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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. vol.61 no.4 Concepción dez. 2016 





a Departamento de Química,Facultad de Ciencias, Universidad de Chile, Santiago, Chile
Facultad de Ciencias Básicas, Universidad Católica del Maule, Talca, Chile
* e-mail:


Polycrystalline Cu2ZnSnS4-xSex (X=1, 2, 3) compounds were synthesized by conventional solid-state reactions. The samples were characterized by powder X-ray diffraction (XRD), energy-dispersive X-ray analysis (SEM-EDS), Raman spectroscopy, diffuse reflectance UV-vis and Photoluminescence. All ofphases crystallize in the tetragonal kesterite-type structure. The powder X-ray diffraction (XRD) pattems were indexed in the space group . No secondary phases were detected in XRD pattems. The results from diffuse reflectance show band gap between 1.26 - 1.17 eV, when S is gradually replaced by Se. The PL spectrum of Cu2ZnSnS4-xSex phases shows nearly symmetrical band, which shifted linearly to the lower energy with increasing Se content. The selenized (CZTSSe) phases are promising candidates to be used as absorbing material in solar cells



In recent decades, the study of new photovoltaic (PV) materials for the development of more efficient and lower-cost solar cells has become the subject of an impressive field of research in physics, and solid state chemistry [1].

The quaternary semiconductor Copper-Zinc-Tin-Sulfur Cu2ZnSnS4 (CZTS) is the focus of new materials with great potential for its application as absorbent materials for solar cells [2-6]. The CZTS phase has interesting physical properties suitable for photovoltaic applications, namely, optimum band gap of ~1. 5 eV and its absorption coefficient of ~104 cm-1 [7-11]. These values are similar to those of Cu(In,Ga)(S,Se)2 (CIGS), that is one of them more successful thin-film PV materials, of commercial use, with power conversion efficiencies (PCEs) of nearly 20% [12-14]. However, unlike the CIGS, the CZTS phase is formed by elements that are abundant, cheap and of low toxicity, in addition, the CZTS-PV devices shown PCEs of up to 6.7%. [13-15].

In the last years, has been informed the preparation of CZTS nanoinks, for the preparation of high quality thin-films of CZTS and CZTSSe, suitable for use in solar cells [16]. The devices fabricate with thin-films nanoparticles of CZTS have shown the power conversion efficiency (PCE) of 8.4% [17-18]. Also, it has been reported that Wurtzite nanoparticle films of CZTS presents a phase transformation to kesterite phase, when sintered in selenium vapor at 500°C. The resulting thin-film of the selenized CuZnSn(S,Se)4 (CZTSSe) presents a significant improvement in performance as dye solar cells [19]. The device built with selenized kesterite CZTSSe nanoparticle inks presents a PCE of 9.15% and this value has been improved to 9.4% by partial doping of Sn with Ge [20-22].

We have previously reported on the synthesis, structural characterization and magnetic properties of; Cu2Mn1-xCoxSnS4 with kesterite structure, Cu2MnxFe1-xSnS4 and Cu2Fe1-xCoxSnS4 solid solutions [23-25]. The aim of this work is to study the effect on the physics properties of the Selenium content in the phase CZTS. In particular we report here the synthesis, characterizations and the optical properties of the Cu2ZnSnS3Se; Cu2ZnSnS2Se2 and Cu2ZnSnSSe3.



The synthesis of Polycrystalline Cu2ZnSnS4-xSex (with x = 1, 2, 3) compounds were performed on solid state by the ceramic method. Cu2ZnSnS3Se; Cu2ZnSnS2Se2 and Cu2ZnSnSSe3 were prepared by reaction of the high-purity element powders (99.99%, Aldrich) in stoichiometric amounts. All manipulations were carried out under an argon atmosphere. The reaction mixtures were sealed in evacuated quartz ampoules, placed in a programmable furnace, and heated at 850 °C for 72 h, and then cooled by quenching in liquid nitrogen. The reaction products appeared to be air' and moisture'stable over several weeks.

SEM-EDS analysis

The chemical compositions of the powder samples were determined by scanning electron microscopy (SEM) with the aid of energy-dispersive x-ray analysis (SEM-EDS) using a JEOL 400 system equipped with an Oxford Link ISIS microanalyzer. The working distance was 35 mm and the accelerating voltage was 22.5 kV. The samples were mounted on double-sided carbon tape, which adhered to an aluminum specimen holder. The EDS spectra were collected for 60s.

Powder X-ray diffraction measurements

Powder xRD patterns were collected at room temperature using a Bruker D8 advanced powder diffractometer, with CuKa radiation (λ = 1.541Å) in the range of 5° < 20 < 80° at 0.01°/s.

Raman spectroscopy

The Raman spectra of powder samples were recorded in the frequency range 100-1800 cm-1 using a micro-Raman Renishaw system 1000 equipped with a microscope Leica-DMLM. The spectra data were collected at room temperature with laser line of 633 nm and laser power of ~1 mW.

Diffuse reflectance UV-vis measurements

The diffuse reflectance UV-vis spectra were recorded using a Perkin Elmer UV-visible spectrophotometer. BaSO4 powder was used as reference at all energies (100% reflectance). Reflectance measurements were converted to absorption spectra using the Kubelka-Munk function.

Photoluminescence measurements (PL)

PL spectra were recorded by using a LabRam HR800-UV Horiba Jobin Yvon spectrometer with a laser excitation source solid state laser (line 532 nm and 1064 nm) system with CCD detector (InGaAs diode). In all cases, spectra were measured in backscattering configuration; excitation and light collection was made through an Olympus metallographic microscope, with a laser spot on the sample of ~1 μm.


The chemical compositions of the powder samples were determined using EDS analysis on polished surfaces of the pelletized samples. The backscattered image and EDS analysis (chemical maps of several areas) revealed that the samples were uniform throughout the scanned region (Fig 1). It was found that the average concentrations of Cu, Zn, Sn, S and Se elements were close to the nominal compositions (Table 1).


Fig. 1 (SEM) micrograph: Backscattering electron image (left) and the
corresponding EDS spectral analysis (right) ofthe solid solutions Cu2ZnSnS4-xSex


Table 1. Chemical composition analysis (% mass) of Cu2ZnSnS4-xSex and end-members


It is known that CZTS crystallizes in a tetragonal kesterite stmcture (space group ) > which consisting a ccp array of sulfur anions, with metal cations occupying one half of the tetrahedral interstitial sites within the S sublattice. The crystal structure of Cu2ZnSnS3Se, Cu2ZnSnS2Se2 and Cu2ZnSnSSe3 can be represented by the cation-centered tetrahedral, MQ4 (with M = Cu, Zn and Sn; Q = S, Se), arranged in such a way that all polyhedra are oriented in the same direction and connected to each other at the corners, as illustrated in Figure 2. Moreover, S and Se atoms present a disorder in the occupation (same crystallographic site) at the corner of the MQ4 tetrahedral. The experimental XRD patterns of the polycrystalline Cu2ZnSnS4-xSex phases (Fig. 3) were fully indexed in I-4 space group. All phases are isostructural and adopted the kesterite-type structure. The final structural parameters are summarized in Table 2. As expected, the cell parameters obey Vegard-slaw (Fig. 4), in according with the gradual increase of the volume of the cell lattice as sulfur was replaced by selenium, in line with the increased of the anions radii. No secondary phases or impurity peaks were detected in xRD pattern of Cu2ZnSnS4-xSex.


Fig. 2. Unit cell of the Cu2ZnSnS4 structure viewed along [010]


Fig. 3. Experimental X-ray powder diffraction patterns of Cu2ZnSnS4-xSex


Table 2 Cell parameters data for Cu2ZnSnS4-xSex and end-members.


Fig. 4. Vegard’slaw of Cu2ZnSnS4-xSex phases


The Raman spectra of powder samples at room temperature are shown in figure 5. The spectra of CZTS is characterized by one strong line centered around 335 cm-1, and a weak at 285 cm-1, which can be assigned to the A1 vibration mode, aside from a weak contribution at 370 cm-1 assigned to the B2 mode [26, 27]. The vibrations of the A1 mode can be viewed as a "breathing" mode of MS4 (M = Cu, Zn and Sn) tetrahedra. This vibrational mode can be understood as the expansion or contraction of the M-S bonds, simultaneously triggered by sulfur atoms placed at the vertices of the tetrahedra. The Raman spectrum of CZTSe is characterized by two main peaks, which also can be assigned to the A1 vibration mode, at 175 cm-1 and 145 cm-1, while the sulfoselenide, which contains both S and Se at the anion sites in the crystal lattice, presents broadening peaks corresponding to A1 modes from both end-members.


Fig. 5. Room temperature Raman spectrum of polycrystalline
2ZnSn(S,Se)4 phases.


Fig. 6. The optical absorption spectrum of Cu2ZnSnS4-xSex phases


Fig. 7. Photoluminescence PL spectra of of polycrystalline
Cu2ZnSn(S,Se)4 phases.


The optical properties of the Cu2ZnSn(S,Se)4 phases were measured by UV-vis. The results from spectra for Cu2ZnSnSSe3, Cu2ZnSnS2Se2 and Cu2ZnSnS3Se phases show band gaps between 1.17-1.26 eV, when Se is gradually replaced by S, these values are consistent with the reported values of approximately 1.0 eV for CZTSe and 1.44 eV for CZTS [26, 28].

The PL spectrum of Cu2ZnSnS4-xSex shows nearly symmetrical band, it is possible to observe that there is a shift from 1.39 eV to 1.26 eV of the bands, consistent with the increasing of selenium, these values are according with the data obtained from of UV-Vis measurements, and also with the corresponding values reported in literature for the quaternary Cu2ZnSnS4 and Cu2ZnSnSe4 phases [4]. Tanaka et al. [29] attributed the detected broad PL band between 1.1-1.45 eV to donor-acceptor pair recombination


The solid Cu2ZnSnS4 XSeX ( x = 1,2,3) phases were obtained by conventional solid-state synthesis method. The SEM-EDS analysis confirms the chemical compositions. The Raman spectra were dominated by the peaks assigned to the A1 vibration mode, the broadening of these peaks can be attributed to the disorder effects related to SnQ4 and MQ4 (M = Cu2+ and Zn2+; Q = S, Se) tetrahedra. The results of diffuse reflectance combined with photoluminescence show band-gap values Eg between 1.26 to 1.17 eV, the experimental observation have showed that the band gap of CZTS decreased linearly when S is gradually replaced by Se. Thus, the S/Se ratio can be used to adjust an optimal band gap in the CZTSSe phase.


The authors thank the financial support of FONDECYT Post-doctoral project 3140520, and the Vibrational Spectroscopy Laboratory of the Facultad de Ciencias, Universidad of Chile.


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