Fe0.986Ru0.014S2 Thin Films, Having Desired Properties for Photovoltaic Application, Synthesized by Spray Pyrolysis Technique

Abstract: A good choice of the ruthenium doping percentage has given high-quality Fe0.986Ru0.014S2 thin films having desired properties for several applications, such as photovoltaic. These layers have been grown by a simple and non toxic technique: spray pyrolysis. In a first step, for an adequate molar ratio of (FeCl3.6H2O : RuCl3.3H2O), amorphous Ru-doped iron oxide thin films were deposited on glass substrate pre-heated at 350°C. Then, a heat treatment at 450°C under sulfur atmosphere (~10-4 torr) in sealed-tube transformed the as obtained films into Fe0.986Ru0.014S2 films. XRD analysis confirmed the FeS2 phase. The estimated compositions were tested by the EDX analysis. Surface morphology is studied by SEM and AFM and the obtained images showed an inhomogeneous granular structure and a rough surface. However, the granular size varies from 150 nm to 300 nm. The optical measurements showed a high absorption coefficient ( > 3.5x10 cm for wave lengths lower than 800 nm). A direct band gad corresponding to a photon energy of 1.55 eV was obtained after plotting (h) versus the photon energy (h); which is an estimated value for photovoltaic application. The electron mobility in Fe0.986Ru0.014S2 films has been obtained from Hall Effect measurement where an initial current of 1A and a magnetic field, B = 0.554T, are applied. Our layers show n-type conduction. Electrical measurements (bulk concentration =1.0209x10 cm, Sheet Concentration=2.766510 cm) encourage their use as good low cost materials for solar cells. Indeed, the carrier mobility was measured of about 0.21215 cm/(V.s), the resistivity was of 2.8821 Ω.cm, and the conductivity was of 0.34696 Ω1.cm-1. The bulk concentration was of 1.0209×10cm.


INTRODUCTION:
Nowadays, researchers are of great interest to innovate new low cost materials for solar cells [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Several research works focused on the Pyrite films fabrication because of their low cost, but unconvincing results were obtained. However, for their use in this area, it needs to improve their properties (optical and electrical). Thus, researchers thought about their doping in the aim to improve their optical and electrical properties to make it more effective as a photovoltaic material. Different dopants were chosen As, Ni, Co, Cu, Al, Ni, Zn, Os, Ru, Mg, Ba, etc [6, [15][16][17][18][19][20][21][22][23][24]. Knowing that, the ruthenium has been treated as a good candidate for doping FeS2 thin films in the aim to improve their properties since they were chosen among the low cost materials for solar cells. Indeed, in a previous work [19], a good study was done concerning the Ru-alloyed FeS2 thin films for solar cells application wherein estimated optical results were obtained for the ratio RuCl3.3H2O:FeCl3.6H2O of about 1.56% (high absorbance and a direct band gap corresponding to photon energy of 1.48 eV). In order to fabricate Ru-doped FeS2 films having good properties for solar cells application, which means a good choice of the ratio: RuCl3.3H2O:FeCl3.6H2O; we tried with different ratios close to 1.56% [19]. Hence, a well chosen ratio: RuCl3.3H2O:FeCl3.6H2O of 1.46% allowed us to fabricate Fe0.986Ru0.014S2 thin films having desired optical and electrical properties for solar cells application.
The as obtained amorphous Ru-doped iron oxide films were transformed into Fe(1-x)RuxS2 films after heat treatment under sulfur atmosphere (~10 -4 torr) at Tsulfuration = 450°C for 3h. Dark films with good adherence to the substrate were obtained. Therefore, the prepared films are treated by XRD to identify the obtained phase, and then their optical properties are obtained after reflectivity and transmittance measurements. Electrical properties are obtained by Hall Effect technique. The surface morphology is investigated by SEM and AFM.

Structural properties a-X-ray diffraction
The adequate molar ratio (33 : 65 : 2) of (FeCl3.6H2O : S : RuCl3.3H2O) was used to synthesize Ru-doped FeS2 material. The obtained phase was investigated by powder X-ray diffraction (PXRD) with a Bruker D8 Advance diffractometer equipped with a lynxeye detector. The dichromatic copper radiation (CuKα1/α2) was used by the θ-2θ scan in the 2θ range of 5-70°. Full diffraction power of 40mA/40kV with 0.018°  step and 2s time-step were used to record the diffraction pattern represented in Fig.1. The search-match phase identification procedure was applied using the Powder Diffraction File (PDF) database and the PanAlytical XpertPro Highscore software. Two phases were identified in the diffraction pattern: FeS2 (JCPDS card n°: 00-024-0076) and Fe2O3. The sulfuration of the pre-deposited Ru-doped iron oxide layers under vacuum (~10 -4 torr) takes 3h at 450°C. Compared to the initial pyrite peak position, the observed shift towards the low 2 values of the obtained thin film proofs the ruthenium doping of the pyrite material and the synthesis of Ru-doped FeS2. Since the ruthenium atomic radius is larger than that of the iron, replacing Fe by Ru will increase the cubic cell parameter explaining the observed shift of the Bragg peaks position. The refined cubic cell parameter of the Ru-doped FeS2 is a = 5.4180 Å.

b-Energy Dispersive X-ray Spectroscopy (EDX or EDS) analysis:
To determine the compositions of the Ru-doped FeS2 stacked layers, EDS analysis were carried out using a Thermo Scientific UltraDry Detector. The measured values are summarized in Table.1.

Surface morphology Scanning-electron-microscopy
The surface morphology of the Fe0.986Ru0.014S2 films is visualized by a Thermo Scientific Q250 SEM. Fig.2 shows SEM micrographs surface views at different magnifications of Ru-alloyed pyrite films annealed at 450 °C under vacuum (∼10 −4 torr) for 3h. Porous surface and inhomogeneous granular structure were shown. The grain size could reach 300 nm. These properties encourage more their application as solar cells.

Atomic-Force-Microscopy
To further investigate the morphology of the obtained films (Fe0.986Ru0.014S2) a characterization by AFM (Dimension 3100 controller NanoscopeIIIa, Digital Instruments, Veeco, Metrology Group) in contact mode at ambient temperature was done. Root mean square (rms) surface roughness value was determined on an area of 2m x 2m for the studied films. The rms was calculated using the roughness analyses software provided by the instrument manufacturer. Fig. 3 shows AFM surface morphology of the Fe0.986Ru0.014S2 film. A granular structure was observed confirming the SEM observation (Fig. 2). The AFM images of the obtained films show porous and rough surface (Fig. 3), a strong roughness was observed. AFM images (Fig. 3) are in agreement with SEM views, when irregular spherical crystallites and a large grain size dimensions were observed.

Optical Properties
Optical absorption spectra of the as-prepared non-doped and Ru-doped pyrite thin films were recorded using a SHIMADZU 3100S spectrophotometer. Fig.4 depicts the optical absorption spectra versus photon energy (h). The transmission and reflectivity variations versus the wavelength (400 nm-1800nm) of the obtained films are shown in Fig.4 (a,b). It appears that the transmission and reflectivity coefficients measurements of the Fe0.986Ru0.014S2 films (Fig.4(a)) have shown a remarkable decrease comparing to those of the non doped FeS2 films ( Fig.4 (b)). However, the obtained Fe0.986Ru0.014S2 films showed a high absorption coefficient ( >2x10 4 cm -1 for  < 1900nm) (Fig.4(c)). Plots of (h) n/2 versus the photon energy h, with n=1 and n=4 for these obtained layers are given simultaneously in Fig.4(d) and Fig.4 (e). Three transitions are depicted: an indirect one corresponding to photon energy of about 2.8 eV and two direct ones corresponding to the energies 1.55 eV and 3.72 eV. Thus, the lower energy is attributed to a direct band gap of the Fe0.986Ru0.014S2 phase comparing to the values range of the Fe(1-x)RuxS2 band gaps obtained previously [13]. The two other transitions, direct and indirect, corresponding to the energies 2.8 eV and 3.72 eV could be attributed to the iron oxide phases.

Electrical transport properties
To assess the electrical quality of the Ru-doped FeS2 thin films, especially of the Fe0.986Ru0.014S2 ones, Hall mobility measurements were performed and analyzed to extract carrier mobility and carrier concentration. The obtained carrier density (n) and mobility (n) of an n-type semiconductor can be determined from the measured Hall coefficient (RH(B)) and the resistivity ((B)) using the relations: Where q is the electronic charge, B is the applied magnetic field, and rH is the Hall factor. In fact, the Hall coefficient RH is given by the following equation: Recall that when electrons are the charge carriers, RH is negative and when holes are the charge carriers, RH is positive.
In the present work the electron mobility in the as obtained Fe0.98535Ru0.01465S2 films has been obtained from Hall Effect measurement where an initial current of 1A and a magnetic field, B = 0.554T are applied. Our layers show n-type conduction.
The electrical measurements such as: the bulk concentration that is about 1.0209x10 +19 cm -3 , and the sheet concentration that is of about 2.766510 +15 cm -2 , encourage their use as good low cost materials for solar cells. Indeed, the carrier mobility was measured of about 0.21215 cm 2 /(V.s), the resistivity was of 2.8821 Ω.cm, and the conductivity was of 0.34696 Ω -1 .cm -1 .
4. CONCLUSION: An adequate molar ratio was chosen to fabricate Fe0.986Ru0.014S2 thin films having good properties for solar cells application. The morphology study showed a granular structure with grain sizes reaching 300 nm, which means that our layers present a good porosity. A strong roughness was observed. The optical properties of these layers showed a high absorption coefficient ( >2x10 4 cm -1 for  < 1900nm) and a direct band gap corresponding to photon energy of 1.55 eV; which is considered as a desired band gap value for solar cells application. In addition, the obtained electrical properties of the Fe0.986Ru0.014S2 layers were optimal for the intended application. Thus, one can say that the obtained Fe0.986Ru0.014S2 thin layers could be used as a material for low cost solar cells application. It could be considered as an interesting plus in the field of low cost materials for solar cells application.