Crystallochemical Characterizations, Raman Spectroscopy and Studies Nuclear Magnetic Resonance (NMR) of Cu₂Zn(Sn, Si)S₄ Compounds for Photovoltaic Applications

Mohamed Hamdi¹, Messaoud Benamira²

¹Northern Border University, Arar, Turaif, Saudi Arabia.
²Laboratory of Interaction Materials and Environment (LIME), University of Mohamed Seddik Ben Yahia-Jijel, Jijel, Algeria.

DOI: 10.4236/msce.2022.101002                

Abstract

In this study, Si-doped Cu₂ZnSnS₄ compounds (Cu₂ZnSn_1-xSi_xS₄, 0 ≤ x ≤ 1) were prepared by solid state reaction method for use of materials for photovoltaic cells. The structural and spectroscopic properties of the as-prepared compounds were studied by X-ray diffraction (XRD), ¹¹⁹Sn, ²⁹Si and ⁶⁵Cu Magic Angle Spinning nuclear magnetic resonance (MAS NMR) and Raman spectroscopy. The Si-substitution in the Sn-site induces three different types of XRD patterns which depend largely on the Si content in the compound. For 0 ≤ x ≤ 0.5, XRD analysis reveals the presence of a pure tetragonal phase of solid solution with I-42m as a space group. Mixed tetragonal and orthorhombic phases were observed for 0.5 < x < 0.8, followed by a pure orthorhombic structure with a space group Pmn2₁ at high content of Si (x ≥ 0.8). ¹¹⁹Sn MAS NMR spectra show the presence of Sn/Si disorder as a function of the Si content. The ⁶⁵Cu MAS NMR spectra of the quadratic solid solution confirm the presence of the two copper sites (Cu-2a and Cu-2c) at 780 ppm while in the case of the orthorhombic solid solution samples, a very broad band is observed. The optical properties were investigated of all compounds by UV-Vis diffuse reflectance and the obtained optical band gap values (1.31 to 2.43 eV) confirm a semiconductor character.

Keywords

Photovoltaic Cells, Cu₂ZnSnS₄, Nuclear Magnetic Resonance, Raman Spectroscopy

1. Introduction

Nowadays, the use of clean energy resources such as solar energy becomes one of the relevant challenges of researchers in order to limit the damage caused by the use of fossil energy. The use of solar cells as a source of electricity generation has been increasing in the last decade due to the decrease in the cost of the fabrication materials and the increase in efficiency. The manufacture of solar cells based on non-toxic and abundant materials is a major challenge for the development of renewable energies in general, and photovoltaic applications in particular. One of the most promising ways is the use of thin layers of semiconductor materials based on the compound Cu₂ZnSnS₄ (CZTS) as a light absorber instead of Cu(In, Ga)(S, Se)₂ alloys which contain chemical elements with a limited availability [1] - [7]. Its use is associated with the presence of local disorder which can be beneficial for high performance. CZTS semiconductor compound greater than 30% [8] and an energy band gap (E_g) value which varies between 1.4 and 1.7 eV depend on the method used to determine E_g value from the UV-Vis diffuse reflectance spectra and also the preparation condition of this material [9] [10] [11]. Malerba et al. prepared CZTS thin films using sulfurization method with different compositions of the initial precursors of ZnS, Cu and Sn. The obtained band gap values, between 1.48 and 1.63 eV, are influenced by the increase of the [Sn]/[Cu] ratio responsible for the formation of additional defects [12] - [17].

Nitsche et al. report the first study about the synthesis of CZTS compound using iodine vapor transport method [18]. Different attempt was conducted to synthesize CZTS thin layers using several methods such as thermal evaporation, atomic layer deposition, sol gel, radio frequency magnetron sputtering and spray method [19] [20] [21] [22] [23].

In this study, we investigate the effect of the partial chemical substitution of Sn by Si in Cu₂ZnSnS₄ semiconductor on the structural, spectroscopic and optical properties. Our main objective is to expand the family of these quaternary chalcogenides (CZTS) by gradually replacing tin with silicon (Cu₂ZnSn_1-xSi_xS₄, 0 ≤ x ≤ 1). The substitution of Sn by Si leads to a decrease in lattice volume due to the difference in atomic radii between Sn and Si. Moreover, the two limit compounds of this series have two different crystal structures, Cu₂ZnSnS₄ with tetragonal phase and Cu₂ZnSiS₄ with orthorhombic phase.

2. Experimental Section

Synthesis and chemical analyses: the synthesis route was adapted from the one proposed by L. Choubrac et al. [24]. Namely, the compounds were prepared at 900˚C by solid state reactions from elemental Cu, Zn, Sn, Si, S precursors weighted in the appropriate ratios respectively 2:1:1-x:x:4 in dry box for 1 week. The reagents were weighted in stoichiometric ratio and put together into a fused silica tube. The tube was flame sealed under vacuum, heated to 900˚C (with intermediate temperature steps at 250˚C and 450˚C for two hours) and held at that temperature for 1 week. The as-obtained samples were then ground in an agate mortar and annealed at the synthesis temperature for 96 h to improve the homogeneity and the crystallization of the powders, then subsequently quenched in an iced bath [25]. At this stage, X-ray powder diffraction analyses did not detect any sub-products.

The elemental composition of the studied samples is a critical point for this study, and many efforts were made to ensure both accuracy and precision in these measurements. Practically, powders were embedded in epoxy and polished to get a perfect flat surface of the grains. Additionally, the systematic use of standards (Cu, Zn, Sn, CaSiO₃, FeS₂) ensures that the obtained chemical compositions are both precise and accurate. Because the silicon weight percentage remains low in most of the studied compounds, the atomic silicon content is not very accurate.

3. Results and Discussion

3.1. Structural Analysis

The XRD patterns of Cu₂Zn(Sn_1-xSi_x)S₄ (0 ≤ x ≤ 1) were obtained after calcination. The Si-substitution of Cu₂ZnSnS₄ in the Sn-site induces three different types of XRD patterns which depend largely on the content of Si in the compound [25]. For 0 ≤ x ≤ 0.5, XRD analysis reveals the presence of pure phase of solid solution in which the peak positions shift for more positive values of θ while Si content increases in the kesterite structure. This is related with the difference in atomic radii between Sn and Si (r_Sn (1.62 Å) > r_Si (1.32 Å)) [25]. All XRD diffraction maxima can be indexed in a tetragonal phase with I-42m space group. The lattice parameters were obtained using Le Bail refinement in Fullprof software, the results are regrouped in Table 1. The associated lattice parameters decrease with the increase of Si content which can be attributed to the low atomic radii of Si compared to Sn. The study of the evolution of the lattice parameters of the tetragonal structure as a function of the Si content indicates a linear fit and follows Vegard’s law [26] (Figure 1). It should be noted that the variation in lattice parameters is less than 2%.

In the case of 0.5 < x < 0.8, XRD patterns reveal the presence of diffraction maxima attributed to tetragonal phase and also the presence of new diffraction maxima attributed to the orthorhombic structure with space group Pmn2₁. Moreover, the lattice parameters are practically unchanged with the Si content in this region which is a typical signature of a two-phase domain between two solid solutions transition. However, while x is ≥ 0.8, All XRD diffraction maxima are indexed with the pure and single orthorhombic phase. The lattice parameters of Cu₂ZnSiS₄ (x = 1.0) obtained in this study are in accordance with those reported in the literature [27]. This study confirms that Cu₂Zn(Sn_1-xSi_x)S₄ compounds present pure tetragonal phase for low content of Si (x ≤ 0.5), the increase of Si content is associated with the formation of an additional orthorhombic phase before the transition phase observed for x = 0.8 and the formation of single and pure orthorhombic phase.

Figure 1. Variation of lattice parameters (a, c and volume v) as a function of the silicon content in the solid solution (0 ≤ x ≤ 0.5). The Si content is deduced from the measured volume and the equation of Vegard’s law.

Table 1. Tetragonal unit cell parameters evolution versus the targeted Si-content, x = Si/ (Sn/Si), in the Cu2Zn(Sn, Si)S₄ series from the PXRD investigation.

3.2. Raman Analysis of Cu₂Zn(Sn_1-xSi_x)S₄ Compounds

In order to correlate properties with structure, Raman analysis was conducted. According to the literature, stoichiometric CZTS compounds with kesterite structure have 27 optically active modes [28] [29]. Optimization of measurement conditions by choosing the appropriate equipment, power, laser, acquisition time, powder or crystal sample were the first step of this study. Preliminary measurements made on the visible dispersive Raman of the “Jobin-Yvon T64000” type, containing a CCD detector cooled by liquid nitrogen, require long counting times that are not always compatible with the need to carry out a large number of measurements. Visible Raman assembly dedicated to mapping “Renishaw In Via reflex” was used and the detection is optimized for short counting times (a few seconds). All measurements were done with green excitation (514.5 nm laser), 0.15 mW power and an acquisition time of 30 seconds.

Recently, Caballero et al. [30] suggested that the structural disorder, due to deviations from the stoichiometry or to the copper/zinc disorder which increases the width of the peaks, is the origin of a characteristic peak around 332 cm⁻¹ and the peak at 338 cm⁻¹ is the signature of the ordered kesterite structure.

Figure 2 shows the Raman spectra of the sample Si00 with rapid cooling (quenching, Si00-21) and slow cooling (Si00-31) with a rate of 20˚C/h. The width of the majority of peaks is increased and in particular, those observed at 132, 287, 336, 355, 410 and 671 cm⁻¹. The peaks shift to the left with the appearance of a new peak at around 208 cm⁻¹. The sample with rapid cooling presents a disordered kesterite structure (I-42m) and the sample with slow cooling has a weakly disordered structure. However, the peak observed at 336 cm⁻¹ remains the main peak.

The addition of Si to the structure shifts the peaks towards high values of the wavenumber (Figure 3); for example, the most intense peak evolves from 337 (Si00) to 343 cm⁻¹ for Si50 sample.

In order to quantitatively monitor the effect of silicon insertion on the position on the positions and intensity of the peaks, the deconvolution of Raman spectra was carried out using LabSpec5 software. Figure 4(a) & Figure 4(b) shows the deconvolution of Raman spectra of Si00-21 and Si50-A1 samples. A displacement of the majority peaks of Si00-21 sample from 255, 286, 337, 352, 373, 410, 610 et 669 cm⁻¹ to 271, 294, 343, 358, 370, 415, 623 et 687 cm⁻¹ when 50% Sn is replaced by 50%Si (Si50-A1). However, some peaks remain in the same position (147 and 314 cm⁻¹) and the addition of silicon has no effect on the peak positions. Moreover, some peaks (471 and 581 cm⁻¹) exist only in sample without Sn. The evolution of the position of the intense peak as a function of Si content for the tetragonal solid solution (0 ≤ x ≤ 0.5) is shown in Figure 5, and confirms the shift of this peak to high value with the increase of Si content.

Figure 2. Raman spectra of compounds Si00-21 (rapid cooling) and Si00-31 (slow cooling).

Figure 3. Raman spectra of compounds at different silicon compositions.

(a)(b)

Figure 4. (a). Raman spectrum deconvolution recorded with a wavelength of 514 nm of Si00-21 sample. (b). Raman spectrum deconvolution recorded with a wavelength of 514 nm of Si50-A1 sample.

Figure 5. Evolution of the position of the intense peak as a function of x.

However, the deconvolution of Raman spectra of orthorhombic solid solution (x ≥ 0.8) presented in Figure 6 reveals that the intense peak of the sample with 80% of Si appears at 350 cm⁻¹ while that of x = 1 appears at 393 cm⁻¹. As observed in tetragonal solid solution, new peaks appear only in the sample without Sn content (194, 243 and 393 cm⁻¹) and also in the sample with Sn content, 831, 429, 528 and 700 cm⁻¹ (Figure 6(c)). All peaks obtained in Raman spectrum of the sample without Sn (Si100-21) are similar and in accordance to those found in the literature [31].

(a)(b)(c)

Figure 6. (a). Raman spectrum deconvolution of Si80-11 sample. (b). Raman spectrum deconvolution of Si100-21 sample. (c). Superposition of the two Raman spectra (Si80-11 and Si100-21) of the orthorhombic solid solution.

3.3. Nuclear Magnetic Resonance (NMR)

Among the various spectroscopic methods using to access to the microscopic information about the local structures, NMR of solid is regarded as one of the most important. Magic Angle Spinning (MAS) technique is commonly used to explore polycrystalline powders. In this study, the close range chemical environments of our synthesized compounds were studied by using the solid state ¹¹⁹Sn, ⁶⁵Cu and ²⁹Si MAS NMR spectroscopy. The chemical shift of the Sn isotope (¹¹⁹Sn) with the spin of 1/2 is very sensitive to its chemical surrounding; the atoms located beyond the first coordination sphere have an observable impact on the chemical shift interaction.

In the ordered kesterite structure of CZTS compound, the all Sn atoms occupy the same crystallographic site and have strictly the same environment with 4 S atoms having as environment: Cu (2a), Sn (2b), Cu (2c), Zn (2d). They must all produce an NMR signal with the same chemical shift, which must consist of a fine single and symmetrical signal located at −122 ppm in agreement with the reported values for kesterite structure [32]. ¹¹⁹Sn MAS NMR spectra for different content of Si are displayed in Figure 7(a).

The insertion of Si in the structure reveals certain criteria. Firstly, the chemical shift increases gradually with the increase of Si content. Secondly, the lines widen remarkably from x ≥ 0.1 which can be attributed to the insertion effect of Si in the structure. ¹¹⁹Sn MAS NMR spectra show the presence of Sn/Si disorder as a function of the Si content in the structure, this disorder is originate from the difference in atomic radii between Sn and Si occupying the same crystallographic site. The desorder is more pronoucend in the case of samples with orthorombic structure.

It should be noted that the quadrupole interaction is sensitive to the site geometry which is described by two parameters, the quadrupolar constant (C_Q) and the asymmetry (η_Q), where Q is the electric quadrupole moment. In the CZTS kesterite structure, all copper and zinc atoms occupy tetrahedral sites formed by four sulfur atoms. The more these tetrahedral are distorted, the higher the value of C_Q. However, the copper atoms are distributed in equal quantity on two distinct sites (Cu_2a et Cu_2c), which explains the presence of two lines on the ⁶⁵Cu MAS NMR spectrum (Figure 7(b)).

According to a Choubrac et al. [32] [33] [34], the Cu/Zn disorder in Cu₂ZnSnS₄ compound is most important with sample rapidly cooled than that of sample slowly cooled sample. The ⁶⁵Cu MAS NMR spectrum with wide and distorted band is observed (Figure 7(b)). Nevertheless, the substitution of Sn by Si brings another Si/Sn structural disorder.

The ⁶⁵Cu MAS NMR spectra of the quadratic solid solution (0 ≤ x ≤ 0.5) confirm the presence of the two copper sites (Cu-2a and Cu-2c) at 780 ppm for the samples with x < 0.3 [32]. For x ≥ 0.3, the insertion of Si in the structure modifies the Cu environment with wider band which explains the presence of a distribution of chemical shifts and gradient of electric fields. It is concluded that the Sn/Si disorder is much greater than that provided by Cu/Zn.

While in the case of the orthorhombic solid solution samples (0.8 ≤ x ≤ 1), we observed the presence of a very broad band compared to that observed for quadratic solid solution. Cu₂ZnSiS₄ sample (x = 1) present two peaks which may be to the presence of impurities and/or to the environment around the copper atoms (Figure 7(c)).

(a)(b)(c)

Figure 7. (a). ¹¹⁹Sn NMR spectra at different compositions. (b). ⁶⁵Cu NMR spectra at different compositions of the tetragonal solid solution. (c). ⁶⁵Cu NMR spectra at different compositions of the orthorhombic solid solution.

The ²⁹Si MAS NMR spectrum which has 1/2 as spin is shown in Figure 8. The Cu₂ZnSiS₄ sample indicates the presence of one band at −4.4 ppm. For x = 0.8 (Si80-11), an enlargement of the band is observed which is due probably to the new environment with the presence of the structural disorder Sn/Si. However, for quadratic solid solution two peaks are observed and the intensity of each peak is not regular. For low content of Si (Si25-31), the left peak is more intense while for the other samples with more Si content (Si35-11 and Si50-A1) the right one is more intense. Thus can be attributed to the insertion effect of Si in the structure and to the two very close silicon sites (Si-S4).

3.4. Optical Properties

The optical gap values (E_g) of the as-prepared Cu₂ZnSn_1-xSi_xS₄ (0 ≤ x ≤ 1) are determined from the measurement of the UV-Vis diffuse reflectance spectra. The band gap can be determined by extrapolation to the energy axis of the linear plots of (1 − R)²/2R as a function of the photon energy (E (eV)).

Figure 9(a) illustrates the UV-Vis diffused reflectance spectra obtained for the quadratic solid solution. The optical band gap energies (E_g) for the as-prepared Cu₂ZnSn_1-xSi_xS₄ compounds confirm a semiconductors character, and the band gap values increase from 1.38 to 2.08 eV with the increase of Si content [25] [35] [36]. For 0.5 < x < 0.8 (Figure 9(b)), UV-Vis diffused reflectance spectra reveal the presence of two different linear regions in according with the presence of two phases. The optical band gap values are 1.75 and 1.78 eV for Si60-11 and Si75-11 samples, respectively. These values are closed to those obtained for quadratic solid solution. The E_g values of the second region are 2.35 and 2.43 eV in accordance with E_g values reported in the literature for orthorhombic solid solution [37].

For x ≥ 0.8, the samples present orthorhombic (wurtzite-derived) structure and the optical band gap values are 2.08 and 2.6 eV for x = 0.8 and x = 1.0, respectively (Figure 9(c)). The Eg value of Cu₂ZnSiS₄ (x = 1.0) is closed to that found in the literature [38] [39]. In summary, the band gap values increase with the substitution of tin by silicon and rise from 1.31 eV for Cu₂ZnSnS₄ (x = 0) to 2.6 eV for Cu₂ZnSiS₄ (x = 1).

In addition to the composition of the sample which influences the band gap, the grain size can be an interesting parameter to monitor the band gap value. Si50-31 sample was used to study the effect of the different milling modes on the Eg value. Figure 10 shows the obtained UV-Vis diffused reflectance spectra of Cu₂ZnSn_0.5Si_0.5S₄ using hand grinding in a mortar and ball milling in a planetary crusher for 15 and 180 min.

The milling parameters have a considerable impact on the final value of E_g, Hand grinding and ball milling for 15 min allows to obtain very close value of E_g, 1.74 and 1.72 eV, respectively (Figure 11). However, the planetary milling for 180 min decrease the band gap value to 1.48 eV which can be attributed to the fine grain microstructure obtained by ball milling process and confirmed by the XRD patterns. The peaks of the XRD pattern of the powder ball milled for 180 min are wide compared to that obtained for 15 min.

Figure 8. ²⁹Si NMR spectra at different rapidly cooled silicon compositions.

(a)(b)(c)

Figure 9. (a). Diffuse reflectance spectra of compounds of the quadratic solid solution. (b). The diffuse reflectance spectra of the compounds Si60-11 and Si75-11 or the presence of two gaps. (c). Diffuse reflectance spectra of compounds in the orthorhombic solid solution.

Figure 10. Diffuse reflectance spectra as a function of the wavelength of the Si50-31 sample (grinding effect).

Figure 11. K/S spectra as a function of the energy of the Si50-31 sample (grinding effect).

4. Conclusion

In summary, Cu₂ZnSn_1-xSi_xS₄ (0 ≤ x ≤ 1) compounds were prepared using the solid state reaction method. For low content of Si (x ≤ 0.5), the material conserved its pure tetragonal phase, and the associated lattice parameters decrease with the increase of Si content which can be attributed to the low atomic radii of Si compared to Sn. The evolution of the lattice parameters as a function of the Si content indicates a linear fit and follows Vegard’s law. At high content of Si (0.5 < x < 0.8), mixed tetragonal and orthorhombic phases were observed; followed by a pure orthorhombic phase for x ≥ 0.8. Solid state MAS NMR spectroscopy was used to investigate the cationic disorder. ¹¹⁹Sn MAS NMR spectra show the presence of Sn/Si high disorder as a function of the Si content. The ²⁹Si MAS NMR spectrum of Cu₂ZnSiS₄ indicates the presence of one band at −4.4 ppm which becomes wider for x = 0.8 due to the structural disorder Sn/Si. However, for quadratic solid solution with low content of Si, two peaks are observed. The optical band gap values increase from 1.31 eV for Cu₂ZnSnS₄ (x = 0) to 2.43 eV for Cu₂ZnSiS₄ (x = 1) confirming a semiconductor character of the as-prepared compounds.

Acknowledgements

The author gratefully acknowledges the approval and the support of this research study by the grant no. SAT-2018-3-9-F-7691 from the Deanship of Scientific Research at Northern Border University, Arar, K.S.A.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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