119Sn MAS NMR to Assess the Cationic Disorder and the Anionic

Article pubs.acs.org/JPCC

119

Sn MAS NMR to Assess the Cationic Disorder and the Anionic Distribution in Sulfoselenide Cu2ZnSn(SxSe1−x)4 Compounds Prepared from Colloidal and Ceramic Routes Michael̈ Paris,*,† Gerardo Larramona,‡ Pierre Bais,† Stéphane Bourdais,‡ Alain Lafond,† Christophe Choné,‡ Catherine Guillot-Deudon,† Bruno Delatouche,‡ Camille Moisan,‡ and Gilles Dennler‡ †

Institut des Matériaux Jean Rouxel, Université de Nantes, CNRS, 2 rue de la Houssinière, BP32229, Nantes 44322 Cedex 03, France IMRA Europe S.A.S., 220 rue Albert Caquot, F-06904 Sophia Antipolis, France

‡

S Supporting Information *

ABSTRACT: Crystalline powders of the photovoltaic material candidate copper zinc tin sulfoselenide Cu2ZnSn(SxSe1−x)4 (CZTSSe) with x = S/(S + Se) from 0 to 1 were characterized by 119Sn solid state nuclear magnetic resonance (NMR) and by powder X-ray diffraction (PXRD). Two series of powders were characterized: one synthesized by a ceramic route and having cationic stoichiometry 2:1:1 and another one synthesized by a colloidal route, having a cationic Cu-poor Zn-rich composition and used as precursors for photovoltaic active films. The homogeneous anionic composition of the samples, which is a feature needed for the NMR analysis of the anionic distribution, has been proved by PXRD. The x values determined from the quantitative analysis of the 119Sn spectra are in very good agreement with those deduced by PXRD. In addition, the 119Sn spectra reveal, for the first time, the random distribution of the chalcogen atoms, which seems to be a general process. Finally, a qualitative, but thorough, analysis of the line width of the 119Sn NMR spectra was undertaken to investigate the Cu/Zn cationic disorder, a feature which can seriously affect the optoelectronic properties of CZTSSe. The cationic disorder turns out to be dependent on the type of cationic composition. Indeed, regardless of x ratios and the synthesis methods, our samples containing A-type defect complexes [VCu + ZnCu] are less prone to Cu/Zn disorder.

■

were explored.10,13 Point defects have been characterized by their electronic properties (energy level or ionization energy).14−17 These experimental characterizations showed a good consistency with the theoretical calculations.18,19 Recently, several investigations dealt with the occurrence of a full cationic disorder (involving Cu, Zn, and Sn cations) in stoichiometric CZTS materials. For example, calculations suggested that the lowest energy configurations are encountered when the same kind of cations are separated.20 Moreover, in the case of CZTS nanoparticles, a study combining neutron diffraction and X-ray absorption fine structure (XAFS) investigations showed that local fluctuations in the composition, corresponding to nanoscale domains enriched in certain metal ions and depleted in others, occurred.21 Local fluctuations have also been predicted for the defects originated by the Cu/Zn cationic disorder22 ([CuZn + ZnCu] defect complexes in the planes at z = 1/4 and z = 3/4 of the kesterite structure) which has recently received special attention. The interest in such a structural feature arises from the suspected deleterious effect of the degree of disorder on the

INTRODUCTION Cu2ZnSn(SxSe1−x)4 (CZTSSe) is a potential alternative to CIGS for thin film solar cells, which suffers from the low abundance of indium. Despite the fact that the best cell efficiencies have been obtained with the sulfoselenide CZTSSe absorbers,1 they have been scarcely studied, the most comprehensive characterization and simulation of the material properties being conducted on the pure sulfide CZTS and the pure selenide CZTSe extremes. Powder X-ray diffraction (PXRD) confirmed that the CZTSSe possesses the kesterite structure all along the whole range of the S/(S + Se) anionic ratio.2 The measurement of the S−Se ratio itself has been carried out by PXRD2,3 and Raman spectroscopy.4 The band gap of the material changes with such ratio from ∼1 eV for pure Se to ∼1.5 eV for pure S kesterite. Band gap and characterization of electronic defects by photoluminescence for various anionic ratios have been published.2,5−7 On the other hand, the studies on the pure sulfide and on the pure selenide materials have proved the kesterite structure against the stannite structure in both the stoichiometric and nonstoichiometric compounds.8−12 The local structures of some defect complexes occurring in the Cu-poor Zn-rich materials, like A-type [VCu + ZnCu] or B-type [ZnSn + 2ZnCu], © XXXX American Chemical Society

Received: September 14, 2015 Revised: November 5, 2015

A

DOI: 10.1021/acs.jpcc.5b08938 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

acetonitrile (CH3CN) solution containing the three metal chlorides SnCl4, ZnCl2, and CuCl (∼1 M concentration in total metal cations). They were mixed rapidly at room temperature under magnetic stirring. After stirring for a few additional minutes, the suspension was washed several times in water or in ethanol, by centrifuging the colloid, removing the supernatant, and redispersing the precipitate in the washing solvent. The offstoichiometric Cu−Zn−Sn ratio in the metal precursors was 1.9/1.2/1 (46−29−25%), and the NaHS solution was in ∼20− 40% molar excess with respect to the total metal stoichiometry. The solution preparations and the synthesis steps were carried out in a nitrogen glovebox, and all solvents were deoxygenated by bubbling N2 for 1 h before use. The sulfoselenide CZTSSe colloids and the pure selenide CZTSe colloid were synthesized in a similar manner but using a solution made of a mixture of the chalcogenide precursors NaHS and NaHSe (only the latter for the pure selenide CZTSe colloid). After the last centrifugation step, the precipitate was left drying at room temperature in the glovebox for at least 2 days. Afterward, the resulting powder was placed in a ceramic boat and submitted to an annealing treatment on a hot plate inside the N2 glovebox. For the pure sulfide material, the treatment consisted of a ramp of about 1 h up to 525 °C, a 15 min dwell at such a temperature, and a natural cooling for about 7 h. For the Secontaining materials, the treatment consisted of a ramp of about 1 h up to 550 °C, a 45 min dwell at such a temperature, and a natural cooling for about 8 h. The cooling step was not extremely slow as compared with the one of the ceramic series, but it was chosen in this way in order to keep close to the one used when fabricating active layers deposited on Mo glass substrates which led to photovoltaic devices showing acceptable cell efficiencies.33,36 Only one sample of the colloid series, the 35% S content, was cooled more slowly than the others (6 h down to 350 °C plus 32 h from 350 °C to room temperature). The annealing treatment allowed us to grow the particles and to get a PXRD signature close to that of the active photovoltaic films, the nonannealed dried colloidal powders showing practically no peaks due to the small crystalline domain of the particles prior to the annealing. After NMR analysis of this sample, the powder was heated from room temperature to 250 °C at 50 °C/h, held for 1 h, and then cooled very rapidly by an iced-water quenching. We chose the powders obtained directly from colloid precursors instead of the powders scratched from photovoltaic grade films deposited on Mo glass. This choice was done because of the minimum amount of material (about 150−200 mg) needed for the NMR measurements, which would have implied to scratch a lot of films to reach such an amount. Moreover, we carried out an NMR measurement on scratched powder from Mo/CZTSSe films leading to ∼8% efficiency level and with a S content similar to the colloid sample with target composition x = 0.35. The NMR spectra of the scratched films were very similar to the spectra of the dried colloids. Hence, the present research was done on the colloidal precursor powders in order to save NMR acquisition time. In summary, the ensemble of samples consists of two series of five samples. They are labeled “CO” and “CE” according to the synthesis route, colloidal or ceramic, respectively. The additional number in the code stands for the targeted x = S/(S + Se) ratio. Thus, CE25 refers to the sample with a targeted x value of 0.25 obtained from the ceramic route. The list of the studied samples is given in Table S1. An additional sample cited in this work consists of a slow-cooled Cu-poor Zn-rich CZTSSe ceramic sample, which was labeled CE25_Cu-poor. Finally, the

photovoltaic properties of the material. However, up to date there has been only one report on the effect of the degree of disorder in the cell performance.23 Concerning the bulk material, the Cu/Zn disorder in the kesterite structure has been characterized by neutron diffraction,8 Raman spectroscopy,24−26 nuclear magnetic resonance (NMR),10,12,27 X-ray resonant diffraction,28 and transmission electron microscopy (TEM).29 For example, critical temperatures between Cu/Zn ordered and disordered structures have been recently determined to be 260 ± 10 °C and 200 ± 20 °C for pure sulfide and pure selenide, respectively,30,31 and 195 ± 5 °C for a sulfoselenide with 8% S content.32 Relationships between the Cu/Zn disorder and the cationic composition have also been explored.10 However, all these experimental data mostly refer to the pure sulfide and the pure selenide materials; therefore, there is a lack of information regarding the Cu/Zn disorder in sulfoselenide compounds. The present work intends to extend the knowledge of the cationic disorder into the sulfoselenide CZTSSe materials for different anionic ratios, by exploring the influence of the thermal history (well-known to impact the Cu/Zn disorder) and the cationic composition. For this purpose, this investigation was conducted on two series of CZTSSe powder materials. The first series consists of materials with homogeneous and stoichiometric CZTSSe phases, synthesized by a ceramic route. The second series was synthesized via a colloidal route and led to photovoltaic grade CZTSSe materials. This latter series has been used as precursors in an environmentally friendly method based on colloidal ink spray deposition and leading to 8.6% efficient solar cells.33 In this work, we make use of 119Sn MAS NMR spectroscopy, a technique which was already shown to be able to probe the cationic disorder.10,27 We also demonstrate here that NMR is an efficient technique to measure the S/(S + Se) ratio and to address the question of a hypothetical existence of a S−Se anionic ordering. Although recently suggested by Raman spectroscopy,3 such an anionic ordering has never been studied despite some calculations predicting the alteration of the material band structure.34

■

EXPERIMENTAL SECTION Synthesis. The synthesis of powders via the ceramic route is adapted from Bernardini et al.35 Namely, the compounds were prepared by solid state reactions from elemental Cu, Zn Sn, S, and Se precursors weighted in appropriate ratios. The reactants (total mass of about 1 g) were ground together and pressed into pellets before being heated in evacuated, sealed, fused silica tubes. The tubes were heated to 750 °C (300 °C/h), held there for 4 days, and then cooled to room temperature (100 °C/h). The batch was then separated into two parts which underwent different cooling treatments after annealing at 350 °C for 48 h. One part was quenched from 350 °C into iced-water to maximize the Cu/Zn disorder. The other part was slowly cooled from 350 to 150 °C at 5 °C/h, held at 150 °C for 4 days, and then cooled to room temperature at 10 °C/h. Since the pure sulfide and the pure selenide samples were synthesized for previous investigations,12,27 when order−disorder transition temperatures were unknown, they underwent a different thermal history. In particular, the annealing step at 150 °C, below the order−disorder transition temperature, was absent. The synthesis of the powders via the colloidal route used offstoichiometric precursor ratios. The pure sulfide CZTS colloid was synthesized according to the following procedure:36 an aqueous solution of NaHS (∼0.1 M) was mixed with an B


Article


Energy-Dispersive X-ray (EDX). The elemental composition of the ceramic samples is an important point for this study, and a lot of care was undertaken to ensure both accuracy and precision in these measurements. The composition of the ceramic samples was determined using an EDX-equipped scanning electron microscope (SEM) (JEOL 5800LV) on polished sections of the products embedded in epoxy resin. The operating conditions were an accelerating voltage of 20 kV, a current of 0.300 nA, and a recording time of 30 s/spot. The elemental percentages were calculated using calibrated standards (Cu, Zn, Sn, FeS2, and Se) and subsequently corrected according to the composition of stoichiometric Cu2ZnSnS4 and Cu2ZnSnSe4, previously determined using an electron probe microanalyzer (EPMA). This procedure ensures to get very reliable results and demonstrates the homogeneity of the CZTSSe phase within the studied samples. Secondary phases, such as Zn(S,Se), are often detected. The elemental analysis of the CO series was carried out with an EDX Noran System SIX coupled to an SEM Hitachi S-4700 on the powders adhered to a carbon tube. Due to the absence of a flat surface and no additional calibration, the values of the elemental composition are approximate. Table S1 lists the cationic composition of the samples. The CE25, CE50, CE75, and CE100 samples are stoichiometric, whereas CE0 and CE25_Cu-poor samples are close to A-type, hence contain [VCu + ZnCu] defect complexes. The substitution type derivation is not possible for the CO series due to the lack of accuracy in the EDX measurements. Nevertheless, from Table S1, it appears that CO35, CO50, and CO75 are clearly Cu-poor Zn-rich, whereas CO0 and CO100 are closer to the stoichiometry 2:1:1. Powder X-ray Diffraction. The PXRD patterns of the ceramic samples were collected on a Bruker D8-diffractometer in Bragg−Brentano geometry with the monochromatized Cu(Kα1) radiation. For the CE series, the recording conditions were 10−100° 2θ range, step size 0.0105°, 0.4 s/step. For the CO series, the acquisition was carried out in a similar apparatus with the pseudo Bragg−Brentano geometry and a 2θ−θ lockedcoupled mode, and the recording conditions were 10−60° 2θ range, step size 0.04°, and 40 s/step. For all samples, the unit cell parameters were determined by Le Bail refinements (full pattern matching mode) implemented in the JANA2006 program.41 In the case of the CE25 sample, the refinement was carried out using the fundamental approach. In this method, the line profiles are calculated taking into account the diffractometer optics, the distribution of the wavelength, and the contribution of the sample through two physical parameters corresponding to the crystallite size and the microstrains within the sample.

two iced-water quenched samples were labeled CO35-Q and CE50-Q. 119 Sn MAS NMR. The 119Sn NMR spectra were acquired with a Bruker Avance III 300 MHz spectrometer with a 4 mm CP-MAS probe. To obtain an absorption mode only line shape, 119 Sn MAS (14 kHz) spectra of the pure sulfide or the pure selenide compounds were acquired using a full shifted echo acquisition (π/2−τ−π−acq) with τ ranging from 0.57 to 1.9 ms. We also used the CPMG (Carr−Purcell−Meiboom−Gill) approach combined with MAS for sensitivity enhancement of the 119Sn spectra of sulfoselenide samples.37,38 The acquisition time τa for each echo ranged from 0.75 to 1.18 ms depending on the length of the f.i.d. (free induction decay), and up to 240 echoes were acquired. For rotor-synchronization purpose, the MAS frequency was adjusted between 12 693 and 12 732 Hz according to τa, to the refocusing π pulse length, and to additional short delays (40 μs) for reducing the effects of probe ringing. In all cases, we used a radio frequency field of 80 kHz. To ensure quantitative spectra, recycle times between scans were systematically adjusted. Thus, we used delays ranging from 20 s (due to a small amount of CuS) to 120 s. 119Sn spectra were referenced to Me4Sn using Ph4Sn as a secondary reference (−121.15 ppm). In order to allow spectral decompositions (vide inf ra) and to obtain an absorption mode only, 119Sn spectra were constructed by adding the 10 first full echoes of the CPMG acquisitions. An excellent agreement was obtained between the spectra reconstructed from the CPMG acquisitions and those obtained from the (π/2−τ−π−acq) sequence (see examples of the CO35 and CE50 samples in the Supporting Information, Figure S1). The signal-to-noise ratio is enhanced by a factor of about 3 without any change in the relative intensities of the lines. It is worth noting that adding more echoes improves the signal-to-noise ratio enhancement but induces spectral distortions stemming from differences in T2 relaxation time between the lines. Decompositions of the 119Sn MAS spectra into individual components are hindered by the overlapping of many lines whose shapes are asymmetric. As discussed in the text, the widths and the shapes of the lines originate from multiple factors. On the contrary, their intensities are only related to the anionic environments. Therefore, since only line intensities are relevant for the discussion developed along this article, we have used a fitting procedure to obtain accurate relative intensities of all NMR spectra. On one side, due to the small asymmetry exhibited by the 119Sn spectra of the CO series, the experimental spectra were efficiently reproduced by using a limited set of Gaussian/Lorentzian mixed functions.39 On the other side, the larger asymmetry shown by the CE series spectra called for the use of individual lines with unsymmetrical shape. For this purpose, we have chosen the LF(α,β,ω,m) function available in the CasaXPS software40 initially designed to fit XPS spectra where asymmetry can be large. This function is a numerical convolution of a Lorentzian with a Gaussian of width m. The α and β parameters describe the asymmetric profile on either side of the peak position. ω is a damping parameter to force the tail to reduce toward the limits of the integration range. Here, we have used the LF(0.6,1.0,50,50) line shape (unsymmetrical right-hand side of the line with α = 0.6 and symmetrical left-hand side with β = 1.0). An example of such a decomposition for the CE50 sample is given in the Supporting Information (Figure S2).

■

RESULTS AND DISCUSSION This section is organized in three parts. We will first discuss the detailed PXRD characterization of the samples. Here we obtain precise and accurate equations on the linear regressions of the lattice parameters as a function of x = S/(S + Se) obtained by EDX. Their applicability is discussed according to the cationic composition and to the thermal history of the samples. These regressions will then be used to calculate the sulfur content of the CO series. The second and third parts deal with the 119Sn MAS NMR characterization of both the CO and CE series. We will qualitatively interpret the NMR spectra and address the occurrence of the cationic disorder in the sulfoselenide CZTSSe materials. This will be followed by a quantitative C


Article


Figure 1. (Left) Unit cell parameters a and c from PXRD as a function of x = S/(S + Se) accurately determined by EDX of the pure phase, cationic stoichiometric ceramic CE series materials. The linear regressions are added. (Right) Enlargement of the x ≈ 0.25 and x ≈ 0.50 regions for different CE samples showing that the a parameter is less affected than c by thermal treatments or deviation from the cationic stoichiometry. Error bars are within the symbol size.

Figure 2. 119Sn MAS NMR (reconstructed from CPMG for 0 < x < 1) of CZTSSe samples as a function of the S content, x = S/(S + Se), for the series synthesized by the colloidal route “CO” (left) and by the ceramic route “CE” (right). The x value determined by NMR is indicated in italics for each spectrum.

interpretation of the 119Sn NMR spectra in order to extract the x ratios and to investigate the anionic distribution. PXRD Characterization. The PXRD patterns can be indexed in the tetragonal unit cell of the kesterite structure for all the CE and CO samples, in agreement with the existence of a solid solution within the whole x = S/(S + Se) composition range.2,3 Figure 1 (left) shows the plot of the unit cell parameters of the ceramic preannealed ceramic samples versus the corresponding EDX anionic compositions, as well as the linear fits (the Vegard’s law), which lead to the following equations

When considering additional data points such as the CE samples with a quenched cooling treatment (CE50-Q) or with a Cu-poor composition (CE25_Cu-poor), one can see that the a parameter is less scattered than the c parameter (Figure 1, right). This means that the anionic composition of a given sample can be more accurately deduced from the a parameter than from the c parameter, regardless of the copper content and/or the thermal treatment. Therefore, the c/a ratio for a given x composition slightly depends on the nature of the sample. The possible occurrence of clusters with different anionic compositions (i.e., different values of x within a single sample) is a primordial point for the interpretation of the NMR results (vide inf ra). Figure S3 gives the positions of the larger diffraction peaks (112) versus the anionic composition (x) for the two series. When the x values increase, the peaks shift

a(x) = −0.266x + 5.696 and c(x) = −0.516x + 11.353

The standard deviations on the equation parameters are in the range of 1 to 3 on the last digit, and the correlation factors of these linear fits are R = 0.999 in both cases. D


Article

The Journal of Physical Chemistry C Table 1. Isotropic Chemical Shifts δisoa, Relative Intensities I(n), and Assignments of the T(n) Lines from the Spectra of Figure 2 T(4) sample CO0 CO35 CO50 CO75 CO100 CE0 CE25 CE50 CE75 CE100 a

δiso

a

−182c −163 −143 −122

−178 −142 −122

T(3)

T(2)

0.02 0.12 0.45 1.00

−280 −253 −230

0.12 0.32 0.39

−372 −344 −310

0.29 0.35 0.14

−457 −425 −400c

0.39 0.17 0.02

−300 −269 −232

0.04 0.27 0.39

−395 −357 −315

0.19 0.38 0.22

−483 −440 −407c

0.42 0.23 0.03

0.07 0.35 1.00

I

(3)b

δiso

a

(2)b

I

δiso

T(0)

δiso

a

Sn MAS NMR

T(1)

I

(4)b

119

a

(1)b

I

δisoa

I(0)b

−596 −533 −489c

1.00 0.18 0.04

−592 −560 −529c −507c

1.00 0.35 0.05 0.01

Due to line asymmetry, δiso refers here to the position of the maximum ±5 ppm. b±0.02. cValues with larger errors due to low intensities.

variations for the isotropic chemical shift as a function of x is in support to the performed spectral decompositions (see the variation of the 5 T(n) lines versus x in the Supporting Information, Figure S4). The above discussion of the CO series also applies for the CE series. Nevertheless, these two series hugely differ in their 119 Sn NMR line widths, which are now discussed. 119 Sn NMR Line Widths in Sulfoselenide CZTSSe. Prior to the comparison of the CO and CE series, we first discuss the NMR line width variation as a function of x and n. For a given n, the maximum width takes place at intermediate Se contents (x = 0.5), while the minimum width occurs at the extremes (x = 0 or x = 1). This behavior is also observed in Raman spectra.4,6 The NMR width reflects an isotropic chemical shift distribution due to a distribution of angles and bond lengths; this means that each individual T(n) tetrahedron (n and x fixed) shows a slightly different geometric distortion within the structure. The distortions are induced by the other T(p≠n) species and their spatial distribution, which cause local structural disorder and hence line widening with x. Accordingly, if the Cu2ZnSn(S0.5Se0.5)4 compound was only constituted by T(2) tetrahedra (i.e., no anionic disorder), the 119Sn NMR spectrum would consist of a single thin line like those in the cases x = 0 (only T(0)) or x = 1 (only T(4)) shown in Figure 1. On the other hand, for a fixed x, the T(n) lines tend to widen as n decreases. This is particularly visible in the CO75 and CE75 spectra. We ascribe this behavior to the softness of the Sn−Se bond as compared to the Sn−S one. Thus, [SnSnSe4−n] tetrahedra with more Sn−Se bonds would better accommodate the local structural disorder, which would result in line broadening for small n values. Cationic Composition to Control the Cu/Zn Disorder. A final and relevant contribution to the line width is the full cationic disorder (involving Cu, Zn, and Sn cations), which can show up under different forms. For example, in stoichiometric CZTS nanoparticles, local fluctuations of the cation distribution were experimentally observed to extend beyond a few crystallographic unit cells.21 The existence of a full cationic disorder in our samples would likely contribute to the NMR line broadening. However, the level of this full cationic disorder was shown to decrease with annealing,21 in agreement with the energetically unfavorable configurations involved.20 We would expect the CO series to be more affected by the full cation disorder since the CO series samples were cooled more rapidly than the CE series. Yet, the lines of the CE series are broader

toward higher 2θ values. Because the shift is larger than the width, even for the colloid series, we conclude that all the samples have a quite homogeneous anionic composition. In addition, from the fundamental parameter approach used for the CE25 sample, the crystallite size is found at about 700 nm. In the case of the colloid samples, the peaks are broader due to a larger instrumental contribution, but the crystallite sizes can be estimated larger than 60 nm. The lattice parameters of the samples are given in Table S2. Qualitative Analysis of the 119Sn MAS NMR Spectra: Toward the Cationic Disorder. Description of the 119Sn NMR Spectra. Figure 2 shows the 119Sn MAS NMR spectra of the CZTSSe samples as a function of x = S/(S + Se), for the colloidal CO (nonquenched) and ceramic CE (slow-cooled) series. The position of the NMR lines (the isotropic chemical shift δiso) is sensitive to the electronic surrounding of the probed nucleus. Consequently, δiso is very sensitive to the nature of the first neighbors (S or Se) as well as to the bond lengths and angles. In the kesterite structure, the cations are centered within a tetrahedron formed by four chalcogens (anions). Hence, the 119 Sn NMR line shifts from −592 ppm for pure CZTSe ([SnSe4] tetrahedra) to −122 ppm for pure CZTS ([SnS4] tetrahedra). For the intermediate sulfoselenide cases, the line associated with the [SnSnSe4−n] tetrahedra (hereafter referred as T(n)) lies between those two poles with an approximate 90 ppm frequency upshift as n increases. For example, the CO50 spectrum in Figure 2 shows four main lines associated with the T(1), T(2), T(3), and T(4) tetrahedra (the small intensity of the T(0) line makes it hardly observable, see Table 1). Table 1 lists the assignment and position of each line for all spectra in Figure 2. A direct consequence of these spectra is that the anionic distribution cannot be described by a simple anionic ordering scheme since pure [SnS4] and [SnSe4] tetrahedra can coexist within the same CZTSSe grain for intermediate x values. The relationships between the line intensities and the S/Se distribution will be discussed below. From Table 1, it appears that the δiso of a given T(n) line decreases as x decreases, whatever the n value. This reflects the differences in the local geometry of the T(n) tetrahedra belonging to samples with different x values. For instance, the T(4) line shifts from −122 to −143 ppm and then to −163 ppm when going from the spectra of CO100 to CO75 and to CO50, respectively. This means that the bond lengths and angles of the T(4) tetrahedra within the sample CO100 differ from those within the samples CO75 or CO50. The linear E


Article

The Journal of Physical Chemistry C than of the CO series (Figure 2). Hence, we can conclude that the full cation disorder process does not drive the line widths. Due to the low formation energy of the [ZnCu + CuZn] defect complexes,19 the main cationic disorder is the Cu/Zn disorder (i.e., Cu/Zn antisite defects in the planes at z = 1/4 and z = 3/4 of the kesterite structure). Compared with other characterization techniques, the 119Sn MAS NMR technique provides a signature of the Cu/Zn disorder characterized by an asymmetric broadening on the right-hand side of the lines, whose positions slightly downshift.27 Moreover, 119Sn NMR has also confirmed that stoichiometric CZTS samples always contain a fraction of Cu/Zn disorder, regardless of the cooling rate used when going through the order−disorder transition temperature, as recently described by the Vineyard’s model.30−32 Finally, an important finding of these previous NMR investigations on the Cu-poor Zn-rich CZTS and CZTSe materials is the ability of the [VCu + ZnCu] defect complexes (Atype substitution with Cu vacancies) to restrain the level of the Cu/Zn disorder.10,12 We now discuss some additional information on the Cu/Zn disorder which can be extracted from the NMR spectra of Figure 2 in relation with the asymmetric line broadening. The single line of the CO100 spectrum is slightly wider than the CE100 one, though both exhibit an asymmetric shape. This asymmetry is more visible in the CO0 spectrum. In contrast, although quite wide, the line shape of the spectrum of CE0 is not asymmetric in agreement with the presence of the A-type [VCu + ZnCu] defect complexes in this Cu-poor Zn-rich material (see Table S1). In addition to the line width itself, the more intriguing difference between the samples of the two series, for 0 < x < 1, is the asymmetric feature exhibited by the lines of the CE samples. Again, the relationship between the Cu/Zn disorder and the cationic composition can be invoked to explain the asymmetric line broadenings and downshifts since CE25, CE50, and CE75 are purely stoichiometric, whereas CO35, CO50, and CO75 are rather Cu-poor Zn-rich and hence are likely to contain [VCu + ZnCu] defect complexes. This explanation is further supported by the spectra of the quenched samples. Indeed, as mentioned in the Experimental Section, the level of Cu/Zn disorder within a sample can be increased by an additional heating step above the transition temperature followed by iced-water quenching. Figure 3 shows, for one sample of each series, the comparison of the 119Sn NMR spectra before and after quenching. It clearly appears that the spectrum of CO35 is barely affected. In contrast, the spectrum of CE50 is noticeably broadened, with a shift of the lines to lower frequencies. This exactly corresponds to the behavior of the lines after quenching which were previously observed for pure A-type CZTS materials and pure stoichiometric materials.10 A final proof of this finding is given by the 119Sn spectrum of the A-type Cu-poor Zn-rich CZTSSe material (CE25_Cu-poor) which undoubtedly contains [VCu + ZnCu] defect complexes (see comparison of that spectrum with those of CE25 and CO35 in Supporting Information, Figure S5). The NMR lines of such a ceramic sample are not asymmetric anymore and present line widths which compare adequately with those of CO35. Consequently, the ability of the [VCu + ZnCu] defect to decrease the level of Cu/Zn disorder appears as a very general process in kesterite compounds which occurs at any anionic composition. Finally, an intriguing point is the huge difference in line width and asymmetry for the stoichiometric samples at intermediate x content as compared to those at x = 0 and x

Figure 3. Comparison of 119Sn MAS (reconstructed from CPMG) NMR spectra of two samples before (CO35 and CE50) and after quenching (CO35-Q and CE50-Q).

= 1, despite the fact that they were all slow-cooled. This suggests that the formation energy of the [CuZn + ZnCu] defect complexes in stoichiometric compounds is lower for CZTSSe sulfoselenides than for pure sulfide CZTS or pure selenide CZTSe. However, since this explanation is based on a qualitative interpretation of the 119Sn NMR line widths, it should be taken cautiously until a quantitative interpretation is well developed. Quantitative Analysis of the 119Sn MAS NMR Spectra: Toward the Anionic Distribution. In the CZTS kesterite structure, each anion is tetrahedrally coordinated by 2 Cu, 1 Zn, and 1 Sn atoms, thus each cation at a given Wickoff position experiences the same distribution of anions. Hence, the information about the anionic neighbors obtained from the Sn atom is equivalent to that obtained from the Cu or Zn atom. Consequently, we focus on the quantitative interpretation of the 119Sn MAS spectra of the two CZTSSe series, that is, on the extraction of the relative intensities of the lines in order to get the anionic composition and to address the question of the anionic ordering, as well as its possible dependence on the cooling rate. As for the Raman-based method recently proposed,4 the methodology described here assumes a homogeneous anionic composition of the studied samples as derived from the observation of the (112) PXRD peak (vide supra). Determination of x = S/(S + Se). All spectra of Figure 1 have been decomposed according to the methodology described in the Experimental Section. The relative intensities of the T(n) lines, labeled I(n), are listed in Table 1 and meet 4

∑ I (n) = 1 n=0

The x = S/(S + Se) ratio, which is equal to the average number of S atoms as first neighbors of Sn atoms, is therefore determined with the following equation

x=

1 4

4

∑ nI (n) n=0

Table 2 lists all x = S/(S + Se) values obtained from NMR, from EDX, and from PXRD through the linear regression of the F


Article

The Journal of Physical Chemistry C Table 2. Comparison of x = S/(S + Se) Obtained by NMR, PXRD, and EDX for the CO and CE Series

119

sample

NMRa

PXRDb

EDXd

CO35 CO50 CO75 CE25 CE50 CE75

0.35 0.58 0.82 0.23 0.52 0.76

0.358 0.593 0.840 0.235c 0.504c 0.791c

(0.44)e (0.66)e (0.83)e 0.254(3) 0.517(9) 0.79(1)

Sn

a ±0.02. bFrom the linear regression of the a parameter on x shown in Figure 1. cThe corresponding samples have been used to build the linear regression. dDeviation on the last digit. eSemiquantitative values.

a unit cell parameter on x. For the CE series, the x values obtained from NMR are in good agreement with those accurately measured from EDX on polished sections. In contrast, for the CO series, the x values obtained from EDX measurements are much less precise and should only be considered as semiquantitative information. Indeed, they were conducted on free-standing powders of small particle size (not polished sections); furthermore, the particles contained Zn(S,Se) impurities. A better approach to get the x values of the CO series is to make use of our linear regression of the a unit cell parameter on x as described above. The obtained x values are in very good agreement with those deduced from the NMR spectra. Indeed, we found x = 0.840, 0.593, and 0.358 by PXRD versus x = 0.82, 0.58, and 0.35 by NMR for the CO75, CO50, and CO35 samples, respectively. Thus, NMR turns out to be an effective method for the determination of the x = S/(S + Se) ratio in addition to EDX, PXRD, and Raman techniques. By contrast, NMR provides accurate determination of x, and calibration curves or reference compounds are not requested. In addition, because NMR is a selective and a local probe technique, the 119Sn NMR-based method for x determination is not hindered either by the presence of impurities (e.g., Zn(S,Se)) or by the particle size. Anionic Distribution. The NMR approach also offers the possibility to address the question of the S/Se ordering. As previously stated, Figure 2 shows that the anionic distribution does not fit a simple ordering scheme. For example, the CO50 sample is not exclusively constituted by [SnS2Se2] tetrahedra since its spectrum does not consist of a single T(2) line. The signature of the anionic distribution falls within the relative intensities of the T(n) lines. We have compared the experimental I(n) intensities with those predicted for a random distribution of anions, which is governed by the binomial distribution. Considering the 4-coordination of the Sn atoms, the relative intensities of the T(n) lines for a given x are (n) Irand =

Figure 4. Comparison of the experimental relative intensities of the 119 Sn MAS NMR lines of the CO75 sample and those predicted following the binomial distribution for x = 0.82. Inset: rmsd between the two sets of relative intensities versus x (see text). 4

rmsd =

(n) 2 ) ∑ (I (n) − Irand n=0

The inset in Figure 4 gives the rmsd values for the CO75 sample. The curve shows a sharp minimum at x = 0.82, which is in full agreement with the previous calculation (Table 2) and reaches the very low rmsd value of 0.005. Finally, we have carried out the decomposition of the CO35-Q spectrum in order to investigate the effect of the thermal quench treatment on the anionic distribution. The result (see Supporting Information, Figure S7) shows unambiguously that the random distribution of anions still occurs after quenching, that is when the level of Cu/Zn disorder rises.42 The random distribution of anions occurs for the entire set of slow-cooled samples of the CE and CO series, which show yet a very different level of Cu/Zn disorder. Hence, it appears that the Cu/Zn disorder and the S/Se anion distribution are unrelated. Therefore, a random distribution of the S and Se atoms takes place at the 8g Wyckoff position within the kesterite structure of CZTSSe whatever the S/(S + Se) ratio and the level of Cu/Zn disorder are and for two different types of powders with different cation composition. Consequently, it is likely that the random distribution of anions is a very general process.

■

⎛4⎞ n 4−n ⎜ ⎟x (1 − x) ⎝n⎠

CONCLUDING REMARKS In this work, we have provided accurate equations of the Vegard’s law in the Cu2ZnSn(SxSe1−x)4 series and have emphasized the greater ability of the a unit cell parameter to determine x, due to its low sensitivity to the cationic composition and the level of Cu/Zn disorder. In order to determine the x values, we have extended the set of available tools with the 119Sn MAS NMR spectroscopy, which can be used for a large range of CZTSSe powders, with or without the presence of a significant amount of secondary phases like the common Zn(S,Se). However, the most valuable contribution of NMR to the characterization of the anionic subset is to provide a methodology to tackle the question of the S/Se ordering,

⎛4⎞ where ⎜ ⎟ = 4! /(n! (4 − n) ! ) is the binomial coefficient (see ⎝n⎠ also Supporting Information, Figure S6). Figure 4 emphasizes the striking concordance between the experimental and the predicted distribution of the I(n) intensities at x = 0.82 for the CO75 sample (similar figures for other samples in both series are given in the Supporting Information, Figure S7). The quantitative comparison between the predicted and experimental intensities can be more efficiently evaluated by the calculation of the root-mean-square deviation (rmsd), which is defined as follows G


Article


Multiwavelength Excitation Raman Scattering of Cu2ZnSn(SxSe1−x)4 (0 ≤ x ≤ 1) Polycrystalline Thin Films: Vibrational Properties of Sulfoselenide Solid Solutions. Appl. Phys. Lett. 2014, 105, 031913. (4) Dimitrievska, M.; Gurieva, G.; Xie, H.; Carrete, A.; Cabot, A.; Saucedo, E.; Pérez-Rodríguez, A.; Schorr, S.; Izquierdo-Roca, V. Raman Scattering Quantitative Analysis of the Anion Chemical Composition in Kesterite Cu2ZnSn(SxSe1−x)4 Solid Solutions. J. Alloys Compd. 2015, 628, 464−470. (5) Fairbrother, A.; Fontané, X.; Izquierdo-Roca, V.; EspindolaRodriguez, M.; López-Marino, S.; Placidi, M.; López-García, J.; PérezRodríguez, A.; Saucedo, E. Single-Step Sulfo-Selenization Method to Synthesize Cu2ZnSn(SySe1−y)4 Absorbers from Metallic Stack Precursors. ChemPhysChem 2013, 14, 1836−1843. (6) Grossberg, M.; Krustok, J.; Raudoja, J.; Timmo, K.; Altosaar, M.; Raadik, T. Photoluminescence and Raman Study of Cu2ZnSn(SexS1−x)4 Monograins for Photovoltaic Applications. Thin Solid Films 2011, 519, 7403−7406. (7) Kask, E.; Grossberg, M.; Josepson, R.; Salu, P.; Timmo, K.; Krustok, J. Defect Studies in Cu2ZnSnSe4 and Cu2ZnSn(Se0.75S0.25)4 by Admittance and Photoluminescence Spectroscopy. Mater. Sci. Semicond. Process. 2013, 16, 992−996. (8) Schorr, S. The Crystal Structure of Kesterite Type Compounds: A Neutron and X-Ray Diffraction Study. Sol. Energy Mater. Sol. Cells 2011, 95, 1482−1488. (9) Choubrac, L.; Lafond, A.; Guillot-Deudon, C.; Moëlo, Y.; Jobic, S. Structure Flexibility of the Cu2ZnSnS4 Absorber in Low-Cost Photovoltaic Cells: From the Stoichiometric to the Copper-Poor Compounds. Inorg. Chem. 2012, 51, 3346−3348. (10) Paris, M.; Choubrac, L.; Lafond, A.; Guillot-Deudon, C.; Jobic, S. Solid-State NMR and Raman Spectroscopy to Address the Local Structure of Defects and the Tricky Issue of the Cu/Zn Disorder in Cu-Poor, Zn-Rich CZTS Materials. Inorg. Chem. 2014, 53, 8646− 8653. (11) Nateprov, A.; Kravtsov, V. C.; Gurieva, G.; Schorr, S. Single Crystal X-Ray Structure Investigation of Cu2ZnSnSe4. Surf. Eng. Appl. Electrochem 2013, 49, 423−426. (12) Choubrac, L.; Lafond, A.; Paris, M.; Guillot-Deudon, C.; Jobic, S. The Stability Domain of the Selenide Kesterite Photovoltaic Materials and NMR Investigation of the Cu/Zn Disorder in Cu2ZnSnSe4 (CZTSe). Phys. Chem. Chem. Phys. 2015, 17, 15088− 15092. (13) Lafond, A.; Choubrac, L.; Guillot-Deudon, C.; Deniard, P.; Jobic, S. Crystal Structures of Photovoltaic Chalcogenides, an Intricate Puzzle to Solve: The Cases of CIGSe and CZTS Materials. Z. Anorg. Allg. Chem. 2012, 638, 2571−2577. (14) Grossberg, M.; Krustok, J.; Raudoja, J.; Raadik, T. The Role of Structural Properties on Deep Defect States in Cu2ZnSnS4 Studied by Photoluminescence Spectroscopy. Appl. Phys. Lett. 2012, 101, 102102. (15) Grossberg, M.; Krustok, J.; Raadik, T.; Kauk-Kuusik, M.; Raudoja, J. Photoluminescence Study of Disordering in the Cation Sublattice of Cu2ZnSnS4. Curr. Appl. Phys. 2014, 14, 1424−1427. (16) Grossberg, M.; Krustok, J.; Timmo, K.; Altosaar, M. Radiative Recombination in Cu2ZnSnSe4 Monograins Studied by Photoluminescence Spectroscopy. Thin Solid Films 2009, 517, 2489−2492. (17) Levcenko, S.; Tezlevan, V. E.; Arushanov, E.; Schorr, S.; Unold, T. Free-to-Bound Recombination in near Stoichiometric Cu2ZnSnS4 Single Crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 045206. (18) Han, D.; Sun, Y. Y.; Bang, J.; Zhang, Y. Y.; Sun, H.-B.; Li, X.-B.; Zhang, S. B. Deep Electron Traps and Origin of P-Type Conductivity in the Earth-Abundant Solar-Cell Material Cu2ZnSnS4. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 155206. (19) Chen, S.; Walsh, A.; Gong, X.-G.; Wei, S.-H. Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 EarthAbundant Solar Cell Absorbers. Adv. Mater. 2013, 25, 1522−1539. (20) Shang, S.; Wang, Y.; Lindwall, G.; Kelly, N. R.; Anderson, T. J.; Liu, Z.-K. Cation Disorder Regulation by Microstate Configurational Entropy in Photovoltaic Absorber Materials Cu2ZnSn(S,Se)4. J. Phys. Chem. C 2014, 118, 24884−24889.

which can influence the optoelectronic properties. By this methodology, we have for the first time demonstrated the random distribution of the two chalcogen S,Se anions in the CZTSSe materials. This agrees with the fact that all anions in kesterite structure experience the same cationic environment, at least for these cationic compositions. However, this condition might not be always fulfilled in highly Cu-depleted surfaces of nonstoichiometric CZTSSe.43,44 Besides, we have shown that the Cu/Zn disorder occurs over the whole range of CZTS−CZTSe solid solution. Additionally, an important finding of this work is the ability of the [VCu + ZnCu] defect complexes to limit the level of Cu/Zn disorder which could have an important implication on the photovoltaic quality of the CZTSSe materials. Further work is needed to understand the underlying mechanism which could explain such a disorder limitation, that is, to figure out the local structure and the clustering of the low energy formation of [VCu + ZnCu] and [CuZn + ZnCu] defect complexes, as well as their mutual interaction. In either case, such experimental or theoretical works could be carried out on pure sulfide CZTS material instead of CZTSSe sulfoselenides since we have shown that the Cu/Zn disorder and the anionic distribution are unrelated. This would reduce the complexity of such studies by avoiding the loss of spectroscopic resolution or the use of large supercells in the theoretical calculations.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08938. Cationic compositions and lattice parameters of the samples. Reconstruction of the 119Sn MAS NMR spectra and their decompositions. 119Sn isotropic chemical shift as a function of x = Se/(S + Se). Comparison of 119Sn NMR line widths. Comparison between experimental and random chalcogen distribution for all the samples (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank very much N. Fairley for having adapted casaXPS which enabled us to fit our NMR data and for his fruitful remarks. The technical assistance of N. Stéphant for EDX analyses is acknowledged. We are also thankful to R. Lefort for preliminary results and discussions.

■

REFERENCES

(1) Gershon, T.; Gokmen, T.; Gunawan, O.; Haight, R.; Guha, S.; Shin, B. Understanding the Relationship between Cu2ZnSn(S,Se)4 Material Properties and Device Performance. MRS Commun. 2014, 4, 159−170. (2) He, J.; Sun, L.; Chen, S.; Chen, Y.; Yang, P.; Chu, J. Composition Dependence of Structure and Optical Properties of Cu2ZnSn(S,Se)4 Solid Solutions: An Experimental Study. J. Alloys Compd. 2012, 511, 129−132. (3) Dimitrievska, M.; Xie, H.; Fairbrother, A.; Fontané, X.; Gurieva, G.; Saucedo, E.; Pérez-Rodríguez, A.; Schorr, S.; Izquierdo-Roca, V. H


Article


Colloid Synthesized by Instantaneous Reaction. RSC Adv. 2014, 4, 14655−14662. (37) Larsen, F. H.; Jakobsen, H. J.; Ellis, P. D.; Nielsen, N. C. QCPMG-MAS NMR of Half-Integer Quadrupolar Nuclei. J. Magn. Reson. 1998, 131, 144−147. (38) Hung, I.; Gan, Z. On the Practical Aspects of Recording Wideline QCPMG NMR Spectra. J. Magn. Reson. 2010, 204, 256−265. (39) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calvé, S.; Alonso, B.; Durand, J.-O.; Bujoli, B.; Gan, Z.; Hoatson, G. Modelling One- and Two-Dimensional Solid-State NMR Spectra. Magn. Reson. Chem. 2002, 40, 70−76. (40) Fairley, N. Copyright© 2005; Casa Software Ltd., Version 2.3.17dev6.5y, 2005. (41) Petricek, V.; Dusek, M.; Palatinus, L. Crystallographic Computing System JANA2006: General Features. Z. Kristallogr. Cryst. Mater. 2014, 229, 345−352. (42) Unfortunately, due to the strong overlapping of broad lines, the 119 Sn NMR spectra of CE50-Q could not be decomposed. (43) Bär, M.; Schubert, B.-A.; Marsen, B.; Krause, S.; Pookpanratana, S.; Unold, T.; Weinhardt, L.; Heske, C.; Schock, H.-W. Native Oxidation and Cu-Poor Surface Structure of Thin Film Cu2ZnSnS4 Solar Cell Absorbers. Appl. Phys. Lett. 2011, 99, 112103. (44) Harel, S.; Guillot-Deudon, C.; Choubrac, L.; Hamon, J.; Lafond, A. Surface Composition Deviation of Cu2ZnSnS4 Derivative Powdered Samples. Appl. Surf. Sci. 2014, 303, 107−110.

(21) Espinosa-Faller, F. J.; Conradson, D. R.; Riha, S. C.; Martucci, M. B.; Fredrick, S. J.; Vogel, S.; Prieto, A. L.; Conradson, S. D. Neutron Diffraction and X-Ray Absorption Fine Structure Evidence for Local Lattice Distortions and Aperiodic Antisite Substitution in Cu2ZnSnS4 Nanoparticles. J. Phys. Chem. C 2014, 118, 26292−26303. (22) Zawadzki, P.; Zakutayev, A.; Lany, S. Entropy-Driven Clustering in Tetrahedrally Bonded Multinary Materials. Phys. Rev. Appl. 2015, 3, 034007. (23) Redinger, A.; Sendler, J.; Djemour, R.; Weiss, T. P.; Rey, G.; Dale, P. J.; Siebentritt, S. Different Bandgaps in Cu2ZnSnSe4: A High Temperature Coevaporation Study. IEEE J. Photovolt. 2015, 5, 641− 648. (24) Fontané, X.; Izquierdo-Roca, V.; Saucedo, E.; Schorr, S.; Yukhymchuk, V. O.; Valakh, M. Y.; Pérez-Rodríguez, A.; Morante, J. R. Vibrational Properties of Stannite and Kesterite Type Compounds: Raman Scattering Analysis of Cu2(Fe,Zn)SnS4. J. Alloys Compd. 2012, 539, 190−194. (25) Valakh, M. Y.; Kolomys, O. F.; Ponomaryov, S. S.; Yukhymchuk, V. O.; Babichuk, I. S.; Izquierdo-Roca, V.; Saucedo, E.; PerezRodriguez, A.; Morante, J. R.; Schorr, S.; et al. Raman Scattering and Disorder Effect in Cu2ZnSnS4. Phys. Status Solidi RRL 2013, 7, 258− 261. (26) Caballero, R.; Garcia-Llamas, E.; Merino, J. M.; León, M.; Babichuk, I.; Dzhagan, V.; Strelchuk, V.; Valakh, M. NonStoichiometry Effect and Disorder in Cu2ZnSnS4 Thin Films Obtained by Flash Evaporation: Raman Scattering Investigation. Acta Mater. 2014, 65, 412−417. (27) Choubrac, L.; Paris, M.; Lafond, A.; Guillot-Deudon, C.; Rocquefelte, X.; Jobic, S. Multinuclear (67Zn, 119Sn and 65Cu) NMR Spectroscopy − an Ideal Technique to Probe the Cationic Ordering in Cu2ZnSnS4 Photovoltaic Materials. Phys. Chem. Chem. Phys. 2013, 15, 10722−10725. (28) Lafond, A.; Choubrac, L.; Guillot-Deudon, C.; Fertey, P.; Evain, M.; Jobic, S. X-Ray Resonant Single-Crystal Diffraction Technique, a Powerful Tool to Investigate the Kesterite Structure of the Photovoltaic Cu2ZnSnS4 Compound. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2014, 70, 390−394. (29) Mendis, B. G.; Shannon, M. D.; Goodman, M. C.; Major, J. D.; Claridge, R.; Halliday, D. P.; Durose, K. Direct Observation of Cu, Zn Cation Disorder in Cu2ZnSnS4 Solar Cell Absorber Material Using Aberration Corrected Scanning Transmission Electron Microscopy. Prog. Photovoltaics 2014, 22, 24−34. (30) Scragg, J. J. S.; Choubrac, L.; Lafond, A.; Ericson, T.; PlatzerBjörkman, C. A Low-Temperature Order-Disorder Transition in Cu2ZnSnS4 Thin Films. Appl. Phys. Lett. 2014, 104, 041911. (31) Rey, G.; Redinger, A.; Sendler, J.; Weiss, T. P.; Thevenin, M.; Guennou, M.; Adib, B. E.; Siebentritt, S. The Band Gap of Cu2ZnSnSe4: Effect of Order-Disorder. Appl. Phys. Lett. 2014, 105, 112106. (32) Krämmer, C.; Huber, C.; Zimmermann, C.; Lang, M.; Schnabel, T.; Abzieher, T.; Ahlswede, E.; Kalt, H.; Hetterich, M. Reversible Order-Disorder Related Band Gap Changes in Cu2ZnSn(S,Se)4 via Post-Annealing of Solar Cells Measured by Electroreflectance. Appl. Phys. Lett. 2014, 105, 262104. (33) Larramona, G.; Bourdais, S.; Jacob, A.; Choné, C.; Muto, T.; Cuccaro, Y.; Delatouche, B.; Moisan, C.; Péré, D.; Dennler, G. 8.6% Efficient CZTSSe Solar Cells Sprayed from Water−Ethanol CZTS Colloidal Solutions. J. Phys. Chem. Lett. 2014, 5, 3763−3767. (34) Khare, A.; Himmetoglu, B.; Cococcioni, M.; Aydil, E. S. First Principles Calculation of the Electronic Properties and Lattice Dynamics of Cu2ZnSn(S1‑xSex)2. J. Appl. Phys. 2012, 111, 123704. (35) Bernardini, G.; Bonazzi, P.; Corazza, M.; Corsini, F.; Mazzetti, G.; Poggi, L.; Tanelli, G. New Data on the Cu2FeSnS4-Cu2ZnSnS4 Pseudobinary System at 750°C and 550°C. Eur. J. Mineral. 1990, 2, 219−225. (36) Larramona, G.; Bourdais, S.; Jacob, A.; Choné, C.; Muto, T.; Cuccaro, Y.; Delatouche, B.; Moisan, C.; Péré, D.; Dennler, G. Efficient Cu2ZnSnS4 Solar Cells Spray Coated from a Hydro-Alcoholic I


119Sn MAS NMR to Assess the Cationic Disorder and the Anionic

Recommend Documents