A new study of the optical and structural characterization properties of p-type ZnS2 pyrite with an application in solar energy

In this manuscript, we study the grouping of bands and optical properties of p-type ZnS2 pyrite, which helps us in the study of crystal p-type ZnS2 pyrite and its impact in organic photovoltaic cells. Our work has two approaches one theorical where we find the band structure of p− type ZnS2 by using linear muffin-tin orbital method in the atomic-sphere approximation (LMTO-ASA), the second approach is the experimental where we prepare the crystals by chemical vapor transport. The sample was examined by XRD and optical characterizations. Our results show that the p-type ZnS2 pyrite is a direct semiconductor with optical gap about 1.610eV and calculated gap about 1.550eV. As an illustration of our finding, we present an application of our work to photovoltaic devices.


Introduction
From the beginning of crystallography, the crystallographic structure of pyrite type compounds inclines to be a fundamental testing of solid-state physics that is because their interesting relations between structure property [1,2]. The pyrite type MS 2 with = M Fe Cu Ni Zn Ru , , , , have been attracting technological and scientific interest because of a large range of optic electronic and magnetic properties [3][4][5][6][7][8][9]. Some of these compounds are semi-conductor like the diamagnet band gap of ZnS , 2 RuS 2 has indirect band gap of values eV 1.8 [10,11] and FeS 2 has direct band gap of value eV 0.90 [12]. In difference, CuS 2 is a superconducting while CoS 2 becomes a ferromagnetic metal [5,13,14]. At the present state of Knowledge, the most obstacle for a large application of pyrite in broad area electronics is related to the sulfur deficiency [15,16]. Many applications for energy storage of pyrite structure dichalcogenides [17], corrosion protections, thermoelectric and other section a.e. investigated intensively at the moment.
Tiantian Jia et al proved that pyrite ZnS 2 has an excellent thermoelectric performance that improve the parameters lattice and the electronic contributions [12]. They provide that ZnS 2 has indirect band gap about eV 1.41 . But its pyrite structure such as rayon atomic, the sulfur-sulfur distance and the distance Sulfur-zinc still unknown, due to its incompatibility with the concept of ionic radii.
In this manuscript, we studied the interdependence between electronics structures and optical properties to understand the structure of the pyrite ZnS 2 and its impact in organic photovoltaic cells.
For our experimental work, we prepared our sample by the chemical vapor transport. We studied the structure of the sample by using x-ray powder, this method is well described in [18,19]. We start with a description of experimental and calculation method. Then, we provide most details of pyrite structure of ZnS 2 crystals.
For our theoretical calculations we used linear muffin-tin orbital method in the atomic-sphere approximation (LMTO-ASA) which it was developed by Anderson in [20]. We used LMsuite version 7.11. This Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. method is different from the GW calculation [21,22] where they also succeed in finding an agreement between the outcome of experimental work and GW calculations.
ZnS 2 pyrite is a semiconductor that has continued to gain more attention owing to industrial, biological, and agricultural applications [23]. The nanoparticles show significant quantum confinement effects that influence their optical and electrical properties [24]. Hence, its application in photovoltaics, photonic/optoelectronic devices, sensors, and catalysis [25,26].
ZnS 2 pyrite is a good candidate for photovoltaic use because of its suitable properties and its efficiency into electrical energy. As we know conventional photovoltaic materials are not very abundant in Earth's crust. And the economical aspect oftheir application is not sustainable [27,28], which ZnS 2 solar cells though very promising in nature.
The photovoltaic cell is usually made of nanomaterials possessing many semiconductive properties of different shapes. In our application we used the transparent electrode made up of Indium-doped tin oxide (ITO).
In this manuscript, we studied the interdependence between optical properties and electrical and electronic structure to understand the impact of the method of preparation to band gap value and to photovoltaic application of ZnS . 2 The resulting ZnS 2 shows better light absorption and large band gap. Therefore, ZnS 2 pyrite may be promising for the fabrication of pyrite solar cells.
Regenerative energy can also be produced using alloys of ZnS 2 with Fe. We obtained in last work noteworthy experimental and theoretical results [29], which is motivation to better understand ZnS 2 pyrite structure.

Experimental details
Our pyrite crystal prepared by chemical vapor transport, where we used bromine as transporting agent in a silica ampoule with a length of 150 to 200 mm and diameter of 25 mm for temperature gradient from 850 to 950°C [10]. The ampoule contained ZnS 2 powder and 2% sulfur. First, we added -mgCm 3 3 of ZnO 2 to this element. The ampoule was sealed in a Bromine atmosphere mm of Hg 100 . ( ) We obtained ZnO 2 and Br 2 at a pressure of 3 atmospheres ZnOBr 2 at  950 C. Thereafter, we placed the ampoule in a furnace with six Zones. Finally, the solution was situated at a hot temperature  - 870 C 950 C .
( ) The transport time took between h 200 and h 400 , which the slow transport rates was less than mg h 0.2 . / Mono-crystalline ZnS 2 was obtained in the cool zone of the ampoule. We have formed by this method a polycrystalline structure that consists of mono-crystal grains of dimension´ḿm The color of the obtained crystals is light yellow. Figure 1 shows the growth of monocrystal ZnS 2 by Chemical vapor transport .
The sample was analyzed by x-ray diffraction and by transmission electron micrograph. Parameters lattice and the crystal structural are obtained using the Reitveld method were utilized PDXL program. The measurement of powder x-ray diffraction was conducted on crushed crystals utilize a Philips diffractometer with a Cuk radiation for l = 1.5412Å in the usual q q -2 geometry. ) and distance Sulfur -Sulfur. We obtained the optical bandgap by Tauc's equation a n (1)). where A is a constant, n h denotes the photon energy, E g is the energy gap, = n 1 2 / for direct transition, and = n 2 for indirect transition.
The optical absorption spectra of p-type ZnS 2 pyrite obtained by using a SHIMADZU 3100s spectrophotometer. Plots of a n h 2 ( ) versus n h ( n h photon energy) is illustrated in figure 3(a), and the plot of a n h 1 2 ( ) versus n h in figure 3(b) depict an indirect band gap which the indirect band gap is Zero that confirm ZnS 2 pyrite has only direct band gap. According to this procedure, an approximate value of a direct band gap of about eV 1.61 for p-type ZnS 2 pyrite. This result is very important for its application as multispectral photovoltaic cells.
We use values of the experimental lattice parameters constant = a 5.9657Å and Sulfur position = u 0.399. the inter-atomic distance between Sulfur-Sulfur is 2.087, is the shortest. The distance between Zinc and Sulfur is 2.53. It keeps dimer of Sulfur S 2 and not an individual S atom. For the space group T Pa3 , h 6 ( ) we have 24 operations at the point T h that show us the corner of Brillouin Zone is big than more case of Familiar monoatomic. First Brillouin Zone is presented in figure 5, which we can see the lines connecting X with M are of two types, they are not identical like primitive cell of simple cubic (Energies of ¢ Z s different to energies of Z ).

Calculated band energy
The main reason that we are using LMTO-ASA method, is that it is an important tool in band structure calculations for compounds. We advise the reader to see [20,30], where they can find detailed description of such method. In our work, we use this technic because the calculations are done by DFT [31], utilizing the local density approximation [32] and numerical methods which based on the study of electron ion interaction in the pseudopotential approximation [33]. Moreover, the ASA Hamiltonian is mainly knowing by finding the potential parameters. Note that, the potential is between the logarithmic derivative D v at the sphere radius and the energy n E of the wave function j. In general, it is enough to find one which can be used to find the other. Also, notice that knowing D v and n E makes finding the potential P a simple process [34,35]. ∆ and g , 1 are the 'potential parameters' that parameterize P. C 1 corresponds to the band 'center of gravity', 1 ∆ is the 'band width' parameter and g 1 is the 'band distortion parameter'. In general, Small Parameterization is a good way to investigate band structure. We start with finding the potential parameter for all atomic spheres. The muffin-tin potential constant V MTZ was the crossing point of muffin-tin potential around Zn and S, it is equal to −0.790607 a.u.  The muffin-tin MT radii are 2.3483 a. u for Zn, 1.9495a. u for S. The initial sphere packing was equal to 80.7%, scaled to 89.5%. Note that, the empty spheres play an important role in reducing the number of iterations, as well as the reducing of the overlaps between the spheres centered at Zn and S.
The self-consistent energy bands of p-type pyrite is illustrated in figure 6. It shows a full picture of both the conduction band and the valence states, for energies ranging from -20 to eV 8 .We have four p-type ZnS 2 pyrite units in each unit cell. Note that the energy is displayed in order of increasing energy. From eV 16 to eV 11 we have S s 3 character. Next the Zn d 3 levels appear an additional with the basic structure of S p 3 states. This group of bands are occupied with a minimum of energy about -eV 7.2 , in this bands the S p 3 levels related to e g orbitals. The 12 bands just below Fermi levels are associated to Fe dt 3 g 2 bands. The conduction bands are above E F the Fermi level. This conduction bands have S p 3 and Zn de 3 g characters. Our results show that p-type ZnS 2 pyrite is a semiconductor, with a direct gap of eV 1.55 ( Ry 0.114 ). The (ec) conduction band energy is -Ry 0.078 and the (ev) valence band is -Ry 0.192 . The Fermi energy is -Ry 0.192 . The Fermi energy is close to the valence band (ev), this confirms that our sample is p-type and the conduction band is empty while the valence band is filled. In closing, our band gap is direct and in good agreement with our optical measurement of p-type ZnS 2 pyrite. Our results contrasts with the result of Tiantian Jia et al [16], who obtained eV 1.41 indirect band gap. Note that, in this work we succeeded in showing that the band gap of p-type ZnS 2 pyrite is about eV 1.55017 by theoretical calculations and the optical band gap is about eV 1.61 and that leads us to an agreement between theoretical and experimental band gap, where we can deduce that p-type ZnS 2 pyrite is a good candidate for photovoltaic cell.

Conclusion
ZnS 2 Pyrite sample was synthesized via a simple and low cost process by the chemical vapor transport method. Our found provide the favorable optical, crystallographic, and electrical properties of p-type ZnS 2 pyrite. Through the XRD, we determine experimental parameters cell, and we provide that the interatomic crystals Sulfur-Sulfur is shortest and close to distance sulfur-Sulfur in S 2 molecules. In this work, we calculated the band structure energy by using LMTO-ASA method. The optical gap energy value obtained significantly consistent with the gap energy calculated by LMTO-ASA method. In summary, p-type ZnS 2 pyrite was investigated for band gap. Crystal parameters obtained improved large optical and calculated band gap. An increased photoresponse with p-type ZnS 2 pyrite was observed in the organic photovoltaic cells devices resulting from the reduced light current. Our results confirm that p-type ZnS 2 pyrite is a good candidate for photovoltaic applications with optimum band gap ∼ -eV eV 1.61 1.55 . ( )

Acknowledgments
The Author R. AbuMousa and A. F. Alanazi would like to thank Prince Sultan University for paying the publication fees for this work through EWE LAB.

Data availability statement
We are still using this data in different research directions. So, if there is a reasonable request from a researcher we will provide them with the data. The data that support the findings of this study are available upon reasonable request from the authors.