AM 1.5G illumination (blue line). The device structure is glass / Mo / BaCu2SnS4 / CdS / i-ZnO / ITO with a Ni-Al grid.

■ CONCLUSIONS In summary, BCTSSe is demonstrated as a new earth-abundant chalcogenide-based family that offers promise for photovoltaic application. Over the range of S:Se stoichiometries examined, combined experimental and theoretical studies reveal that BaCu2SnSexS4-x 18 ACS Paragon Plus Environment

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compositions with 0 < x ≤ 3 are isostructural to BaCu2SnS4, with space group P31, and exhibit a tunable band gap in the 1.6–2 eV range. The pure selenide BaCu2SnSe4, adopts the analogous SrCu2GeSe4-type structure with space group Ama2 and with a band gap of 1.72 eV. The range of observed band gaps overlaps well with targeted values for both single junction (i.e., 1–1.6 eV) and top cell in multi-junction (i.e., 1.7–2.0 eV) photovoltaic device configurations.50, 51 Full band structure calculations for BaCu2SnSexS4-x (0 ≤ x ≤ 3; P31 space group) predict a weakly indirect band gap, while the fully Se-substituted BaCu2SnSe4 (x = 4; Ama2 space group) shows a direct band gap. The BCTSSe structures provide both difference in ionic size (large for Ba2+ vs small for Cu+ and Sn4+) and coordination environment (square antiprism for Ba2+ vs tetrahedral for Cu+ and Sn4+) to inhibit the level of anti-site disorder and corresponding band tailing relative to, for example, the CZTSSe kesterite structure. Sharp cut-offs in the optical absorption spectra and EQE data for wavelengths above the band gap value, coupled with a sharp photoluminescence peak centered at a wavelength closely corresponding to the band gap, confirm the relative absence of band tailing in the BaCu2SnS4 system. Additionally, for the first time, a prototype BaCu2SnS4-based thin-film solar cell has been successfully demonstrated. Since the device structure and working mechanism of BaCu2SnS4based solar cells are analogous to the well-known Cu(In,Ga)(S,Se)2 and Cu2ZnSn(S,Se)4 systems, additional improvement in device performance can be expected as the electronic character and microstructural quality of the absorber layer are further fine-tuned. For example, fluxing agents such as sodium and/or antimony52, 53 may enhance grain growth, and modification of the post-deposition annealing profile may substantially impact defects and recombination in the material.54, 55 Further structural and spectroscopic studies to elucidate the band alignment in our devices, as well as details of anti-site disorder and recombination are also of great interest.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website. Details of the basis functions used for hybrid DFT calculations, calculated fundamental band gaps of BaCu2SnS4 and Cu2ZnSnS4 as a function of the HSE06 exchange mixing parameter α, 19 ACS Paragon Plus Environment


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comparison of lattice parameters, fundamental gaps, band structures and densities of state for different computationally relaxed geometries, calculated formation energies and effective masses of electrons and holes for BaCu2SnSexS4-x (0 ≤ x ≤ 4), Pawley-fitted power X-ray diffraction pattern for BaCu2SnSe4, direct and indirect band gap model fit of the Tauc plot for BaCu2SnSexS4-x (0 ≤ x ≤ 4), summary of PV performance parameter statistics for BaCu2SnS4 devices, further details of light current density-voltage characteristics with two different scanning directions, and comparison between band gap derived from EQE spectrum and PL peak position, Figures S1-S9 and Tables S1-S4.

■ AUTHOR INFORMATION Corresponding author *D. B. Mitzi: Email: [email protected] *V. Blum: Email: [email protected]

Author Contributions D. Shin and B. Saparov contributed equally. The manuscript was written through contribution of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS This material is based upon work supported by the National Science Foundation under Grant No. 1511737 and by the Duke University Energy Initiative Research Seed Fund. One of the authors (BS) acknowledges support from a Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Award administered by the 20 ACS Paragon Plus Environment

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Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by Oak Ridge Associated Universities (ORAU) under DOE contract number DE-AC05-06OR23100. All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of NSF, DOE, ORAU, or ORISE. The authors gratefully acknowledge computational resources provided by the Argonne Leadership Computing Facility and by the Barcelona Supercomputing Center.

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■ TABLE OF CONTENTS GRAPHIC We report on BaCu2SnSexS4-x as a multinary earth-abundant chalcogenide PV material system with minimal band tailing and the first demonstration of a thin-film solar cell device using BaCu2SnS4.


BaCu2Sn(S,Se)4: Earth-Abundant Chalcogenides for Thin-Film

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