1. Introduction

Antimony sulfide (Sb₂S₃) is a p-type semiconductor that has attracted much attention because of its unique physical properties. It is one of the metal chalcogenide compounds with 443orthorhombic crystal structures [1]. Sb₂S₃ has a direct band gap of approximately 1.5–2.5 eV and exhibits a high optical absorption coefficient of 10⁵ cm⁻¹ [2]. Moreover, Sb₂S₃ has low toxicity and is a cheap, earth abundant, and environmentally friendly material, which makes it a very promising candidate for solar cell applications [3].

Sb₂S₃ is used in many applications, such as photocatalysis [4], solar cells [5], thermoelectric conversion [5], fast ion conductors [6], thermoelectric cooling technologies [7], and Li-ion batteries [8]. Many methods have been used to fabricate Sb₂S₃, such as vacuum evaporation [9], chemical bath deposition (CBD) [10], electrochemical deposition [11], and the hydrothermal method [12]. Among these techniques, the solvothermal method whichis one of the most popular with which to fabricate Sb₂S₃.

The solvothermal technique is reproducible, simple, inexpensive, and does not require the use of a toxic precursor. The materials produced using solvothermal methods have thermodynamically stable structures. The solvothermal method has several advantages: (1) it can use an environmentally friendly solvent to prepare the nanostructure products; (2) the metal Sb and S powder used in solutions are easily prepared and both are inexpensive; (3) the process is easy to control, facile, and can be conducted at low temperature. Several morphologies of Sb₂S₃ superstructures prepared using a hydrothermal method have been reported, such as dendritic-like or feather-like, nanocrystals, microspheres, microtube- and dumbbell-like microstructures, nanoparticles, microcrystals, and nanorods, which have wide applications in batteries and solar cells. Zhang et al. prepared Sb₂S₃ nanocrystals using a hydrothermal method with Polyvinylpyrrolidone (PVP) acting as a complexing agent [12]. Pal et al. synthesized Sb₂S₃nanorodssuperstructures (micro or nano-structures) through a hydrothermal process [7]. However, to date, few attempts have been made to fabricate Sb₂S₃nanobars through a simple one-step solvothermal method.

The solvothermal method usually requires tight control of the complexing agent, temperature, additives, and salt concentration to obtain superstructures with a high crystallinity. It is important to obtain a single phase in high purity without secondary phases such as SbOCl, Sb₂O₃, and Sb(SO₃)₃, which negatively affect the performance of the solar cells owing to an increase in the recombination of charge carriers [13]. In solar cell devices, a p-type absorber can provide a highly effective charge generation and transportation by reducing the effect of factors that hinder the efficiency such as low carrier concentration, high binding energy, recombination at the grain boundary, and high defects trap density. Over the past decade, nanostructure-based thin films have attracted much attention owing to their ability to provide light scattering over a large angular range and excitation of a thin film surface.

The present work demonstrates the fabrication of Sb₂S₃ nanobars through a solvothermal method using different concentrations of SbCl₃ at 180 °C for 14 h. Our study focuses on the characterization of the morphology, structure, and optical properties of the products using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS; energy-dispersive X-ray spectroscopy (EDX) and mapping), UV–visible spectroscopy, and Raman spectroscopy. To the best of our knowledge, limited work has been reported regarding the effect of the SbCl₃ concentration on the formation of nanobars, which is followed by spin coating to fabricate thin film solar cells. The results indicated that the SbCl₃ concentrations have effects on the prepared Sb₂S₃ nanobars, thus they have effect on the solar cell device prepared using Sb₂S₃ nanobars. To evaluate the photovoltaic properties of the spin-coated Sb₂S₃ thin-film-based nanobars, Sb₂S₃ solar cells comprising Mo/Sb₂S₃/CdS/ITO/Ag are fabricated, and the current-voltage characteristics of these solar cells are investigated.

2. Experimental

In a typical synthesis of Sb₂S₃nanobars, four solutions containing differentconcentrations of SbCl₃ (0.75, 0.8, 0.85, 0.9 mmol) weredissolved in 10 ml oleyamin. 1.45 g of thiourea (SC (NH₂)₂) in 20 ml of oleyamin dissolved in another beaker solution. After 20 min ofwell constant stirring, the solution 2 was added drop by drop to solution 1. After 30 minutes, until transparent yellow solutions were formed, the mixture was transferred to four 50 ml Teflon coated autoclaves at 180 °C for 14 h. The autoclave left to cooldown naturally to room temperature. The products were centrifuged at 4500 rpm for 20 min and washed several times using deionized water and ethanol. Finally, the obtained powder was dried at 100 °C in vacuum. To fabricate Sb₂S₃ solar cell devices, 1 µm of Mo coated soda lime glass (25 mm x 10 mm) using radio frequency magnetron (RF) was sputtered at 300 W under vacuumof2.5 m Torr which adhered well on glass. To prepare Sb₂S₃ thin film for solar cell device of Mo/Sb₂S₃/CdS/ITO/Ag structure as p-type, 3 g of polyvinylpyrrolidone (PVP) was dissolved in 50 mL of ethanol, 3 g of Sb₂S₃nanobars dissolved in 30 mL of ethanol followed by centrifuging the sample for 10 min at 3000 rcf. 300 µL of the 6% PVP solution added to the nanobars tube, sonicated to disperse the nanobars. Sb₂S₃ thin film was deposited on Mo coated SLG using spin coating method under spin rotor at a speed of 4500 rpm. Sb₂S₃ thin film annealing and sulfurization in the cylinder vacuum furnace with the presence of nitrogen and sulfur to improve crystallinity was conducted. Furthermore, 80 nm thickness of CdS thin film deposited on Sb₂S₃ thin film from solution bath containing: cadmium nitrate (30 ml of 0.1M), sodium citrate (17 ml of 1M), thiourea (5 ml of 1M), and ammonia (15 ml) at 80 °C for 1 h. Indium tin oxide films (ITO) with thickness 150 nm have been deposited by RF sputtering technique with power density of 2 W cm⁻² and base pressure was 4×10⁻⁶mbar and pure argon have been utilized as sputtering gas. Finally, grids of Ag were applied as a front contact deposited by thermal evaporation with a vacuum system at approx. 10⁻⁶ Torr.

The crystal structure and phase compositions of the nanobars were determined using X-ray diffraction (XRD: Utima –IV, Rigaku, Japan), equipped using graphite monochromatized Cu-K α (λ = 1.5406 Å, 45 kV, 40 mA) at a scan rate of 0.01 ° /s in the range 10–80°. Elements composition and oxidation state of Sb₂S₃ powder was characterized using X-ray photoelectron spectroscopy (XPS PHI5000VersaprobeII) equipped with a mono-chromatic Al Ka, hv = 1486.6 eV X-ray source and10 µm probe Size. The XPS data was calibrated with carbon standard peak position (284.8 eV). The morphology andsize of the samples were performed by Transmission electron microscopy (TEM) (JEOL ARM -200F Tokyo, Japan) operated at 200 kV. Optical properties of Sb₂S₃ powder were studied by (Lambda 750, PerkinElmer) spectrometer. The morphologies and elemental composition as a deposit of the nanobars was measured using FESEM equipped with energy dispersive X-ray Spectroscopy) (FESEM-EDX; JSM 7600F, JEOL, Japan). The Raman spectrawere measured using DXR (Thermo Fisher Scientific) spectrometer, with an excitation wavelength of 785 nm near IR laser light. J-V measurements were investigated using a solar simulator (SS-1000: ORC, France) in Air Mass 1.5G 1 Sun illumination (100 mW/cm²). The effective area of the solar cell device was 0.5 cm².

3. Results and discussion

Figure 1 displays the XRD pattern of the Sb₂S₃ nanobars prepared using different concentrations of the SbCl₃ salt (0.75, 0.8, 0.85, 0.9 mmol). All the reflection peaks were indexed well to a pure orthorhombic phase of Sb₂S₃ according to the JCPDS#42-1393 Card with a strong intensity for the (130) orientation peak [14]. The strongest peaks that appeared at 2θ = 17.33, 24.91, 29.12, 32.14, 36.11, and 46.52 degrees were assigned to the (120), (130), (211), (221), (240), and (501) plane orientations, respectively. These results are in good agreement with the literature [15,16]. The crystallite size was calculated using Scherrer's formula depending on the full-width at half-maximum of the (130) peak [15]. The values determined for the crystallite size were 42.4 nm, 41.8 nm, 36.8 nm, and 26.8 nm for the concentrations of SbCl₃ of 0.90 mmol, 0.85 mmol, 0.80 mmol, and 0.75 mmol, respectively. For the sample prepared using 0.9 mmol of the SbCl₃ salt, the lattice parameters of a = 1.123 nm, b = 1.130 nm, and c = 0.3840 nm matched well with the values previously reported [10,17]. The diffraction peaks corresponding to the sample obtained with 0.90 mmol of SbCl₃ salt appeared to be the most intense and the sharp peaks suggested a crystalline nature compared with those observed from the samples obtained from the other concentrations of the SbCl₃ salt. The absence of secondary phases indicated that our sample was of high purity. Our experimental results indicated that an increase of the SbCl₃concentration to more than 0.9 mmol was associated with the solution viscosity, which increased with the increase of the SbCl₃ concentration and then led to the formation of many white agglomerates that were deposited on the bottom of the beaker.

 

Fig. 1. Powder XRD patterns of the obtained Sb₂S₃nanobars synthesized using different concentrations of the SbCl₃ salt.

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Figure 2 displays the Raman scattering data of the as-deposited Sb₂S₃ nanobars synthesized using different concentrations of the SbCl₃ salt. Raman scattering modes were clearly observed by the appearance of the peaks at 113, 186, 248, 294, 367, and 447 cm⁻¹. These results are in good agreement with the literature [7,18]. The variations of the peak intensities for the different concentrations revealed that the crystallinity increased with an increasing concentration of the SbCl₃ salt. The presence of intense and sharp peaks at 186 and 248 cm⁻¹suggested thatthe products were highly crystalline [19,4]. The peaks at 142, 294, and 367 cm⁻¹ were in accordance with the SbS₃ pyramid units of the material with C_3V symmetry [4]. The weak peak at 113 cm⁻¹was attributed to the S–S vibrations or the symmetric stretching of the Sb–S–S–Sb structural units [4]. The broad peak at 447 cm⁻¹ was attributed to the symmetric stretching of the Sb–S–S–Sb structural unit [20].

Additional details about the nature of the crystal nanostructure were determined using TEM. Figure 3(a) shows the as-prepared Sb₂S₃ nanobars prepared through a solvothermal method using 0.90 mmol of the SbCl₃ salt. The image shows that the width of Sb₂S₃ nanobars was approximately 150 nm. The fringes that appeared in the high-resolution transmission electron microscopy (HRTEM) image (Fig. 3(b)) indicated that the Sb₂S₃ nanobars have a single-phase nature. Furthermore, the lattice fringes with a d-spacing of 0.44 nm were well indexed to the (130) planes of orthorhombic Sb₂S₃ with a [001] preferential growth direction. The selected-area electron diffraction (SAED) results shown in Fig. 3(c) are represented as white spots; these spots showed the single crystalline nature of the Sb₂S₃ nanobars. Furthermore, the spots confirmed the crystallinity that appeared at the XRD peaks (130), (110), (132), and (120). Recently, studies of Sb₂S₃ solar cells have demonstrated a correlation between the photovoltaic performance and crystal plane orientation. Solar cell devices made from thin films with a high degree of [001] orientation show higher short-circuit current density (Jsc) and fill factor (FF) than films with other orientations, which may arise from the more active transport over covalently bonded ribbons in the [001] direction [21–23].

 

Fig. 2. Raman spectra of Sb₂S₃ nanobars prepared using a solvothermal method with concentrations of the SbCl₃ salt of 0.75, 0.80, 0.85, and 0.90 mmol.

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Fig. 3. (a) TEM image, (b) HRTEM image, and (c) SAED pattern of Sb₂S₃ nanobars prepared with 0.90 mmol of SbCl₃ salt.

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The morphology and size of the obtained Sb₂S₃nanobars with 0.90 mmol of SbCl₃ salt are displayed in Fig. 4. The results showed that the nanostructure is mainly composed of Sb₂S₃ nanobars with smooth surfaces. Figure 4(a) shows that the product contained a large quantity of straw-like materials. High magnification measurements displayed that the crystals exhibited a rectangular cross section and the diameters of the bars were approximately 150 nm (0.15 µm). In general, the final shape of the nanostructures was determined by the careful control of external factors, such as reaction media, additives, reaction time, and temperature, which affected the crystal structure during the initial nucleation stage.

 

Fig. 4. (a) Low magnification and (b) high magnification SEM images of the Sb₂S₃ nanobars synthesized by the solvothermal method with 0.90 mmol of SbCl₃ salt at 180 °C for 14 h.

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Table 1 show the atomic percentage of Sb and S with ideal stoichiometry where an exact Sb₂S₃ phase is obtained with a ratio of 2:3. Figure 5 shows the EDX spectrum of the synthesized Sb₂S₃ nanobars with 0.90 mmol of SbCl₃. The results indicated that only peaks corresponding to Sb and S are detected, which reveals that the sample had good purity as it was composed of only Sb and S. EDX analysis (inset table) displayed that the atomicratio was Sb 35.42% to S 64.58%, which is close to 2:3, to yield the sample with a structure of Sb₂S₃. The elemental composition from the EDX results revealed the desirable stoichiometry of Sb₂S₃. The Sb₂S₃ structure was obtained by tight control of the mole concentrations of thiourea (sulfur source) and SbCl₃ in solution, which enabled the desired ratio of S to Sb in the final product. A higher solar cell efficiency has been obtained from molar ratios closer to stoichiometric conditions as a result of the relatively compact layers [24].

 

Fig. 5. EDX spectrum of the obtained Sb₂S₃ nanobars with 0.90 mmol of SbCl₃ salt. The inset is a table of the chemical composition.

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Table 1. Chemical composition of Sb₂S₃nanobars synthesized with different concentrationsof SbCl₃ salt.

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The optical properties of the Sb₂S₃nanobars were studied by absorption spectroscopy for samples in powder form, and the corresponding UV–visible absorption spectra of the nanobars are shown in Fig. 6(a–d). As shown in Fig. 6, the absorption coefficient (α) of all of the samples was larger than 10⁴ cm⁻¹ in the visible spectrum. The optical energy gap was obtained by extrapolating the linear portion of a plot of (αhν)² against the photon energy (hν) (insets of Fig. 6). The optical energy gap (E_g) of the nanobars was calculated from the Tauc equation [24]. The values of the optical energy gaps of the Sb₂S₃ nanobars prepared with different concentrations of SbCl₃ salt of 0.9, 0.85, 0.8, and 0.75 mmol were 1.5, 1.63, 1.69, and 1.74 eV, respectively, which are in agreement with those reportedfor Sb₂S₃ of 1.5–2.2 eV [25]. The change in energy gap was attributed to the crystallinity of the Sb₂S₃ nanobars that improved with an increase of the SbCl₃ concentration. However, the band gap was suitable for the material to be used for photovoltaic conversion.

 

Fig. 6. UV-Visible absorption spectrum measured with respect to wavelength for Sb₂S₃ nanobars that werefabricated using solvothermal method with different concentrations of SbCl₃ salt of (a) 0.9, (b) 0.8, (c) 0.85 and (d) 0.75 mmol. Inset shows a plot of (αhν)² vs. hν used to estimate the band gap energy.

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The chemical purity and the composition of Sb₂S₃ nanobars prepared with a concentration of 0.90 mmol ofSbCl₃ saltwere determined using XPS analysis. From the typical XPS survey spectrum (Fig. 7(a)), only peaks arising from Sb and S were observed, which indicated the high purity of the sample. High-resolution spectra of the Sb 3d and S 2p core levels were investigated with respect to 385 eV [15]. The peaks of S that appeared at 165.04 and 161.18 eV (Fig. 7(b)) originated from the S 2p_3/2 and S 2p_1/2, respectively, corresponding to S in metal sulfides (S²⁻in the S-Sb bonding structure), which was consistent with the standard splitting of 3.86 eV [26]. The peaks of Sb 3d_5/2 at 529.21 and 3d_3/2 at 538.53 eV appeared with a splitting energy of 9.32 eV, which corresponded to the value of Sb³⁺ [27]. The peaks were consistent with those for Sb₂S₃ reported in the literature. In short, the XPS investigation showed that the valence states of the ingredient elements that existed in the sample Sb₂S₃ were Sb³⁺ and S²⁻, which confirmed the EDX results (Fig. 5 and Table 1). Thus, XPS confirmed the component composition desired for the formation of Sb₂S₃ in a stoichiometric ratio and proved the oxidation state of the component atoms.

 

Fig. 7. (a) XPS survey spectrum of Sb₂S₃ nanobars prepared by solvothermal method with a concentration of 0.90 mmol of the SbCl₃ salt, (b) S 2p core level, and (c) Sb 3d core level.

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Figure 8 shows the scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM–EDS) elemental maps of Sb₂S₃ nanobars prepared with a concentration of 0.9 mmol of SbCl₃ salt. As seen from the analytical results, both elements were present in the sample and the elements exhibited a homogeneous distribution within the sample.

 

Fig. 8. STEM–EDS elemental map of Sb₂S₃ microbars prepared utilizing solvothermal method using 0.90 mmol of SbCl₃ salt.

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Figure 9 shows the field emission (FE)-SEM images of Sb₂S₃ thin films prepared using different concentrations of SbCl₃ salt of (a) 0.75, (b) 0.8, (c) 0.85, and (d) 0.9 mmol using spin coating followed by sulfurization at 350 °C. In Fig. 9(a), with a SbCl₃ concentration of 0.75 mmol. Thesurface of the film was coarse with a lot of aggregates, many holes, high roughness, and the size of the particles varied between 800 nm and 1 µm. By increasing the SbCl₃ concentration to 0.8 mmol, the grain size increased and some networks grew across the substrate surface (Fig. 9(b)). When the SbCl₃ concentration was increased to 0.85 mmol, the grain size of the Sb₂S₃ film grew to 1 µm (Fig. 9(c)). As the SbCl₃ concentration was further increased to 0.9 mmol, the Sb₂S₃ thin film was compact with a smooth surface and large grains of more than 3 µm were formed (Fig. 9(d)). In general, the nucleation and grain size of the Sb₂S₃ film increased by increasing the SbCl₃ concentration. The increase of the grain size was proposed to arise from the Sb crystal lattice deformation, which was attributed to reacting with S atoms during the sulfurization process. In solar cell devices, a larger grain size is important to decrease the number of grain boundaries that will reduce the carrier recombination [28].

 

Fig. 9. FE-SEM images of Sb₂ S₃ thin films fabricated using different concentrations of SbCl₃ salt of (a) 0.75, (b) 0.8, (c) 0.85 and (d) 0.9 mmol.

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Figure 10 shows the cross-section image of Sb₂S₃ solar cells with the structure Mo/Sb₂S₃/CdS/ITO. A p-type solar cell was fabricated by using a Sb₂S₃ thin film prepared with SbCl₃ having a concentration of 0.9 mmol. The Sb₂S₃ thin film had an optimal thickness of approximately 2 µm and the film compact layer had a large grain size and good adherence to the Mo and CdS layers (n-type).

For the solar cells prepared with SbCl₃ with a concentration of 0.9 mmol, the performance improvement from the employment of a high SbCl₃ concentration was proposed to arise from the improved excited carrier collection and decreased recombination in the absorber layer.

 

Fig. 10. Cross-sectional FE-SEM image of the fabricated Sb₂S₃ solar cell

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Figure 11 shows the current density versus voltage (J–V) characteristics for different solar cell devices with a configuration of Mo/Sb₂S₃/CdS/ITO/Ag that were prepared based on differentconcentrations of SbCl₃ salt of (a) 0.75, (b) 0.8, (c) 0.85, and (d) 0.9 mmol. Table 2 shows their parameters measured under 100 mW/cm² of AM 1.5 simulated solar cell irradiation. The best device with a concentration of SbCl₃ of 0.9mmolexhibited a Jsc of 12.47 mA/cm²,Voc of 451 mV, a FF of 0.61, and a solar efficiency of 3.46%. The improved efficiency of the device obtained with a concentration of SbCl₃ of 0.9 mmolwas proposed to arise from the formation of a porous structure surface on the samples prepared with a low concentration of SbCl₃, which thus improved the contact area between the back contact and the Sb₂S₃ thin film, and increased the electron transport medium to reduce the electron–hole recombination. The Sb₂S₃ surface prepared with a SbCl₃concentration of 0.9 mmol exhibited a small energy gap and large absorption coefficient, which also enhanced the light absorption on the surface. However, the lower value ofVoc for all of the devices was attributed to pin hole formation, roughness, and weak contact that led to an increased recombination at the CdS buffer layer interface. Figure 12 shows theperformance statistics parameters of Sb₂S₃solar cells, including open-circuit voltage, short circuit current, fill factor, efficiency, shunt resistance, and series resistance. In Fig. 12, V_oc decreases with increasing the SbCl₃ concentration in the absorber layer. Then the value of V_oc rises from 416 to 451 mV. The downward trend ofVoc although the Rs and Rsh values are improving with the SbCl₃ concentration, may be due to band alignment with other layers or the changing electrical properties of Sb₂S₃ with SbCl₃concentration. It can be also seen that J_sc and Rsh increased with increasing the SbCl₃concentration, which may be due decrease the carrier recombination, thus increasing the life time of the excess electron-hole pairs.Rs decreases from 45.6 to 23.9 Ω cm² with the increasing the SbCl₃ concentration; the increased Jsc and decreased Rs for suggest a lower charge-transport resistance and higher hole extraction efficiency from active layer upon increasing the SbCl₃concentration. Moreover, the conversion efficiency of the proposed solar cell increased as the defect density is increased for device with a concentration of SbCl₃ of 0.9 mmol up to 3.46%.

 

Fig. 11. J-V characteristics of Sb₂S₃ solar cell devices fabricated using different concentrations of SbCl₃ salt.

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Fig. 12. Statistics parameters of (a) PCE, (b)FF,(c) Voc, (d) Jsc (d) PCE, (e) Rs, and (f) Rsh for the Sb₂S₃ solar cell device prepared under different SbCl₃ concentration.

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Table 2. PV parameters of the Sb₂S₃solar cell prepared using different SbCl₃ concentrations

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The increased J_SC value was proposed to arise from the improvement in grain size with the increase in SbCl₃ concentration, which thus enhanced the efficiency of the Sb₂S₃ solar cells that was dependent on the lower roughness and well-developed grain size of the film. Solar cell efficiency is generally described by the two resistance parameters of shunt resistance (R_Sh) and series resistance (Rs). For an ideal performance, the series resistance should be minimized and the shunt resistance should be maximized [29]. Our results indicated that the shunt resistance of the devices was increased with the increase of the SbCl₃ concentration, which led to increase in short circuit current of the samples.

In the present study, the SbCl₃ concentration is shown to play an important role in controlling the formation of Sb₂S₃ nanobars, which affects the crystallinity, purity of single-phase formation, and optimum energy gap, and enhances the grain size.

3. Conclusions

In summary, Sb₂S₃ nanobars were fabricated by the solvothermal method using different concentrations of SbCl_3. The structural, morphological, and optical properties of the nanobars were studied using XRD, SEM, TEM, UV–vis, and Raman spectroscopy. Raman spectroscopy indicated the formation of a highly crystalline product and confirmed the absence of secondary phases. The XRD pattern of the samples matched well with a pure orthorhombic phase, and showed an increase in the crystal size from 26.8 nm to 42.4 nm with the increase in the concentration of SbCl₃. TEM studies indicated the nanobars natureof Sb₂S₃. UV–vis absorption measurements indicated that the energy gap was between 1.5 eV and 1.74 eV with a high absorption coefficient of 10⁴ cm⁻¹. EDX spectra indicated that only Sb and S peaks were observed in this sample, the purity and composition of which were further confirmed by XPS. We realized that the smooth and pinhole-free, large grain sized Sb₂S₃ film with improved grain boundary was related to lowering the recombination probability. The solar cell prepared with a thin-film-based SbCl₃ concentration of 0.9 mmolachieves a powerconversion efficiency of 3.46%.