Cu₂Zn(Sn_1−xSi_x)Se₄: Structural Characterization, Vibrational and Physical Properties of CZTSe-Derivatives

Abstract

Herein, we report the structural characterization and vibrational and physical properties of Cu₂ZnSn_1−xSi_xSe₄ solid solutions synthesized using the ceramic method. X-ray diffraction analysis and Rietveld analysis of the samples indicated that by increasing the x value from 0 to 0.8, the volume of the unit cell decreased because the ionic radius of silicon is smaller than that of tin. Simultaneously, a phase transition between stannite and wurtz-stannite was observed. The Raman peaks were analyzed by fitting the spectra to identify the vibrational modes by comparison with the experimental data from Cu₂ZnSnSe₄ and Cu₂ZnSiSe₄. The spectra of Cu₂Zn(Sn_1−xSi_x)Se₄ (x = 0.2 and 0.3) show two dominant peaks at approximately 172 and 195 cm⁻¹, which are assigned to the A1 mode of the stannite structure. The optical band gaps for Cu₂Zn(Sn_0.8Si_0.2)Se₄ and Cu₂Zn(Sn_0.2Si_0.8)Se₄ were 1.30 and 1.74 eV, respectively. These values were intermediate to those of the end members. Electrical properties of Cu₂Zn(Sn_0.8Si_0.2)Se₄ revealed p-type conductivity behavior with a carrier concentration of approximately ~+3.50 × 10⁻¹⁹ cm⁻³ and electrical mobility of 2.64 cm²/V·s.

1. Introduction

In the face of the global energy crisis, research on CZTS-type semiconductor materials has grown in the last decade. These compounds are potential absorber materials for solar cells that use accessible materials with low toxicity, a suitable band gap (1.5 eV), and a high absorption coefficient (10⁴ cm⁻¹) [1,2]. Therefore, solid solutions of CZTS-type semiconductors are highly significant for photovoltaic applications [3,4].

CZTS-derivatives have been shown to exist in various phases such as kesterite (KS), stannite (ST), and wurtzite (WZ) [5,6,7]. The coexistence of the first two phases is possible because of the similar lattice parameters, bonding, and total energy of the kesterite and stannite tetragonal crystal structures. The wurtzite phase had a hexagonal crystal cell. The kesterite family includes Cu₂ZnSnSe₄ (CZTSe), Cu₂ZnSn(S,Se)₄ (CZTSSe), and Cu₂ZnSnS₄ (CZTS), and has a band gap between 1.0–1.5 eV that can be tuned by changing the [S]/[S + Se] ratio or realizing cationic substitution in the initial compound [3,8,9]. The ideal bandgap that the materials must possess for the optimal interconversion of energy and the application of solar cells has been established at approximately 1.5 eV.

The cationic substitution in the CZTS derivatives can generate evident changes in the electrical properties of the initial compound, but not in the structural changes, particularly when the powder X-ray technique is used. In this last case, Raman spectroscopy becomes a powerful tool to distinguish different crystal structures. This is because the spectrum is sensitive to the local atomic environment, the strength of chemical bonds, masses, and charges of the constituent elements. This allows similar structures to be distinguished from one another. For example, three dominant peaks characterize the spectra of wurtz-stannite crystals, whereas stannite or kesterite crystalline structures present two dominant peaks [10,11,12]. In terms of electrical properties, cationic substitution can change the electrical behavior because it modifies the electrical mobility, and the carrier concentration and conductivity are important parameters for solar cell performance. In-CZTSe exhibited p-type electrical conductivity with a conductivity of 11.7 S·cm⁻¹ and a mobility of approximately 8.0 cm²·V⁻¹·s⁻¹ [13]; whereas, the Mg-doped CZTSe showed n-type behavior with an electrical conductivity and mobility of 24.6 S·cm⁻¹ and 120 cm²·V⁻¹·s⁻¹, respectively [14], and both materials were found to be attractive as absorber layer materials.

The Si substitution of Sn in CZTS has attracted significant interest for its potential applications because Si exhibits important semiconductor properties, and this type of compound is a p-type semiconductor. The band gap in CZTSe-derivative, where tin has been completely replaced by silicon, has demonstrated experimental values of 2.71 eV [15] for Cu₂ZnSiS₄ and 2.33 eV [16] for Cu₂ZnSiSe₄. However, when a partial substitution of tin for silicon occurs, a band gap of 1.7 eV is obtained, which is close to the optimal values for a solar cell [4]. Another substitution is the Al substitution of Sn in CZTSe to form Cu-deficient Cu_1.75Zn(Sn_1−xAl_x)Se₄ where a decrease in the lattice constants a and c occurs with an increase in Al content because aluminum is much smaller than tin, which provides an approach for enhancing carrier mobility. The enhanced mobility can reduce carrier recombination and is important for further improving solar-cell efficiency. When x = 0.4 in Al-CZTSe manifests an electrical conductivity of 57.2 S·cm⁻¹ with a hole mobility of 32.5 cm²/V·s [17]. Conversely, the Ge-substitution of Sn in CZTS can lead to a tunable band gap, in this case, from 1.4 to 2.2 eV by adjusting the Ge/Sn ratio. Increasing the amount of Ge causes a decrease in the lattice parameters, which can be explained by the replacement of Sn atoms by small Ge atoms, which can be followed by X-ray Powder Diffraction (PXRD) and Raman changes [9,18,19,20].

Herein, the structural characterization, as well as the effect on spectroscopic, optical, and conductivity properties of compounds derived from Cu₂ZnSnSe₄ (CZTSe), where tin is partially substituted by silicon to form Cu₂Zn(Sn_1−xSi_x)Se₄, were explored. The compound Cu₂ZnSnSe₄ adopts a tetragonal stannite structure, whereas Cu₂ZnSiSe₄ crystallizes in the wurtz-stannite structure. Considering that the two end members of the Cu₂Zn(Sn_1−xSi_x)Se₄ series crystallize in different structural arrangements, a transition can be expected at a given substitution value, similar to the CZTS sulfur phase. Vibrational analysis showed that the A₁ modes in the stannite structure ($I\underset{\_}{4}2m$) and A₁ and B₁ modes in wurtz-stannite ($Pmm{2}_1$) were Raman-active. The signals were compared excellently with the Raman spectra of the end members. Rietveld analyses, XRD patterns, backscattered images, and energy-dispersive X-ray spectroscopy (EDS) revealed that the samples were homogenous within the detection limits for x = 0.2 and 0.3 in Cu₂Zn(Sn_1−xSi_x)Se₄. The electrical measurements suggested p-type semiconductor behavior.

2. Materials and Methods

2.1. Synthesis

Polycrystalline Cu₂ZnSn_1−xSi_xSe₄ compounds were prepared by directly combining high-purity elemental powders (99.99%, purity, Sigma-Aldrich, St. Louis, MO, USA) in stoichiometric amounts. All manipulations were performed under an argon atmosphere. The reaction mixtures were sealed in evacuated quartz ampoules and placed in a programmable furnace. The ampoules then were gradually heated to 900 °C at 150 °C/h, maintained at this temperature for ~72 h, and slowly cooled from 900 °C to room temperature at a rate of 25 °C/min.

2.2. Powder X-ray Diffraction Measurements

Powder X-ray diffraction (PXRD) patterns were collected at room temperature using a Bruker D8 Advance diffractometer (Bruker, Billerica, MA, USA) equipped with a Cu Kα radiation source in the range of 5° < 2θ < 80°.

2.3. SEM-EDS Analysis

The chemical compositions were determined by energy-dispersive X-ray analysis using a Bruker Vega 3 Tescan system equipped with a Quantax 400 (EDS, Oxford LinK ISIS microanalyzer, Oxford Instruments, Abingdon, UK)) microanalyzer (SEM, JEOL 5400 system, Tokyo, Japan). The samples were mounted onto a double-sided carbon tape, which was adhered to an aluminum specimen holder.

2.4. Raman Scattering Measurements

Raman scattering measurements were performed using an RM1000 Renishaw micro-Raman spectrometer with CCD detectors in combination with a Leica LM/PM microscope using 514 nm wavelength excitation. The spectrometer was calibrated using a reference single-crystal Si sample (Raman peak at 520.7 cm⁻¹). Spectral data were collected at room temperature in the backscattering configuration in the spectral range of 100–500 cm⁻¹ with a laser spot on a sample of approximately 1 μm and a laser power of 2 mW.

2.5. Diffuse Reflectance Measurements

Diffuse reflectance spectra were recorded on a Sciencetech, NIR spectrophotometer. The measurements were carried out between 300 and 1800 nm using the Acquisition software SciSpec 10.0.1.0, Analysis software SciAnalysis 1.1.2.0, FGS-20-01c: Grey Fluorilon (Diffuse Reflectance 20%) as a Standard Reference, Si (180–1100 nm) detector.

2.6. Electrical Properties

Hall-effect measurements were performed using ECOPIA HMS 2000 (ECOPIA, Anyang-city, Gyeonggi-do, South Korea) equipment. The pellets for the electrical measurements were uniaxially pressed at ~2.5 tons, resulting in cylindrical pellets with a 9.00 mm diameter and thickness of 0.93 mm. The Hall coefficient, ±0.556 T, was obtained from a linear fit of the Hall resistivity.

3. Results and Discussion

3.1. X-ray Powder Diffraction and Compositional Characterization

The PXRD patterns of polycrystalline Cu₂ZnSn_1−xSi_xSe₄ (x = 0.2, 0.3, 0.5, 0.7, and 0.8) are shown in Figure 1. These phases were fully indexed in the stannite group I$\underset{\_}{4}2m$ and wurtz-stannite or Enargite group $Pmn{2}_1$ and the patterns were refined by the Rietveld method (Figure 2).

The final structural parameters are summarized in

Table 1. Rietveld refinement of cell parameters for Cu₂ZnSn_1−xSi_xSe₄.

Composition	Cell Parameters (Å)	Volume (Å3)	%Stannite I42m	%Wurtz-Stannite $\mathit{P}\mathit{m}\mathit{n}{2}_1$
Cu2ZnSn0.8Si0.2Se4	a = b = 5.6751(3) c = 11.2991(9)	363.907(2)	100.0	0
Cu2ZnSn0.7Si0.3Se4	a = b = 5.6651(4) c = 11.1837(9)	358.922(2)	100.0	0
Cu2ZnSn0.5Si0.5Se4	a = b = 5.6515(9) c = 11.1837(3)	357.201(3)	85.12	14.88
Cu2ZnSn0.3Si0.7Se4	a = 7.8378(8) b = 6.7498(7) c = 6.4682(6)	342.191(3)	45.35	54.65
Cu2ZnSn0.2Si0.8Se4	a = 7.8382(6) b = 6.7480(5) c = 6.4666(4)	342.033(1)	29.25	70.75

. Samples with a lower amount of Sn present a higher indexing percentage in the wurtz-stannite group [21], and as the Sn percentage increases, a higher indexing is achieved in stannite but still with signals belonging to the $Pmn{2}_1$ group.

As shown in Table 1, as the amount of silicon increases, the volume of the unit cell decreases because the ionic radius of silicon (0.26 Å) is smaller than that of tin (0.55 Å) [22]. In addition, it is observed that there is a phase transition in the samples with a greater amount of Sn; they have a stannite-type structure, and when the Si content increases, they pass to a wurtzite-type structure. For x = 0.5, two phases were detected (tetragonal and orthorhombic) with cell parameters that remained almost constant. Table 1 lists the cell parameters of the major components. The samples were single-phase, with x < 0.3. In all other samples, two phases coexist: the tetragonal stannite structure ($I\underset{\_}{4}2m$) and orthorhombic wurtz-stannite ($Pmm{2}_1$). The evolution of the unit cell volume for x = 0.2, 0.3, and 0.5 follows Vegard’s law, which demonstrates that the members of the Si-poor content belong to distinguishable solid solutions.

The chemical compositions of the powder samples were determined by EDS analysis of the surfaces of the sample pellets. The backscattered images and EDS analysis (chemical maps of several areas) revealed that the samples were uniform throughout the scanned region (Figure S1). The experimental results reveal that the ratio Cu/Zn/Sn/Si/Se is 2.12/1.05/0.18/0.75/3.91, for example, to Cu₂ZnSn_0.2Si_0.8Se₄ nominal composition, as shown in Figure 3.

All the nominal compositions of the samples are presented in

Table 2. Chemical composition of the samples Cu₂ZnSn_1−xSi_xSe₄.

Sample	Mass (%)					Experimental
	Cu	Zn	Sn	Si	Se
Cu2ZnSn0.8Si0.2Se4	20.2	9.92	14.9	0.75	57.9	Cu1.9Zn0.92Sn0.77Si0.17Se4.5
Cu2ZnSn0.7Si0.3Se4	21.1	10.7	13.3	1.21	58.1	Cu2.0Zn0.99Sn0.68Si0.26Se4.5
Cu2ZnSn0.5Si0.5Se4	23.3	12.6	9.84	2.24	51.9	Cu2.1Zn1.1Sn0.48Si0.46Se3.8
Cu2ZnSn0.3Si0.7Se4	24.7	11.1	2.86	7.26	54.1	Cu2.2Zn1.0Sn0.25Si0.65Se4.8
Cu2ZnSn0.2Si0.8Se4	24.3	12.36	3.81	3.84	55.7	Cu2.1Zn1.1Sn0.18Si0.75Se3.9

.

3.2. Raman Analysis and Diffuse Reflectance Measurements

We measured the Raman spectra of polycrystalline samples of the compounds Cu₂Zn(Sn_1−xSi_x)Se₄ (x = 0.2, 0.3, 0.7, and 0.8), which present a phase transition from the stannite structure ($I\underset{\_}{4}2m$) to the structure type wurtz-stannite ($Pmm{2}_1$), by replacing Sn atoms with silicon, which indicates a dependence on the chemical composition.

It has been reported that Cu₂ZnSnQ₄ (Q = S, Se) quaternary phases crystallize in the tetragonal stannite structure. In this structure, only Cu atoms form one plane; whereas, the next plane consists of Zn and Sn atoms. In contrast, the Cu₂ZnSiQ₄ (Q = S, Se) quaternary phases crystallized in an orthorhombic structure called wurtzite, where the layers formed by Zn and Si are separated by layers of Cu atoms. The Raman spectra of wurtz-stannite are characterized by three dominant signals, which contrasts with the spectra reported for similar quaternary compounds that crystallize in a stannite-like structure where two dominant signals of the A1 vibrational mode are observed [11].

Figure 4 shows the spectra measured for Cu₂Zn(Sn_1−xSi_x)Se₄ compounds with x = 0.2 and 0.3. In them, it is possible to observe 7 signals, characterized by the presence of two dominant peaks at approximately 172 and 195 cm⁻¹, which were assigned to the A1 mode of the stannite structure. The adjustment of the spectrum through Lorentzian curves has allowed the identification of the weakest vibrational modes and the frequencies of these peaks (see

Table 3. Frequency and proposed mode assignment of Raman peaks from solid solutions Cu₂Zn(Sn_1−xSi_x)Se₄ (x = 0.2, 0.3) and Cu₂ZnSnSe_4.

Mode	Cu2Zn(Sn1−xSix)Se4		Cu2ZnSnSe4
	x = 0.2	x = 0.3	Ref. [23]
E	---	120	160.9
B2	134	137	163.1
A1	171	173	180.0
	194	196	194.6
B2	238	240	233.0
	248		240.3
	385	388	353.7
	439	438	---

), which agrees with the values reported by Gürel et al. [23] and Fontané et al. [24] for quaternary compounds CZTSe (S).

For its part, Figure 5 shows the Raman spectra of the Cu₂Zn(Sn_1−xSi_x)Se₄ compounds for x = 0.7 and 0.8. By adjusting the Lorentzian curves, six signals were identified, of which three were assigned to the tetragonal crystal structure and the rest to the orthorhombic crystal structure. The spectra were dominated by a wide and intense split signal at ~200 cm⁻¹ which was attributed to three bands centered at wavenumbers of ~219, 204, and 187 cm⁻¹. Detailed analysis of this signal and considering the data obtained from the Rietveld refinement for these phases allows us to suggest that the peak at 218 cm⁻¹ (x = 0.7) and 219 cm⁻¹ (x = 0.8) of the spectra is attributed to the A1 vibration mode of the wurzt-stannite structure; simultaneously, in both spectra, a harmonic overtone of the A1 mode is observed at ~412 cm⁻¹, a broad and weak peak, approximately twice the peak at ~219 cm⁻¹, showed good agreement with the results reported by Levcenco et al. [25] and Guc et al. [11] for the quaternary compound CZSiSe.

In contrast, the bands observed at 187 and 204 cm⁻¹, which contribute to the dominant signal of both spectra (see Figure 5), are assigned to the A1 dominant mode of the stannite structure, demonstrating an excellent relationship with the peaks observed at ~172 and 195 cm⁻¹ of the compositions x = 0.2 and 0.3 for our compound (Cu₂Zn(Sn_1−xSi_x)Se₄, see Figure 4). This reveals the results of the Rietveld refinement for Cu₂Zn(Sn_0.3Si_0.7)Se₄ and Cu₂Zn(Sn_0.2Si_0.8)Se₄, which determines the mixture between the tetragonal ($I\underset{\_}{4}2m$) and orthorhombic ($Pmm{2}_1$) crystal structures.

The bands of the weakest contributions at ~132 and 248 cm⁻¹ (see Figure 5) were assigned to the A1 symmetry modes of the wurtz-stannite structure and B2 of the stannite structure, showing good agreement with the data reported by Levcenco et al. [25] and Guc et al. [11] for CZSiSe compounds and Gürel et al. [23] for CZTSe compounds.

Measured reflectance spectra for Cu₂Zn(Sn_1−xSi_x)Se₄ are performed in the range of 300–1800 nm. The Tauc relationship was used to determine the optical band gap, Eg, (Equation (1)) [26,27] where α and hν correspond to the absorption coefficient of the material and the incident photon energy, respectively. While the term A is a constant and Eg is the band gap.

$$\alpha h\nu =A{\left(h\nu -Eg\right)}^n$$

(1)

In this study, a value of n = ½ is used, which corresponds to a semiconductor with direct band transitions (VB to CB). The linear fit of (αhν)² vs. hν allowed us to determine the values Eg (Figure 6).

The Eg values for Cu₂Zn(Sn_0.8Si_0.2)Se₄ and Cu₂Zn(Sn_0.2Si_0.8)Se₄ were 1.30 and 1.74 eV, respectively. These results show a good agreement with the values determined theoretically and experimentally for the limit and intermediate phases, corresponding to ~0.90 eV for Cu₂ZnSnSe₄ [28,29], ~2.2 eV for Cu₂ZnSiSe₄ [16,30,31] and ~1.7 eV for Cu₂ZnSn_0.5Si_0.5S₄ [4]. Chen et al. reported that the I₂-II-IV-VI₄ semiconductors of the kieserite, stannite, and wurtz-stannite families present the following band structure characteristics: (i) semiconductors usually in Γ point present direct band gaps, (ii) The top valence band considers antibonding p-d hybridization between the group-VI anion and group-I cation as the main constituent; (iii) the bottom conduction band considers the antibonding s-s and s-p hybridization between the group-IV cation and group-VI anion as the main constituent, except for those containing Si, and group-I and -II cations also have significant contributions to the bottom conduction band as well as Si and group-VI anions [32,33].

The results of our study show that partially replacing tin atoms with silicon (from x = 0.2 to x = 0.8) generates a shift towards higher energy; this implies an increase in the bandgap gap as the size of the IVA group cation decreases (Sn⁴⁺:0.55 Å to Si⁴⁺:0.26 Å). This would favor s-s and s-p level repulsion between the elements of groups IV and VI, causing the minimum of the antibonding conduction band to decrease from Cu₂ZnSn_0.2Si_0.8S₄ to Cu₂ZnSn_0.8Si_0.2S₄.

3.3. Electrical Properties

The Van-der-Pauw technique is used to measure the electrical properties at room temperature (R.T.). The density (ρ) was calculated using the mass and dimensions of the sample, which corresponded to an average pellet density of ~89% (experimental density/crystallographic density). Electrical conductivity (σ), mobility (m), and carrier concentration (n) measurements of Cu₂Zn(Sn_0.8Si_0.2)Se₄ revealed p-type conductivity, which is consistent with previous reports on Cu-deficient Cu₂ZnSnSe₄ (with different Zn/Sn ratios) and Mg-, Ag-, and In-doped Cu₂ZnSnSe₄ phases [14,34,35,36,37]. The Hall coefficients of Cu₂Zn(Sn_0.8Si_0.2)Se₄ were positive at R.T with a carrier concentration of ~+3.50 × 10⁻¹⁹ cm⁻³ (in this work). The electrical hole mobility of Cu₂Zn(Sn_0.8Si_0.2)Se₄ (2.64 cm²/V·s) was smaller than the value reported for Mg-doped Cu₂ZnSnSe₄. However, this value is comparable to the mobilities observed for the Cu-deficient Cu₂ZnSnSe₄. Electrical mobility is an important property for evaluating electrical behavior. The electrical conductivity σ in the Ar atmosphere of Cu₂Zn(Sn_0.8Si_0.2)Se₄ was ~13.2 S·cm⁻¹ at 298 K. This value is approximately two times higher than that of Cu₂ZnSnSe₄, as reported by Kuo D.-H. et al. (~7.20 S·cm⁻¹) at 300 K. These values are also comparable to those of Cu₂Zn(Sn_0.3In_0.6)Se₄ systems (~10.4 S·cm⁻¹ at R.T.) [14]. The carrier concentration n of Cu₂Zn(Sn_0.8Si_0.2)Se₄ is approximately ten times higher than that of Mg-doped Cu₂ZnSnSe (~+10⁻¹⁸) and is comparable to the carrier concentration n of Cu₂ZnSnSe₄ with different Sn/Zn chemical ratios at 300 K (~+8.29 × 10⁻¹⁸ to +2.02 × 10⁻¹⁹ cm⁻³) [13,34]. This indicates that σ and n are sensitive to the Sn/Si ratio. The above-mentioned electrical properties indicate semiconductor behavior. Electrical measurements by complex impedance and Dc methods on Cu₂Zn(Sn_1−xSi_x)S₄ (x = 0.1 and 0.4) showed semiconductor behavior in temperature range of 160–300 K [37]. Theoretical calculations suggest that among the various defects formed in CZTX (X = S, Se), copper vacancies (V_Cu and V_Zn) and antisite defects (CuZn, CuSn, and ZnSn) are the dominant cause of intrinsic p-type [33,38,39].

4. Conclusions

Solid Cu₂ZnSn_1−xSi_xSe₄ (0.2 ≤ x ≤ 0.8) phases were obtained by conventional solid-state synthesis, and their chemical compositions were confirmed by the SEM-EDS technique. X-ray powder diffraction and Rietveld refinement showed a phase transition between wurtz-stannite and stannite as the amount of silicon in the substitution increased. The Raman spectrum confirmed the phase transition owing to changes in the dominant signals that exhibited different crystalline structures. The samples were single-phase, with x < 0.3. In all other samples, two phases coexisted: a tetragonal stannite structure ($I\underset{\_}{4}2m$) and an orthorhombic wurtzite ($Pmm{2}_1$). Electrical measurements indicate a p-type semiconductor behavior, and the band gap values increase with increasing amounts of silicon. These results generate great interest in these materials due to their potential applications as solar absorbers or semiconductor materials.