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Feb 10, 2017 - (KS) structural model of the Li-free CZTS in the space group I4̅.25 In ...... (1) Wadia, C.; Alivisatos, A. P.; Kammen, D. M. Material...
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Substitution of Li for Cu in Cu2ZnSnS4: Toward Wide Band Gap Absorbers with Low Cation Disorder for Thin Film Solar Cells A. Lafond,*,† C. Guillot-Deudon,† J. Vidal,‡,§,∥ M. Paris,† C. La,⊥ and S. Jobic† †

Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France ‡ EDF R&D, Departement EFESE, 6 Quai Watier, 78401 Chatou, France § Institute for Research and Development of Photovoltaic Energy (IRDEP), UMR 7174 CNRS/EDF R&D/Chimie ParisTech-PSL, 6 quai Watier, 78401 Chatou, France ∥ Institut Photovoltaïque d’Ile-de-France (IPVF), 8 rue de la Renaissance, 92160 Antony, France ⊥ Laboratoire de Planétologie et Géodynamique, LPG Nantes, CNRS UMR 6112, Université de Nantes, Nantes, France S Supporting Information *

ABSTRACT: The substitution of lithium for copper in Cu2ZnSnS4 (CZTS) has been experimentally and theoretically investigated. Formally, the (Cu1−xLix)ZnSnS4 system exhibits two well-defined solid solutions. Indeed, single crystal structural analyses demonstrate that the low (x < 0.4) and high (x > 0.6) lithium-content compounds adopt the kesterite structure and the wurtz-kesterite structure, respectively. For x between 0.4 and 0.6, the two aforementioned structure types coexist. Moreover, 119Sn NMR analyses carried out on a (Cu0.7Li0.3)2ZnSnS4 sample clearly indicate that lithium replaces copper preferentially on two of the three available 2-fold crystallographic sites commonly occupied by Cu and Zn in disordered kesterite. Furthermore, the observed individual lines in the NMR spectrum suggest that the propensity of Cu and Zn atoms to be randomly distributed over the 2c and 2d crystallographic sites is lowered when lithium is partially substituted for copper. Additionally, the first-principles calculations provide insights into the arrangement of Li atoms as a function of the Cu/Zn disorder and its effect on the structural (lattice parameters) and optical properties of CZTS (band gap evolution). Those calculations agree with the experimental observations and account for the evolutions of the unit cell parameters as well as for the increase of band gap when the Li-content increases. The calculation of the formation enthalpy of point defect unambiguously indicates that Li modifies the Cu/Zn disorder in a manner similar to the change of Cu/Zn disorder induced by Ag alloying. Overall, it was found that Li alloying is a versatile way of tuning the optoelectronic properties of CZTS making it a good candidate as wide band gap materials for the top cells of tandem solar cells.

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INTRODUCTION The field of photovoltaic solar cells has recently turned its attention to the development of Earth-abundant materials which would eventually permit the Terawatt scale deployment while decreasing the cost of solar cell primary constituents.1 Up to now, the kesterite based Cu2ZnSnS4 derived compounds (hereafter labeled CZTS) have been considered as one of the promising candidates as an absorber for thin film solar cells with record power conversion efficiency up to 12.6%.2 Yet, the photovoltaic performances of the CZTS-based devices remain far below those of the CIGS-based technology. The corresponding open circuit voltage (Voc) deficit (ΔVoc = Eg/e − Voc where Eg represents the bang gap) is very high and is the main limitation of CZTS thin-film solar cells performances. The connection between this Voc loss and the cation disorder within the crystal structure of CZTS is still an open question.3,4 However, the Cu/Zn random distribution on 2c and 2d © XXXX American Chemical Society

crystallographic sites (hereafter abbreviated by Cu/Zn disorder) in kesterite has been pointed out to strongly impact the optical properties of such materials.5 Meanwhile, new routes toward high power conversion efficiency have been proposed relying on existing high performance technology such as Si-based or thin film based solar cells: tandem cells has emerged as one of the most promising solutions usually combining a top cell with a standard silicon-based bottom cell.6 Because of the very small lattice mismatch between CZTS and Si, CZTS has been envisioned as a top cell photoactive material for a Si-based tandem cell.7 The technical requirement for the optimal functioning of such a device has been recently derived8,9 and points to a large band gap, i.e., ∼1.7 eV for the photovoltaic Received: November 29, 2016

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DOI: 10.1021/acs.inorgchem.6b02865 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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material forming the top cell. As CZTS and CZTSe present a band gap of 0.95 and 1.45 eV, respectively , cation alloying (also called substitution) is required in order to obtain a CZTSbased material whose band gap exceeds 1.5 eV. Beyond the band gap tuning, it will also be appealing, for the purpose of device design, to tune the absolute position of the valence band and conduction band independently. This can be achieved within the CZTS system due to the decoupling of the cationic nature of the valence band maximum (VBM) (formed by Cu orbitals) and the conduction band minimum (mostly formed by Sn orbitals). Thus, substitution on the Cu site will result in a change of the absolute position of the VBM except for some strain induced effects which may affect all the electronic states of the material: the chemical nature of the bonding between anion p orbitals and Cu d orbitals results in an upward shift of the VBM (the so-called p−d repulsion)10 and a much lower ionization potential compared to standard s−p bonding semiconductor.11 Therefore, the replacement of Cu by an element not belonging to the d-block of the Periodic Table will drastically open band gaps by acting on the position of the VBM. One should notice that tuning of the absolute position of bands is not only important for band alignment but also for passivation purposes. In the case of the Cu(In,Ga)Se2 (CIGS) solar cell, a thin layer of CIGS-based material with a higher ionization potential at the interface between the absorbing layer and the buffer layer is known to decrease recombination in this part of the device.12 Additionally, on the basis of steric effects, partial substitutions on the Cu or Zn sites could reduce the disorder in kesterites. In natural stannite-kesterite chalcogenides, many substitutions are observed such as Ag for Cu and Fe or Cd for Zn. On the other hand, synthetic materials have been prepared with Ge or Si replacing Sn.13,14 Recently, it has been shown that the use of silver instead of copper could decrease the cationic disorder because the energy formation of [AgZn + ZnAg] defect complexes in Ag2ZnSn(S,Se)4 is much higher than that of [CuZn + ZnCu] in Cu2ZnSn(S,Se)4.15 This paper deals with the substitution of lithium for copper in CZTS. The incorporation of lithium in CZTS has already been studied in the case of thin film materials either at a doping level16 after a LiF treatment or for higher lithium-contents but limited to the poor-lithium side of the Cu2ZnSnS4−Li2ZnSnS4 system.17 Here, we present the first solid state chemistry investigation of lithium for copper substitution in Cu2ZnSnS4 compound in the whole composition range, i.e., 0 ≤ x = Li/(Cu + Li) ≤ 1. This study is based on an experimental approach coupled with theoretical calculations. It is well accepted that the substitution of an element B for an element A in a compound is possible when the elements A and B have quite close radii and the same oxidation state. Additionally, if compounds exist both for pure A and B derivatives the substitution is expected to be possible in the whole A1−xBx composition range. In the case of Li and Cu in (Cu,Li)2ZnSnS4 compounds the above conditions are fulfilled: (i) the bond valence parameters are 1.86 and 1.94 Å for Cu and Li, respectively,18,19 (ii) both Cu and Li are monovalent in chalcogenides, and (iii) both the Li-free and Cu-free compounds exist. Thus, substitution of Li for Cu is expected to occur in such pseudoquaternary compounds of general composition of (Cu1−xLix)2ZnSnS4, with 0 ≤ x ≤ 1. However, since there are slight crystal structure differences between Cu2ZnSnS4 (kesterite) and Li2ZnSnS4 (würtz-kesterite), a miscibility gap is expected to exist.

Article

EXPERIMENTAL SECTION

The details of the synthesis procedure, chemical analyses (EDX and ICP-EOS), powder and single-crystal X-ray diffraction (XRD) investigations, nuclear magnetic resonance (NMR) spectroscopy, and theoretical calculations are given in the Supporting Information. Chemical Analyses. The EDX and WDS techniques are often used to determine the chemical composition of CZTS compounds.20−23 Unfortunately, lithium cannot be detected through these techniques due to its too low atomic number. In this study, the Li-content has been indirectly determined from EDX composition (Cu, Zn, Sn, and S) using the charge balance equilibrium condition with the standard oxidation states of the elements: Cu(+I), Li(+I), Zn(+II), Sn(+IV), and S(−II). This condition is well fulfilled in Li-free CZTS.24 However, since the oxidation states of Zn and Sn are much higher than that of Li, the Li-content is highly sensitive to the measured contents of these two elements. Thus, the standard estimated deviation on Li concentration in CZTS derivatives is around 0.05. Moreover, as the EDX analyses might be impacted by the presence of an additional element (Li) (via the ZAF corrections) the compositions of some of the studied samples were checked through the ICP-OES technique. For this purpose, powdered samples with the highest purity, on the basis of XRD and EDX analyses, were selected. These samples have been analyzed in two sets. For one of them, there was an incomplete dissolution of sulfur. The corresponding chemical analyses showed a clear deficit in the sulfur contents so only the cationic compositions (normalized to 1 Sn per formula unit) are given for these samples. Tables 1 and 2 sum up the EDX and ICP-OES analyses carried out on prepared samples. Results are discussed below. Refining the Single Crystal Structure. Our goal was to investigate the single crystal structure of samples on both sides of the miscibility gap, i.e., for low and high Li-contents. Unfortunately, most of the tested crystals were of poor quality, specifically for low Licontent samples. A recrystallization process with iodine as a transport agent was then used (sample 9 with the Cu1.20Li0.80ZnSnS4 target composition) to get better crystals. For the Li-rich side, a crystal suitable for X-ray analysis was picked up from powdered sample 7 (with the Cu0.5Li1.5ZnSnS4 target composition). The crystal from sample 9 was found to be of tetragonal symmetry, while the diffraction pattern of the crystal 7 has been indexed in a monoclinic unit cell which is very close to an orthorhombic one. Just as a comment, no specific cooling procedure was applied for these two samples. Let us start with the crystal from sample 9. The diffraction pattern showed that this crystal was slightly twinned. Fortunately, it was possible to index the most intense diffraction peaks in the standard tetragonal unit cell. Then, the choice was done to ignore the twinning. The first step of the structure refinement was done with the kesterite (KS) structural model of the Li-free CZTS in the space group I4̅.25 In this structure, the cations are located on 2-fold special positions, i.e., Cu(2a), Sn(2b), Cu(2c), and Zn(2d), whereas the sulfur atoms occupy the general position 8g close to (3/4, 1/4, 1/8). Because this sample did not experience a very slow cooling procedure at the end of the recrystallization process, it is likely that Cu and Zn are randomly distributed on 2c and 2d sites.26 With the so-called disordered kesterite structure as the starting model, the residual factors and goodness of fit (GOF) converged to Robs/wRobs = 6.67/14.9 and GOF = 1.94 for 655 observed reflections and 19 refined parameters. At this step it was clear that the atomic displacement parameter (ADP) of the 2a site was substantially larger than those of the other positions. Thus, the 2a site occupancy factor (s.o.f.) was refined and converged to 0.87 corresponding to a better solution with Robs/wRobs = 5.76/12.45 and GOF = 1.69 (20 parameters). The ADPs were more homogeneous but remained higher for Cu and Zn atoms compared to those of Sn and S. The simultaneous refinement of the s.o.f. for all the 2a, 2c, and 2d sites definitely indicated that the 2c site remains with a full occupancy of Cu/Zn, while both 2a and 2d sites are partially occupied. Unfortunately, there are several limitations in a laboratory-based Xray diffraction technique which hinder accurate determination of the actual cationic distribution in this structure: (i) copper and zinc atoms B

DOI: 10.1021/acs.inorgchem.6b02865 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Target and EDX Compositions and Unit Cell Volumes Deduced from PXRD Analysesa samples

target composition

Li-content (xTarg)

EDX composition

Li-content (xEDX)

V (Å)d

1 2 3 4 5b 6 Li-poor 6′ Li-rich 7 8b 9b,c

Cu2ZnSnS4 Cu1.80Li0.20ZnSnS4 Cu1.5Li0.5ZnSnS4 Cu1.25Li0.75ZnSnS4 Cu1.20Li0.80ZnSnS4 CuLiZnSnS4 CuLiZnSnS4 Cu0.5Li1.5ZnSnS4 Li2ZnSnS4 Cu1.20Li0.80ZnSnS4

0 0.10 0.25 0.375 0.40 0.50 0.50 0.75 1 0.40

Cu2.01(3)Zn1.04(2)Sn1.01(2)S4.00(2) Cu1.85(1)Li0.02Zn1.04(1)Sn1.01(1)S4.00(1) Cu1.68(2)Li0.16Zn1.05(1)Sn1.01(1)S4.00(1) Cu1.43(2)Li0.41Zn1.07(2)Sn1.00(2)S4.00(2) Cu1.30(1)Li0.56Zn1.05(2)Sn1.01(1)S4.00(2) Cu1.22(3)Li0.68Zn1.05(2)Sn1.00(1)S4.00(2) Cu0.83(2)Li1.1Zn1.02(1)Sn1.01(1)S4.00(2) Cu0.43(2)Li1.4Zn1.07(1)Sn1.01(1)S4.00(2) Li1.9Zn0.97(1)Sn1.03(1)S4.00(2) Cu1.65(2)Li0.33Zn1.00(1)Sn1.006(9)S4.00(2)

0 0.01 0.09 0.22 0.30 0.36 0.57 0.76 1 0.17

319.71(2)d 320.87(2)d 322.04(1)d 323.40(1)d 324.69(1)d 325.61(1)d 330.57(2)e 334.00(2)e 340.38(1)e 322.15(1)d

a The lithium content, xEDX = Li/(Cu + Li), is estimated from the charge balance. No ESD values are given on the lithium content. bThese samples have been synthesized through the sulfurization route. cIodine transport for crystallization (see synthesis section in Supporting Information). d Tetragonal unit cell. eMonoclinic unit cell.

Table 2. Comparison of the Cationic Compositions of Some of the Studied Samples Deduced from the EDX and ICP-AES Analyses

a

samples

target composition

xTarg

EDX composition

xEDX

ICP composition (see footnotes)

xICP

1 2 10 11

Cu2ZnSnS4 Cu1.80Li0.20ZnSnS4 Cu1.60Li0.40ZnSnS4 Cu1.25Li0.75ZnSnS4

0 0.10 0.20 0.375

Cu2.01(3)Zn1.04(2)Sn1.01(2)S4.00(2) Cu1.85(1)Li0.02Zn1.04(1)Sn1.01(1)S4.00(1) Cu1.64(2)Li0.13Zn1.06(1)Sn1.03(1)S4.00(2) Cu1.28(1)Li0.55Zn1.05(2)Sn1.02(1)S4.00(1)

0 0.01 0.07 0.30

Cu2.05(4)Zn1.01(2)Sn0.96(5)S4.00(3) Cu1.87(2)Li0.15Zn1.05(2)Sn1.00(1)a Cu1.66(2)Li0.39(13)Zn1.06(2)Sn0.99(4)S4.00(3) Cu1.30(2)Li0.71(6)Zn1.05(1)Sn1.00(2)a

0 0.07 0.19 0.35

Incomplete dissolution of sulfur. The cationic composition is normalized on 1 Sn atom per formula unit.

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have very close atomic scattering factors and cannot be distinguished in common X-ray diffraction experiments; (ii) the KS structure is known to be prone to the occurrence of cationic vacancies, especially on the 2a site, at least in the copper-poor and zinc-rich compounds; (iii) due to its low atomic number, the contribution of the lithium atoms to the diffraction intensities is very low compared to those of the other atoms. Thus, the last refinement was done with a simplified structural model assuming a full occupation of all the cationic sites and a stoichiometry of 1 Zn atom per formula unit, Zn being equally distributed on both 2c and 2d sites. On both 2a and 2d sites, lithium partially replaces copper. Then, the refinement led to exactly the same residual factors (Robs/wRobs = 5.41/11.48 and GOF = 1.56 for 21 parameters) as previously, and the Fourier-difference map showed the same peaks (2.1/−1.1 e−/Å3) as for the vacancy-based structural model. Thus, with the limitations of this simplified structural model, a chemical composition similar to the one extracted from the EDX analyses was obtained (see Table 1), providing reliance to our refinement procedure. In the case of the Li-rich single crystal the starting model for the refinement was that of the wurtz-kesterite (WKS) structure of Li2ZnSnS427 in the space group Pn (No. 7). The same twinning law than for Li2ZnSnS4 has been found (1 0 0 0 1̅ 0 0 0 1̅) which is common in a monoclinic crystal structure with β ≈ 90°.28 In this structure, all the atoms are located on 2a Wyckoff positions with two different positions for lithium atoms. Considering these two sites are fully occupied by lithium, the corresponding atomic displacement parameters converged to negative values, and the maximal density in the Fourier-difference map was about 8 e−/Å3. This is in perfect agreement with the expectation that Li and Cu share the same positions. Thus, the site occupancy factors (s.o.f.) of these sites were both refined with the constraint s.o.f. (Li) + s.o.f. (Cu) = 1 for each site. Then, the refinement converged to Robs/wRobs = 2.76/3.98 and GOF = 1.04 (77 refined parameters and 2098 observed reflections), the Fourier-difference map being featureless. For each crystal, the corresponding space group is noncentrosymmetric so the absolute configuration has to be determined. For that purpose, the last refinement has also been done for the inverted structure, and the best solution has been retained.

RESULTS AND DISCUSSION Cu2ZnSnS4−Li2ZnSnS4 Phase Diagram. Table 1 gives the compositions of the studied compounds from EDX analyses. It is worth noting that the samples very often appear to be both Zn-rich and Sn-rich regardless of the target Li-contents as well as the synthetic routes (i.e., ceramic route or sulfurization under H2S gas). The corresponding estimated compositions are then (Cu,Li)-poor, which is reminiscent to the ability to the kesterite compounds to present off-stoichiometry such as copper-poor/ zinc-rich.20,22 The ICP-OES results are of quite good precision and are close to those obtained from EDX analyses for Cu, Zn, and Sn (see Table 2). They confirm that most of the studied samples are zinc-rich as observed from EDX. It is worth noting that the lithium-contents, determined from ICP measurements, are very close to the target Li-contents unlike what is found from EDX analyses. This result could be explained by the presence of a very small amount of lithium-based secondary phases which were not detected by the EDX analyses. The first evidence of the existence of mixed Cu−Li CZTSbased compounds was obtained from our preliminary powder X-ray diffraction (PXRD) investigations. For low target Licontents, the PXRD patterns are indexed in tetragonal unit cells with parameters close to those of the pure copper kesterite Cu2ZnSnS4. Figure 1 shows that the unit cell volume increases monotonously with the target Li-content (xTarg). This behavior is in perfect agreement with the larger bond valence parameters for Li than for Cu leading to expected bond distances, for tetrahedral coordination, of 2.37 and 2.45 Å for Cu−S and Li− S, respectively. However, a clear discontinuity occurs in the unit cell volume increasing for the target lithium-content of x = 0.50. The corresponding PXRD pattern was fitted with two unit cells (tetragonal and monoclinic) with volumes of 325.610(6) Å3 and 330.57(2) Å3, which clearly indicates that the sample is a mixture of a Li-poor phase and a Li-rich one (labeled 6 and 6′ in Table 3). According to the general trend within the series, the larger unit cell volume has been attributed to the Li-rich C

DOI: 10.1021/acs.inorgchem.6b02865 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Evolution of the unit cell volume of (Cu1−xLix)2ZnSnS4 versus the estimated Li-content from EDX analyses (xEDX) compounds in the extent of x = 0 to 1. Between roughly x = 0.4 and 0.6 a phaseseparation domain occurs. The straight line is for guiding the eyes only.

Figure 1. Evolution of the tetragonal unit cell volume of (Cu1−xLix)2ZnSnS4 compounds versus the target Li-composition (xTarg). There is a discontinuity for the target composition of 0.50 because the corresponding sample consists of a mixture of Li-poor and Li-rich compounds.

phase. For higher Li-contents, the PXRD patterns can be indexed with monoclinic unit cells which are reminiscent of the crystal structure of Li2ZnSnS4.27 The raw X-ray diffraction patterns for the studied samples and an example of a Rietveld plot (sample 2) are displayed in Figures S1 and S2 (Supporting Information). Figure 2 displays the evolution of the unit cell volume as a function of the EDX estimated Li-content showing the existence of a phase-separation domain between roughly x = 0.4 and 0.6, corresponding to the limits of the miscibility gap. The regular unit cell volume increasing, when the Li-content increases, in both Li-poor and Li-rich regions, indicates that the Li-content is correctly estimated from EDX analyses. Table 3 and Figure 3 show that the modification of the tetragonal unit cell in the low Li-content solid solution (x < 0.4) is quite complex since the a and c unit cell parameters evolve in the opposite way: a gradually increases with x while c shrinks. In ZnS sphalerite-related tetragonal structures, the tetragonal distortion is related to the deviation of the c/2a ratio from the ideal value of 1.29 For the studied compounds with x < 0.4, the tetragonal distortion is enhanced going from xEDX = 0 to xEDX = 0.36 with a c/2a ratio varying from 0.9977 to 0.9850.

Figure 3. Evolution of the tetragonal unit cell parameters (a and c/2) and the c/(2a) ratio of (Cu1−xLix)2ZnSnS4 compounds versus the Licomposition determined from EDX analyses (xEDX) for low Li-content (tetragonal solid solution).

Table 3. Unit Cell Parameters as a Function of the EDX Li-Contenta samples

Li-content (xTarg)

Li-content (xEDX)

symmetry

a (Å)

1 2 3 4 5 6 6′ 7 8 9b

0 0.10 0.25 0.375 0.40 0.50 0.50 0.75 1 0.40

0 0.01 0.09 0.22 0.30 0.36 0.57 0.76 1 0.17

T T T T T T M M M T

5.43137(8) 5.440747(1) 5.45259(6) 5.46566(4) 5.47872(7) 5.48789(6) 6.3421(1) 6.34822(13) 6.36519(9) 5.45501(10)

b (Å)

6.6778(1) 6.69139(14) 6.7258(1)

c (Å)

V (Å3)

c/2a

10.8377(2) 10.8397(2) 10.8319(2) 10.82580(10) 10.8170(2) 10.81152(14) 7.8055(2) 7.8628(2) 7.95084(14) 10.8259(3)

319.71(2) 320.87(2) 322.04(1) 323.40(1) 324.69(1) 325.61(1) 330.57(2) 334.00(2) 340.38(1) 322.15 (1)

0.9977 0.9962 0.9933 0.9903 0.9872 0.9850

0.9923

a

For the low Li-contents, the crystal structures are tetragonal (kesterite) while monoclinic symmetry (wurtz-kesterite) occurs for Li-rich compounds. The monoclinic unit cells are very close to orthorhombic ones, i.e., β very close to 90.0. bSingle crystal for structural investigation was picked up from this powdered sample after iodine transport. D

DOI: 10.1021/acs.inorgchem.6b02865 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

and 8 (xEDX = 1). The spectra of samples 1 and 8 exhibit a single line at −121 and +50 ppm, respectively. Although the crystallographic structures of samples 1 and 8 are slightly different, the difference of 171 ppm in isotropic chemical shifts is mostly dictated by the substitution of the 8 Li for Cu as second neighbors. Therefore, substituting one Li for one Cu atom in the second coordination shell of a Sn atom (from [Zn4LinCu8‑n] to [Zn4Lin+1Cu8−(n+1)]) modifies the isotropic chemical shift by ∼ +21 ppm. The spectrum of sample 5 shows 7 lines at −112, −88, −65, −43, −21, 0, and 21 ppm (labeled L(0) to L(6)) assigned to Sn atoms with 0 to 6 Li atoms as second neighbors, respectively (i.e., from [Zn4Cu8] to [Zn4Li6Cu2]).30 In addition to the X-ray results, the observed spacing between the lines underpins the substitution of Li for Cu rather than the incorporation of lithium in interstitial sites of the kesterite structure. Moreover, the existence of L(5) and L(6) lines proves that, even at moderate Li content (xEDX = 0.30), Li atoms are not restricted to a single Cu site occupancy. This is in full agreement with the single crystal investigation (vide supra) suggesting partial occupations of the 2d or even 2c sites in addition to 2a sites. Finally, the lines of sample 5 appear broader than those of the samples 1 (xEDX = 0) and 8 (xEDX = 1). This symmetrical widening reflects an isotropic chemical shift distribution having two origins. The first one is a chemical disorder due to many possibilities to share out the lithium atoms on the partially occupied 2a, 2c, and 2d sites of the kesterite structure. Thus, all the configurations (Li atoms arrangements) associated with a given secondary coordination shell ([Zn4LinCu8−n] with fixed n have slight differences in isotropic chemical shift which subsequently give rise to the broadening of each individual lines spaced by 21 ppm. The second origin of the isotropic chemical shift distribution lies in the spatial distribution of all the existing configurations within the structure. It adds a geometric disorder which induces slight differences inside each family of configuration (given number and position of lithium in the second coordination shell of Sn), leading to an additional spread in isotropic chemical shift. The high sensitivity of the 119Sn isotropic chemical shift to the Li content of its second coordination shell offers the capability to determine the whole Li content within the (Cu1−xLix)2ZnSnS4 phases of the samples from the relative intensities of the NMR lines. The 119Sn NMR approach appears very valuable because it turns out to be more precise than the EDX approach which suffers from the indirect determination of lithium-content as previously discussed. In a similar way that we obtained the S/(S + Se) content in Cu2ZnSn(SySe1−y)4 compounds,31 x = Li/(Li + Cu) values are given by

The increase of the unit cell volume with the lithium-content is in agreement with the already published results on thin film (Cu,Li)2ZnSn(S,Se)4 materials.17 To sum up, the single crystal investigation presented above supports very well the existence of two solid solutions within the Cu2ZnSnS4−Li2ZnSnS4 system, i.e., the kesterite structure for low Li-content and wurtz-kesterite structure for Li-rich composition. Tables S2, S3, S4, and S5 (Supporting Information) give the main structure parameters for the two studied single crystals. In the case of the crystal with the kesterite structure, the refined structural formula is Cu1.65Li0.35ZnSnS4. The lithium atoms are found to be distributed mostly on the 2a site and also with smaller occupancy factors on 2d site. Because the cation distributions as well as the ADPs of the 2c and 2d sites are different, one can conclude that these two sites are not equivalent, and thus the crystal structure of this compound has to be described in the I4̅ space group. However, because Cu and Zn cannot be distinguished, no definitive conclusion can be drawn about the distribution of these cations on the 2c and 2d sites and consequently on the Cu/Zn disorder. Let us now give the results for the Li-rich single crystal. In the wurtz-kesterite structure of Li2ZnSnS427 there are two distinct sites for the monovalent cation. For the mixed Li/Cu compounds, this study demonstrates that copper partially replaces lithium on these two sites with occupancy factors of 0.21 and 0.32, the zinc site being not affected (see Table S4). The corresponding structural formula is Cu0.525Li1.475ZnSnS4 (i.e., x(Li) = 0.74 for xTarg = 0.75). Both powder and single crystal X-ray studies definitely show that the incorporation of lithium in CZTS occurs as cationic substitution on tetrahedral sites and not on interstitial ones. 119 Sn NMR Spectroscopy. For a heavy nucleus like 119Sn (S = 1/2), the isotropic chemical shift is very sensitive to the chemical surrounding even beyond the second coordination sphere. In both the kesterite and wurtz-kesterite structures, the tin atom is coordinated to four sulfur atoms, and its second coordination shell is made of 12 cations, i.e., 4 Zn and 8 Cu (or Li), labeled as [Zn4(Cu,Li)8] hereafter. Figure 4 shows the 119 Sn MAS spectra of samples 1 (xEDX = 0), 5 (xEDX = 0.30),

x=

1 8

8

∑ nI (n) n=0

where I(n) are the relative intensities of the L(n) lines with ΣI(n) = 1. The decomposition of the spectrum of sample 5 is given in the Figure S3 (Supporting Information). The deduced intensities lead to xNMR = 0.30 ± 0.02 in full agreement with the Li-content of the (Cu1−xLix)2ZnSnS4 phase of sample 5 determined by EDX (xEDX = 0.30 ± 0.05). In addition to the quantitative analysis presented above, 119 Sn MAS NMR offers a way to assess the Cu/Zn disorder occurring in the Cu2ZnSnS4 compounds. Indeed, in previous investigations,32,33 we have shown that 119Sn MAS NMR provides a signature of the Cu/Zn disorder through the

Figure 4. 119Sn MAS NMR spectra of the end members of the (Cu1−xLix)2ZnSnS4 series and for sample 5 (with composition of Cu1.30Li0.56Zn1.05Sn1.01S4.00), member of the kesterite solid solution. E

DOI: 10.1021/acs.inorgchem.6b02865 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

quaternary family of compounds and subsequently a correct description of alloy energetics. One should first note that enthalpy of mixing of (Li,Cu) alloy is rather low over the full range of Li-content with typical enthalpy of mixing in the range of 2−3 meV/atom. Furthermore, depending on the location of the Li atoms in the unit cell, i.e., on the 2a or 2c Wyckoff position, the enthalpy of mixing is significantly changed, revealing the experimentally observed tendency of Li atoms to occupy preferentially the 2a Wyckoff position instead of the 2c or 2d one. Such a trend is further confirmed by the variation of the lattice constant c predicted by DFT (see Figure 6): the

asymmetric broadening of the right-side of the lines. This line shape asymmetry demonstrates that the Cu/Zn disorder, at local scale, differs from a simple random distribution of Cu and Zn atoms on 2c and 2d sites which would lead to symmetric 119 Sn NMR lines. The spectrum of sample 1 (xEDX = 0) exhibiting such a broadening is partly disordered because of the low energy formation of the [CuZn + ZnCu] defect complexes. In contrast, the spectrum of sample 8 (xEDX = 1) consists of a symmetric line in accordance with the much higher energy formation of [LiZn + ZnLi] defects which drastically limits the occurrence of Li/Zn disorder. Finally, although they overlap, the 7 lines of the spectrum of sample 5 appear symmetrical as witnessed by the spectral decomposition shown in the Figure S1 (Supporting Information). Therefore, the process associated with the Cu/Zn disorder in Cu2ZnSnS4 compounds does not exist for the sample 5 even though Li distribution occurs on the 2a, 2d, and possibly 2c sites. Theoretical Calculations. The enthalpy of mixing of (Li,Cu) alloy has been estimated via density functional theory (DFT) calculations including full structural relaxation and is shown in Figure 5. The dependence of the mixing enthalpy has

Figure 6. Evolution of the calculated and experimental tetragonal unit cell parameters (a and c/2) versus the Li-content when Li replaces Cu on the 2a site. In order to account for the small overestimation of the unit cell parameters through DFT-GGA calculations, the theoretical parameters values have been plotted after shifts of −0.0426 Å and −0.0392 Å for a and c/2, respectively.

decrease of c parameter observed experimentally is only consistent with a large occupation of 2a Wyckoff position by Li atoms. The preferential occupation of 2a Wyckoff position by Li has also the consequence to reduce the bowing of the lattice parameter and consequently induces a nearly linear increase of the lattice parameter a as a function of Li content. On the other hand, the mixing enthalpy of Ag,Cu alloy has also been estimated: the configuration where Ag occupies exclusively the 2a was found to be the lowest mixing enthalpy configuration but as displayed in Figure 5, the values obtained in this case are remarkably larger than the ones obtained for Li alloying, displaying a much steadier increase upon increasing Ag content despite the much closer chemical similarity of Cu and Ag. However, as already observed experimentally through advanced structural analysis, CZTS compounds usually show Cu/Zn disorder and significant off-stoichiometry originating from Cu vacancies. Such intrinsic defects are not accounted for in the present calculations and would be difficult to model as total energy will become dependent on the Fermi level. Instead of the alloy model used to determine the preferential site of Li, we turn here into the analysis of point defect and clusters of point defects. Such an approach has been previously used to explain the lower cation disorder resulting from completely replacing Cu by Ag in CZTS.15 The formation enthalpies of defect complexes [CuZn + ZnCu], [AgZn + ZnAg], and [LiZn + ZnLi] have been calculated at high concentration (i.e.,

Figure 5. Enthalpy of mixing of Li in kesterite CZTS as a function of Li content and for different Wyckoff positions of Li in the unit cell. For comparison, the red broken line refers to the mixing enthalpy of Ag,Cu alloy with Ag exclusively sitting on 2a Wyckoff position which is the lowest configuration for ordered kesterite phase.

been represented from 0% to 50% Li-content. Indeed, the current DFT calculations are not able to explain the crossover between kesterite and wurtz-kesterite structures for Li-content in between 40−60%. Instead, the kesterite phase remains more stable than the wurtz-kesterite phase up to 90% of Li content. Such an apparent discrepancy can be traced back to the very small energy difference between wurtz-kesterite and kesterite phase for Li2ZnSnS4. Chen et al. estimated the DFT energy difference for a large family of quaternary I2−II−IV−VI4 materials.34 They showed that the 0 K energy differences between wurtzite and zinc blende phase calculated with DFT were often much less than 1 meV/atom, that is below the accuracy of DFT calculations. Because of such a small energy difference, the use of more advanced ab initio techniques35 or the inclusion of temperature dependent contributions (configurational or vibrational) may be required to obtain a quantitative description of phase energetics in such a F

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kesterite Cu2ZnSnS4 phase. The values of energy difference are in agreement with the formation energy of complex defects calculated by Huang et al.36 Two different alloying configurations are investigated: configuration I corresponding to the alloying of Li and Cu on both 2a and 2d Wyckoff positions, and configuration II corresponding to the alloying of Li and Cu on 2a Wyckoff position only. In the case of Cu/Zn disorder, the alloy mixing enthalpy is found to be extremely dependent on the random configuration chosen to describe the disorder. Alloy mixing enthalpy down to 25 meV is observed though in both cases. It is foreseen that lower ΔE configurations may exist displaying even lower mixing enthalpy. Nevertheless, the present enumerative approach allows one to obtain an upper bound of the mixing enthalpy. The experimental observation of the configuration Li on 2a and 2d Wyckoff positions together with Cu/Zn disorder appears to be driven by temperature effects as the calculated 0 K mixing enthalpy favors the situation where Li sits exclusively on 2a Wyckoff position with Cu/Zn order. The inclusion of the full temperature dependence is extremely tedious for such a system and out of the scope of this study. However, one could consider the perfect solid solution configurational entropy to estimate the stability of the different scenarii. The configuration with Li on 2a and 2d Wyckoff positions present the advantages to make 2c and 2d Wyckoff positions distinguishable. As a consequence, configuration I can be seen as a high configurational entropy with respect to the (Li,Cu) alloy ordered kesterite phase. Crossover between configuration I and configuration II occurs at temperature 300 K, while crossover between configuration I and the Cu/Zn ordered (Li,Cu) alloy occurs at 850 K. Therefore, it is possible that configuration I with the unexpected occupation of the 2d Wyckoff position by Li is more stable than other configurations such as Li occupation of the 2a Wyckoff position due to configurational entropy argument. It is also clear that the present entropy-driven stabilization of disordered phase is only possible because of the low mixing enthalpy of Li,Cu alloy within a large range of Li content. Thus, such an effect might be absent from Ag,Cu alloy because of its intrinsically larger mixing enthalpy and point defect energetics. Figure 8 shows the evolution of the band gap upon Li alloying. Because of the DFT inability to predict accurate band gaps, ΔEg (Eg(x) − EgCZTS) has been represented, and the band gap variation with respect to the Li concentration x can be expressed as

corresponding to one complex defect in a 64 atom supercell containing 8 Cu(2c)-Zn(2d) pairs) and are 0.20 eV, 0.63 eV (in good agreement with previous studies15), and 0.41 eV, respectively, indicating that both Li and Ag are effectively reducing the Li/Zn or Ag/Zn disorder in Li-pure and Ag-pure quaternary compounds. Extending such an analysis to the case of Li and Ag alloying, the formation of complex point defects involving both alloying atom Li or Ag and CuZn+ZnCu was investigated in CZTS. First, Cu/Zn disorder is found to be more favorable on the 2c/2d sites than on the 2a/2d sites: the formation enthalpy of [CuZn + ZnCu] is 10 meV lower in the former case. If one were to consider the effect of Li or Ag substitution on a [CuZn + ZnCu] antisite defect (i.e., Li or Ag atom occupying the 2d site forming a [(Ag,Li)Zn + ZnCu] defect), the formation enthalpy of LiCu and AgCu is increased respectively by 75 and 100 meV with respect to the case of the perfectly ordered kesterite phase. Similarly, if one considers LiCu or AgCu sitting on a 2a Wyckoff position next to a [CuZn + ZnCu] defect (forming a [LiCu + CuZn + ZnCu] complex defect), the formation enthalpy of the antisite LiCu or AgCu is lowered by 34 and 67 meV with respect to the situation described previously (i.e., Li/Ag sitting in a 2d Wyckoff positions), yet still less favorable than the formation of an antisite LiCu or AgCu in a purely ordered material. Overall, such a systematic increase of formation enthalpy of antisite defects can be interpreted as an indication of a reduced disorder in the case of Li or Ag alloying with CZTS. Turning to the disorder energetics, the effect of Cu/Zn disorder on the mixing enthalpy of Li,Cu has been investigated for one specific alloying concentration, the one of sample 3. The space of possible configurations has been sampled by 50 different random configurations of Cu/Zn disorder occurring on the 2c/2d Wyckoff position. The mixing enthalpy of each configuration was computed with respect to the wurtz-kesterite Li2ZnSnS4 phase and the random configuration kesterite phase without the Li atom. Figure 7 shows the calculated formation enthalpy of (Li,Cu) alloying with respect to the energy between the Cu-pure random configuration and the perfectly ordered

ΔEg = ΔEgLZTS − CZTSx − bx(1 − x)

The bowing parameter has been obtained through the numerical fitting of the band gap values obtained from DFT for 15 different configurations of (Li,Cu) with Li sitting on the 2a site and is estimated at approximately 0.56 eV, which is considerably larger than the one obtained for CZT(S,Se) alloying 0.1 eV37 or (Fe,Zn) alloying 0.01 eV.38 Such large bowing parameter originates from the strong chemical difference between Cu and Li. Indeed, the large relaxation of the orbital p−d repulsion defining the position of the valence band maximum in kesterite will induce a large band offset between CZTS and LZTS and consequently a large band gap bowing.39 The band gap was also found to be only loosely affected by the alloy configuration as very small standard deviations are obtained for the set of 15 random considered structures (typically in the range of 1−4 meV). The Cu/Zn disorder impacts considerably the magnitude of the band gap,

Figure 7. Mixing enthalpy corresponding to concentration x = Li/(Cu + Li) = 0.16 for configuration I (Li on both 2a and 2d Wyckoff positions) and configuration II (Li only on 2a) versus energy difference between the random configuration and the ordered kesterite phase. G

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Figure 9. Absorption spectra (K/S) for different samples of (Cu1−xLix)2ZnSnS4 series after the Kubelka−Munk transformation K/S = (1 − R)2/(2R). To facilitate comparison, the spectra have been normalized between 0 and 1.

Figure 8. Calculated band gap modification (ΔEg = Eg(x) − Eg(x = 0)) versus the lithium-content in the (Cu1−xLix)2ZnSnS4 series for x ≤ 0.5 for the kesterite structural model when Li replaces Cu only on 2a site. The blue points correspond to the band gap opening with respect to the Cu/Zn disordered kesterite phase for Li content x ≈ 0.16, configuration I (see text). For comparison, the case of Cu,Ag alloy is also displayed.

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CONCLUDING REMARKS This study demonstrates that lithium can replace copper in Cu2ZnSnS4. Hence, within the (Cu1−xLix)2ZnSnS4 series, the kesterite structure type of Cu2ZnSnS4 is retained up to x ≈ 0.4, while the wurtz-kesterite structure of Li2ZnSnS4 is stable beyond x ≈ 0.6. In between, a miscibility gap exists. Single crystal X-ray diffraction and ab initio calculations conclude that the 2a site (i.e., the site commonly filled by Cu only even in the disordered-kesterite structure) is preferentially occupied by the alkali metal, while the 2d or 2c sites would be also occupied by Li but in a much lesser extent. More interestingly, from 119Sn NMR investigations, the presence of lithium in materials within the kesterite domain appears to reduce the Cu/Zn disorder which could be beneficial to the photovoltaic performances. Calculations show that this trend would originate from the formation enthalpies of defects significantly higher for [LiZn+ZnLi] complexes than for [CuZn + ZnCu] ones. On the basis of chemical arguments, this observation was difficult to predict because the radii of Li+, Cu+, and Zn2+ cations are quite similar. Anyway, the substitution of monovalent cations (e.g., Li+, Ag+) for Cu+ cations may be an avenue worth exploring to reduce the Cu/Zn disorder in CZTS materials and their derivatives in the forthcoming studies. The Cu/Li substitution appears as an adequate lever to tune the optical gap in the 1.5−1.9 eV range within the kesterite solid solution. More specifically, (Cu0.7Li0.3)2ZnSnS4 with an absorption threshold at ∼1.7 eV can be really regarded as a very good candidate for top cells of tandem solar cells with a Sibased bottom cell. The conservation of the kesterite structure and a small unit cell volume increase is observed upon alloying limiting the lattice mismatch between (Cu,Li)2ZnSnS4 and Si. Additionally, such a range of band gap might also be of interest for photocatalytic water splitting application.44

resulting in band gap reduction ranging between 0.5 and 0.2 eV for all random configurations considered in this study in agreement with experimental findings.5,40 As DFT + U strongly underestimates the band gap of CZTS (Eg for ordered kesterite calculated within DFT + U is 0.5 eV), only the Cu/Zn configurations displaying the smallest band gap reduction were considered in Figure 8 in order to estimate the band gap opening due to Li,Cu alloying. The Cu/Zn disorder is found to mitigate the band gap opening effect of Li alloying, resulting in overall significantly smaller band gaps. The evolution of the band gap with respect to Ag,Cu alloying is also shown in Figure 8 in order to address the potential of such cation substitutions on band gap opening. Variation of band gap upon Ag alloying is much smaller than in the case of Li. This is primarily due to the small difference between the band gap of the pure Ag phase with calculated value of 1.67 eV15 and the one of CZTS. The band gap opening is also plagued by a large band gap bowing such as in the case of CIGS with value comparable to the one of Li in CZTS.41,42 Therefore, a significant increase of the band gap, up to a value of interest for tandem cell application, will be difficult to achieve with a moderate Ag content. Figure 9 gives the reflectance spectra of some of the studied compounds. There is a clear trend that the absorption edge is blue-shifted when the Li-content increases as predicted by the calculations presented above. This evolution is also consistent with the data for the Li-free and Cu-free compounds, i.e., Cu2ZnSnS443 and Li2ZnSnS4.27 Tandem cell applications assuming a Si-based bottom cell require a 1.7 eV band gap top cell absorber for optimal performances. From the calculations, in the case of Li alloying, it appears unambiguously that such optimal values of band gaps can be obtained for Li content included in the range of stability of the kesterite solid solution. This result is experimentally confirmed since the band gap increases by 0.35 eV within the kesterite solid solution (x = 0 to x = 0.36).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02865. H

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Details of the synthesis procedure, chemical analyses (EDX and ICP-EOS), powder and single-crystal X-ray diffraction investigations, NMR spectroscopy, and theoretical calculations (PDF) Crystallographical information files (CIF1, CIF2)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

A. Lafond: 0000-0001-5981-5480 S. Jobic: 0000-0002-1900-0030 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Dr. Florian Massuyeau (IMN, Nantes France) for his assistance in performing the diffuse reflectance measurements and Stéphane Marcotte (COBRA, Rouen France) for fruitful suggestions on the ICP-OES protocol in the case of CZTS derivatives. This project has been partially supported by the French Government in the frame of the program of investment for the future (Programme d’Investissement d’Avenir - ANR-IEED-002-01).

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