Two-step synthesis of antimony sulfide thin films: enhancement in physical properties through sulfurization

Recently, there has been an increase in the use of antimony sulfide (Sb2S3) in Si-based tandem solar cells as a potential absorber material for top sub-cells. The choice of the material stems from the favoured properties such as appropriate bandgap, simple binary composition, nontoxic elements, and long-term stability. However, the physical properties and practical applicability of these materials depend largely on their synthesis conditions. In this work, we investigate the role of sulfurization on the structural, morphological, compositional, and optical properties of Sb2S3 thin films deposited on soda-lime glass via a thermal evaporation technique. Sulfurization was performed on the as-prepared thin films in a customized Chemical Vapor Deposition (CVD) chamber at five different temperatures. Analysis of the crystallinity of the film using the x-ray diffraction technique illustrates the transformation of the film from impure, poor crystalline phase to phase-pure, and highly crystalline orthorhombic structure due to sulfurization. Scanning electron microscopic investigations of the samples revealed better grains with nanorods on the surface at a temperature of 400 °C. For the samples investigated here, the energy values estimated via density functional theory (DFT) calculations agreed well with the experimental data obtained from UV-visible absorption spectral studies. Additionally, it was observed that the desired near-stoichiometric Sb2S3 thin films could be achieved via sulfurization, and the presence of Sb2S3 in all samples was confirmed via Raman spectroscopic studies. Additionally, the defects and trap states of the prepared films were investigated using photoluminescence studies, and donor and acceptor defects were identified. Our study revealed that sulfur rich Sb2S3 films prepared at a sulfurization temperature of 400 °C produced the desired structure, morphology, and optical qualities for future photovoltaic applications.


Introduction
The burgeoning demand for energy with an increasing population necessitates alternative pathways that are economical and have minimal or little environmental pollution.Amongst the renewable and sustainable technologies, the researchers are vigorously exploring now, solar energy is considered a promising renewable energy source.Therefore, recent efforts in the realm of photovoltaic (PV) technologies have focused on improving the efficiency and reducing costs [1,2].Currently, in addition to silicon-based solar cells, most photovoltaic devices use copper indium gallium diselenide (CIGS) and cadmium telluride (CdTe) thin-film solar cells.Solar energy conversion efficiencies greater than 20% have been achieved using these materials [3].Although silicon is less expensive, it is more expensive to produce power than conventional fossil fuels [3,4].On the other hand, in the case of CIGS and CdTe solar cells, the scarcity of In, Ga, and Te on earth and the toxicity of Cd are major problems.CZTS compounds have been developed by substituting Zinc (Zn) and Tin (Sn) for Indium (In) and Gallium (Ga) in CIGS, resulting in non-toxic and cost-effective chalcogenide light absorbers.However, CZTS-based solar cells have been impaired because of their complexity in controlling the required elemental composition and electrical parameters to achieve better efficiencies.Although remarkable efficiency has been achieved in organic-inorganic hybrid perovskites, these hybrids suffer from long-term stability and thus limit their commercialization applications.This has created the need for new, earth-abundant, low-cost, and stable materials [1,5,6].The quest for novel materials has led to the development of chalcogenides or binary chalcogenides.
The increasing popularity of chalcogenides can be attributed to their low cost, high stability, and abundance.Binary chalcogenides contain earth-abundant elements and desirable band gaps, and are highly stable, making them suitable for solar energy conversion applications [7].Among binary chalcogenides, Sb 2 S 3 has been proven to have great potential as a light-absorbing material for solar cells owing to its distinct features, such as a high absorption coefficient (α = 10 5 cm −1 ), earth-abundant and low-cost compositional elements, and a desirable bandgap (Eg = 1.4−1.8eV), which covers the maximum visible and near-infrared ranges of the solar spectrum [4,[6][7][8].However, the stoichiometric and crystalline defects occurring in the films are challenging to scale up.Various bulk defects, such as sulfur vacancy defects (V S ), antisite defects of Sb and S (Sb S ), and surface oxide (Sb 2 O 3 ), strongly influence the performance of Sb 2 S 3 solar cells [8].Several methods, such as chemical bath deposition, thermal evaporation, flash evaporation, and spray pyrolysis, have been employed for the growth of Sb 2 S 3 thin films [9][10][11][12].Thermal evaporation is one of the most commonly used physical processes for producing homogenous Sb 2 S 3 films [13][14][15][16][17].This technique has the unique advantages of (a) controlled and fast film growth, (b) compact uniform coating, and) possibility of producing multilayers.However, in this vacuum method, because of Sb 2 S 3 ′s low melting point (550 °C) and the high saturated vapor pressure of sulfur, the loss of sulfur in the film is very difficult to avoid [17,18].To address the problem of sulfur deficiency, sulfurization of the as-deposited film can be a remedial approach.It is pertinent to note that previous studies have confirmed that sulfur richness in the film leads to a lower defect density and has great significance in enhancing the efficiency of solar cells [19].Emna Gnenna et al utilized the thermal evaporation method to deposit antimony sulfide films, which were subsequently subjected to vacuum annealing at different temperatures.The study revealed that with an increase in the annealing temperature, there was a concurrent increase in the surface roughness and a reduction in the band gap of the films.However, all films were found to be deficient in sulfur even after subjecting annealing procedure [20].Muhammad Sajid et al investigated the impact of post-annealing in the sulfur atmosphere on the physical properties of antimony sulfide thin films.The study showed that within a specific temperature range, post-annealing resulted in a reduction in the sulfur deficiency.Films annealed at 300 °C exhibited improved crystallinity, larger grain size, and a decreased level of sulfur deficiency compared to films annealed under other conditions [21].Even though there are a few reports on the effect of annealing on thermally evaporated Sb 2 S 3 thin films, studies on the role of sulfurization in reducing defects and enhancing the structural and optical properties are very limited and inconclusive.To the best of our knowledge, there have been no reports of comparative experimental and theoretical investigations to establish the effect of sulfurization on the physical properties of Sb 2 S 3 thin films.
In this article, we report a two-step synthesis method for producing phase-pure Sb 2 S 3 thin films using a combination of two methods: thermal evaporation and chemical vapor deposition.Our sulfurization process could control the Sb 2 S 3 composition to an ideal stoichiometric ratio and simultaneously curb the occurrence of linked defects owing to sulfur vacancies.Herein, we illustrate the role of sulfurization via theoretical [density function theoretical studies] as well as experimental methods including x-ray diffraction studies, Energy Dispersive Spectroscopy [EDS], Raman spectroscopy, UV-vis spectroscopy, and photoluminescence spectroscopy to identify the correlation between the sulfur richness in the film and its structural, morphological, compositional, and optical properties.

Experimental details
Antimony sulfide thin films were deposited via thermal evaporation using a molybdenum boat as the source holder and soda lime glass (SLG) as the substrate.The substrates were initially cleaned with distilled water, washed with soap solution, and ultrasonicated with acetone and isopropyl alcohol for 10 min, followed by drying with nitrogen purging.The thermal co-evaporation chamber was loaded with the cleaned substrates and high-purity Sb 2 S 3 (Powder, Thermo Fischer, 99.9%).The average pressure during deposition was maintained at 5 × 10 -6 mbar with liquid nitrogen assistance.The standoff distance, commonly known as the distance from the source to the substrate, was set at 16 cm.Initially, the loaded precursors were heated and deposited onto SLG.The substrate temperature was maintained at 150 °C.A digital quartz crystal monitor was employed to track and regulate the deposition rate and the thickness of the thin layer.The deposition rate was maintained in the range of 3.5-4.1 Å/sec using a calibrated current of 85 A. The thickness of the as-deposited films was measured using a stylus profilometer and found to be 590 ±10 nm.To address the sulfur deficiency in the films, the as-deposited films were sulfurized at five different temperatures (300, 350, 400, 450, and 500 °C) at a pressure of 900 mbar using a customized Chemical Vapor Deposition [CVD] setup [Refer graphical abstract].Sulfur (100 mg [Thermo Fischer, 99.9998% purity] was evaporated in a single-zone furnace in a nitrogen atmosphere.After sulfurization, the films were cooled to room temperature under vacuum.The films exhibited excellent adhesion, except for the film sulfurized at 500 °C.The 500 °C sample was fully evaporated, which can be ascribed to the melting point of the Sb 2 S 3 precursor.The films were labelled S for the as-deposited films and S-300, S-350, S-400, and S-450 for the films sulfurized at 300, 350, 400, and 450 °C, respectively.The final thickness of the sulfurized films was 900 ± 10 nm.

Characterization details
X-ray diffraction (XRD) was used to perform a structural investigation of the synthesized Sb 2 S 3 thin films using Cu Kα radiation with an [λ = 1.5405Å] energy level at 40 kV and 15 mA in the 2θ range of 10-70°(Rigaku Miniflex 600).Using an Nd: YVO 4 diode-pumped solid-state laser source with an excitation wavelength of 785 nm, phase formation details were analyzed using Raman spectroscopy in a backscattering setup.The surface morphology was determined using scanning electron microscopy (SEM) at an acceleration voltage of 10 kV (Zeiss EVOMA18 with Zeiss SEM EVO18).The elemental compositions of the synthesized Sb 2 S 3 thin films were analyzed using energy-dispersive x-ray analysis (EDAX) (Zeiss SEM EVO18).The UV-visible spectrophotometer was used to acquire the absorption spectra (Scan range 190-1100 nm) (Shimadzu1900 UV).Photoluminescence was measured with a fluorescence spectrometer using an Nd: YVO 4 diode-pumped solidstate laser source with excitation wavelengths of 360, 420, and 610 nm.DFT calculations were conducted using Quantum Espresso (QE) software, specifically employing the plane-wave self-consistent field (PWscf) approach [18,22].The calculations used the HSE06 approximation [23,24].The calculations were performed using 2 × 2 × 1 supercells for (a) pure Sb 2 S 3 , (b) S-rich Sb 2 S 3 , and (c) S-poor Sb 2 S 3 .A k-mesh with dimensions of 6 × 6 × 1 was used within the first Brillouin [20,21,25].The relaxing phase of both pure and deficient structures occurs after the energy convergence reaches 2 × 10-5 eV and continues until the force per atom reduce to 0.02 eV [26][27][28].

Compositional analysis
Elemental analysis of the as-deposited and sulfurized films was performed by energy-dispersive spectroscopy (EDS).The atomic ratio of Sb:S varied with the sulfurization temperature.The as-deposited films exhibited a Sb: S ratio of 1:1.The films sulfurized at 300°, and 450 °C exhibited Sb:S ratios of 3:2.The samples that were sulfurized at 350 °C and 400 °C show a composition ratio of nearly 2:3, which is close to the ideal stoichiometry [29,30].Figure 1(a) shows the EDS spectrum of the S-400 sample.Table 1 lists the atomic percentages of the asdeposited and sulfurized Sb 2 S 3 thin films.Figure 1(b) shows a graphical representation of the calculated atomic ratio of the Sb 2 S 3 films.The dotted line represents the expected stoichiometric ratio of the Sb 2 S 3 films.It can be observed that we could get slightly sulfur rich composition for S-400.In the observed temperature range from S-300 to S-400, the composition of the samples exhibited variation, transitioning from being antimony (Sb) rich to sulfur (S) rich.However, an interesting deviation occurred at S-450, where the composition once again became antimony (Sb) rich.This finding contradicts an earlier report by Sajid et al, where sulfur-rich Sb 2 S 3 films were obtained at 300 °C [21].Our results indicate that 400 °C is the most favorable temperature for obtaining S-rich Sb 2 S 3 films.
To see the effect of sulfurization in Sb 2 S 3 , theoretical computations have been conducted to determine the formation energies [31] of (a) pure Sb 2 S 3 (b) S-rich and (c) S-poor Sb 2 S 3 samples.Table 2 clearly demonstrates that the formation energies of S-rich are more favourable than other cases, which also confirms that the presence of S is quite larger in amount than Sb.The findings of this study align completely with our experimental results.

X-ray diffraction analysis
The XRD patterns of sulfurized and as-deposited antimony sulfide thin films are shown in figure 2. The XRD patterns for S and S-300 samples exhibit the dominant peak at 24.5°corresponding to the orthorhombic Sb 2 S 3 phase (JCPDS:42-1393) [32,33], and the peak at 48.5°corresponds to the rhombohedral Sb phase (JCPDS:35-0732).In the S and S-300 samples, the appearance of the Sb phase indicates sulfur deficiency.This was because there was still a metallic Sb phase in the sample sulfurized at 300 °C.With an increase in the sulfurization temperature, the Sb phase began to fade.From the XRD patterns, it can be observed that all the films exhibit a polycrystalline nature, along with an orthorhombic crystal structure of Sb 2 S 3 phase.For sample S-400, the obtained pattern matched with pure orthorhombic Sb 2 S 3 (JCPDS:42-1393), and no other elemental peaks of Sb or S were observed.This is in line with elemental composition obtained using EDS for S-400.The peak located at 24.5°was assigned to the (130) diffraction plane.For samples S-350 and S-450, the peak is located at 25.9°with the corresponding plane (310).From the XRD patterns, we can observe that as the sulfurization temperature changes the preferred orientation changes from the (130) plane to the (310) plane [34].Upon sulfurization, the crystallinity of the films improves.These improved films exhibit peaks corresponding to reflections from the (0 2 0), (1 2 0), (1 3 0)/(3 1 0), and (2 2 1) planes, which match those of stibnite (Sb 2 S 3 ) [35].The absence of secondary peaks corresponding to SbN in both the XRD pattern and Raman spectra, along with the lack of atomic percentage of nitrogen in the EDS spectra, indicates that annealing the film in a sulfur and nitrogen atmosphere does not result in the formation of SbN.This is in agreement with other published reports [21,36].
To obtain further structural details, the crystallite size (D), microstrain, and dislocation density of the Sb 2 S 3 thin films were calculated using the (130) diffraction plane.The classical Scherrer formula was used to calculate the crystallite size, as follows [37]: The microstrain and dislocation densities were calculated using the following equations [38] .
Where D is the average crystallite size (in nm), β is the full-width at half maximum (FWHM) of the diffraction peak (in radians), K is a constant nearly equal to 0.9, λ is the wavelength of incident radiation approximately equal to 1.542 Å, and θ is Bragg's angle (in radians).ε is lattice strain.δ is dislocation density.
The interplanar distance (d hkl ) was calculated using the following equation [33].
where a, b, and c are lattice parameters, and h, k, and l are Miller indices.Table 3 lists the structural parameters of the as-deposited and sulfurized Sb 2 S 3 thin films.From the table, we can see that the variation in crystallite size is a function of sulfurization temperature.As the sulfurization temperature rose, the size of the crystallites grew while the microstrain and dislocation density gradually decreased.However, at higher temperatures, the crystallite size started to decrease again, and the microstrain and dislocation density increased.This phenomenon was attributed to variations in the lattice parameter [18].Moreover, it also indicates a grain formation process as discussed in earlier reports [39].

Raman spectroscopy analysis
Figure 3 shows the Raman scattering spectra for the as-deposited and sulfurized Sb 2 S 3 thin films.The reported Raman modes for Sb 2 S 3 appear at 180, 189, 235, 280, 300 and 307 cm −1 [9,12,30].Weak peaks arise at 180, 189, 234, 280, 301, and 306 cm −1 , indicating the progression of Raman scattering modes.The vibration modes and peak position of sulfurized and deposited Sb 2 S 3 thin films are displayed in table 4. Raman modes at 189 and 239 cm −1 can be attributed to the characteristic asymmetric and symmetric bending vibration of S-Sb-S.Sb-S symmetric and asymmetric stretching vibrations were observed at 282 and 306 cm −1 , respectively [40,41].Raman active modes constituting 10 A g , 5B 1g , 10 B 2g, and 5B 3g modes are also identified [42].The majority of the lines in the corresponding Raman spectra are indicated at 189, 280, and 306 cm −1 , which are Sb-S stretching modes and attributed to A g modes, respectively [35,37,38,43].Because Raman spectroscopy is unpolarized, the B 1g /B 3g modes at 234 cm −1 were also observed [44].The significantly greater number of A g modes compared to the other modes is obviously related to the strong preference for the orientation of the Sb 2 S 3 thin films [45].With an increase in the sulfurization temperature, the intensity of the peaks increased, indicating that the films changed from an amorphous to a crystalline phase [39,40,46,47].XRD and Raman analysis confirmed that sulfurization is essential to obtain single-phase Sb 2 S 3 .

Morphological analysis
Figure 4 displays the scanning electron micrograph obtained using the SEM.The films had a homogeneous and amorphous surface when they were deposited.Yet, grain growth began for the sample that was sulfurized at 300 °C.At 350 °C, grains and nanorods started to form.The surface developed into excellent grains with nanorods at 400 °C.At 450 °C, the grains and nanorods started to break down.The film sulfurized at 400 °C is seen to exhibit grains with nanorods, which is advantageous for solar applications [49].

UV-visible spectroscopy analysis
One of the most effective methods for determining the band structure and energy gap of materials is the analysis of optical absorption spectra.For the as-deposited and sulfurized Sb 2 S 3 thin film, the optical absorption coefficient was evaluated using absorbance and thickness data using the following formula [50], where α is the absorption coefficient, A is the absorbance, and t is the thickness of the film.Figures 5(a) and (b) depict the variation in absorbance as a function of wavelength and the absorption coefficient as a function of photon energy for the sulfurized and as-deposited Sb 2 S 3 thin films.As the sulfurization temperature increased, a change in absorption at lower wavelengths was observed.The absorption coefficient of S is estimated to be 1.25 × 10 5 cm −1 and that of S-400 is 1 × 10 5 cm −1 [21].The data shown in table 1 clearly indicate that the presence of antimony rich (Sb) in the S-300 sample could be the cause of the saturation shown in absorbance and absorption coefficient.Additionally, the opaque nature of the particular sample may contribute to this phenomenon.Moreover, the absorption coefficient of the as-deposited film is higher than that of the sulfurized films because the as-deposited film has poor crystallinity compared to the sulfurized film [51].
The usual Tauc model was utilized to calculate the optical band gap E g using the relation [18], where A is a constant depends on the probability of the transition and hν is the photon energy (eV).Figure 5(c) shows the variation in (αhν) 2 as a function of the photon energy for the as-deposited and sulfurized films.The band gap of as-deposited and sulfurized Sb 2 S 3 films is mentioned in table 5.As seen in table 5, increasing the sulfurization temperature to 300 °C decreases the direct band gap from 1.3-1 eV.This decrease in bandgap can be attributed to an increase in crystallite size as we see in table 3 [38].At 350 °C, the band gap increases to 2.6 eV.This increase is because the decrease in crystallite size.For the S-400, we have obtained the desired bandgap of 1.7 eV.The improvement in crystalline quality and growth in grain size, which is supported by the XRD and  SEM results, might be viewed as the reason for this bandgap [34].Quantum confinement restricts the motion of electrons and holes in semiconductor materials at the nanoscale, resulting in the formation of distinct energy levels known as quantum dots or quantum wells.These quantized levels result in a narrower band gap when the crystallite size is less than the exciton Bohr radius.As the crystallite size increases, these levels merge into bulk energy bands, decreasing the band gap.Quantum confinement is vital in shaping the electronic properties of nanoscale materials [52].From the SEM we can see that the sample S-400 was fully formed grains with a band gap of 1.7 eV which is close to the expected band gap of Sb 2 S 3 thin films [53].Further, the band gap increased from 1.7-2.55eV with the rise of sulfurization temperature to 450 °C, which might be due to the decomposition of the sample S-450 as observed in table 3 and SEM images.Electronic band gap of all possible structures has been done to calculate partial density of states using HSE06 approximation and in turn to see the structural stability and match the experimental bandgap of Sb 2 S 3 .It is observed that in case of vacancy (S-rich and S-poor) in Sb 2 S 3 , transition between conduction band and valence band are also favourable, which was absent in case of other sample (see figure 6).

Photoluminescence analysis
The photoluminescence method was employed to analyze the defects.Figures 7(a)-(c) show the photoluminescence spectra of the as-deposited and sulfurized films at excitation wavelengths of 360, 420, and 610 nm, respectively.
At an excitation wavelength of 360 nm, the emission peaks around the 470 nm range (refer to figure 7(a)) for sulfurized samples, indicating blue emissions.The presence of defects or deep trap states, often attributed to sulfur vacancies, is typically associated with this blue emission [38,54].The broad emission observed for S and S-450 samples may be attributed to the presence of sulfur vacancies, as indicated by compositional analysis.When the excitation wavelength is set to 420 nm, emission peaks are observed with blue emission in the 460 nm range (see figure 7(b)).This blue emission is indicative of donor-acceptor ion transitions, often associated with defects or deep trap states [49,55,56].For an excitation wavelength of 610 nm, emission peaks are observed with red emission in the 725 nm range (refer to figure 7(c)) [39].The significant red shift observed suggests the presence of trap states and band-edge states, aligning with findings reported in other literature [57,58].
Elevating the temperature of sulfurization causes changes in the stoichiometric ratio and crystallite size, ultimately resulting in variations in the intensity of emission.Here the donor ion represents Sb +3 and the acceptor ion represent S −2 .Based on this, we classified the films into two types: antimony-rich and sulfur-rich films.Sb-rich films exhibit three electron trap states called donor defects, whereas S-rich films exhibit two-hole trap states called acceptor defects [20,40].The defects were Sb interstitial (Sb i ), S vacancy (V S ), Sb S antistite, Sb vacancy (V Sb ), and S Sb antisite defects.The increase in V S was initially caused by S deficiency in the Sb-rich Sb 2 S 3 .Consequently, extra Sb preferentially fills the V S rather than reaching the interstitial location.This is because Sb S has a significantly lower formation energy than Sb i .Therefore, Sb-rich Sb 2 S 3 has a higher prevalence of V S and Sb S defects.The S-rich Sb 2 S 3 and S atoms penetrate the lattice to fill the V S .Because of their higher formation energies in an S-rich environment, Sb S and Sb i defects were repressed during their formation.Subsequently, the S-rich state stimulates the synthesis of significant amounts of V Sb because of the decreased formation energy.To preserve the structural stability, a few S atoms may take up residence in the V Sb to generate the S Sb antisite [40].

Conclusion
Phase pure Sb 2 S 3 thin films were synthesized using a two-stage process by combining thermal evaporation and chemical vapor deposition.The impact of sulfurization temperature on the growth properties of Sb 2 S 3 thin films was discussed.Studies on x-ray diffraction revealed that the film had a weak crystalline quality when it was first deposited.On the other hand, the films' sulfurized at 300, 350, 400, and 450 °C enhanced their crystallinity.Interestingly, the film sulfurized at 400 °C showed a pure orthorhombic phase of Sb 2 S 3 .The atomic ratio of the as deposited Sb 2 S 3 thin film was 1:1.However, films sulfurized at 300 and 450 °C displayed a ratio of 3:2, and films sulfurized at 400 °C displayed the expected 2:3 stoichiometric ratio of Sb 2 S 3 , confirming the prominent role of sulfurization in stoichiometry.The morphological analysis showed that sulfurized films produced more nanorods and showed greater grain expansion.Sulfurization was observed to cause a considerable variation in the film's band gap; the film sulfurized at 400 °C showed an expected bandgap of 1.7 eV.Photoluminescence studies show that the as-deposited film exhibits broad emission due to sulfur deficiency.DFT calculation supports the stability of structural and experimental optical bandgap for Sb 2 S 3 .The sulfurized films showed narrow emission with defects, but the film sulfurized at 400 °C showed fewer defects with the formation of Sb 2 S 3 .This study involving both experimental and theoretical investigation demonstrates that Sb 2 S 3 films synthesized at a sulfurization temperature of 400 °C have suitable structural, morphological, and optical properties for photovoltaic applications.

Figure 1 .
Figure 1.(a) Representative EDS image of S-400 sample and (b) graphical representation of the atomic percentages of as-deposited and sulfurized antimony sulfide thin films.

Figure 2 .
Figure 2. X-ray diffraction patterns of as-deposited and sulfurized Antimony sulfide thin films.

Figure 4 .
Figure 4. SEM images of the as-deposited and sulfurized Antimony sulfide thin films.

Figure 6 .
Figure 6.Partial density of states of (a) Pure Sb 2 S 3 (b) S-rich Sb 2 S 3 and (c) S-poor Sb 2 S 3 .

Table 1 .
Atomic percentages of the as-deposited and sulfurized antimony sulfide thin films.

Table 2 .
Estimation of formation energy with different concentration of Selenide (Sb) and Sulfur (S) in Antimony sulfide thin films.

Table 3 .
Structural parameters obtained from the XRD for as-deposited and sulfurized Antimony sulfide thin films.

Table 4 .
Vibration modes and peak position of as-deposited and sulfurized Antimony sulfide thin films.

Table 5 .
Band gap of as-deposited and sulfurized Antimony sulfide thin films.