Antimony sulfide (Sb2S3) nanobars are synthesized by the solvothermal method using different concentrations of the antimony chloride salt (SbCl3)at 180 °C for 14 h. The effects of 0.75, 0.8, 0.85, and 0.9 mmol of SbCl3 on the compositions, morphologies, and phases of the product are investigated. Raman spectroscopy indicates that the product corresponds to the pure orthorhombic phase of Sb2S3. Transmission electron microscopy indicates that the appearance of the as-synthesized Sb2S3 resembles that of nanobars with a typical width of 200–300 nm, which predominantly grow along the  direction. Chemical composition analysis confirms that the sample is composed of S and Sb, and the atomic ratio of Sb/S is close to 2:3, which is confirmed by X-ray photoelectron spectroscopy. The phase-pure Sb2S3 nanobars exhibit an optical energy gap between 1.5 eV and 1.74 eV and an absorption coefficient of approximately 104 cm−1, which would thus be suitable for use in photovoltaic applications. Scanning electron microscopy results indicate that the Sb2S3 thin-film-based nanobars are compact and smooth with a grain size of more than 3 µm. The best results reported here are for the solar cell structure Mo/Sb2S3/CdS/ITO/Ag with an open circuit voltage of 451 mV, short circuit current density of 12.47 mA/cm2, fill factor of 0.61, and conversion efficiency of 3.46%.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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 . Sb2S3 has a direct band gap of approximately 1.5–2.5 eV and exhibits a high optical absorption coefficient of 105 cm−1 . 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 .
Sb2S3 is used in many applications, such as photocatalysis , solar cells , thermoelectric conversion , fast ion conductors , thermoelectric cooling technologies , and Li-ion batteries . Many methods have been used to fabricate Sb2S3, such as vacuum evaporation , chemical bath deposition (CBD) , electrochemical deposition , and the hydrothermal method . 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 . Pal et al. synthesized Sb2S3nanorodssuperstructures (micro or nano-structures) through a hydrothermal process . 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 . 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.
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 . 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 . 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.
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 C3V symmetry . 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 . The broad peak at 447 cm−1 was attributed to the symmetric stretching of the Sb–S–S–Sb structural unit .
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  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  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  direction [21–23].
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.
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 .
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 (Eg) of the nanobars was calculated from the Tauc equation . 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 . 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.
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 . The peaks of S that appeared at 165.04 and 161.18 eV (Fig. 7(b)) originated from the S 2p3/2 and S 2p1/2, 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 . The peaks of Sb 3d5/2 at 529.21 and 3d3/2 at 538.53 eV appeared with a splitting energy of 9.32 eV, which corresponded to the value of Sb3+ . 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.
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.
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 .
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.
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, Voc decreases with increasing the SbCl3 concentration in the absorber layer. Then the value of Voc 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 Jsc 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%.
The increased JSC 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 (RSh) and series resistance (Rs). For an ideal performance, the series resistance should be minimized and the shunt resistance should be maximized . 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.
In summary, Sb2S3 nanobars were fabricated by the solvothermal method using different concentrations of SbCl3. 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%.
King Abdulaziz University.
This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (D-00062997,104-130-1442). The authors, therefore, gratefully acknowledge DSR technical and financial support.
The authors declare that there are no conflicts of interest related to this article.
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