E ﬀ ect of Thioacetamide Concentration on the Preparation of Single-Phase SnS and SnS 2 Thin Films for Optoelectronic Applications

: Eco-friendly tin sulﬁde (SnS) thin ﬁlms were deposited by chemical solution process using varying concentrations of a sulfur precursor (thioacetamide, 0.50–0.75 M). Optimized thioacetamide concentrations of 0.6 and 0.7 M were obtained for the preparation of single-phase SnS and SnS 2 ﬁlms for photovoltaic absorbers and bu ﬀ ers, respectively. The as-deposited SnS and SnS 2 thin ﬁlms were uniform and pinhole-free without any major cracks and satisfactorily adhered to the substrate; they appeared in dark-brown and orange colors, respectively. Thin-ﬁlm studies (compositional, structural, optical, and electrical) revealed that the as-prepared SnS and SnS 2 ﬁlms were polycrystalline in nature; exhibited orthorhombic and hexagonal crystal structures with (111) and (001) peaks as the preferred orientation; had optimal band gaps of 1.28 and 2.92 eV; and exhibited p- and n-type electrical conductivity, respectively. This study presents a step towards the growth of SnS and SnS 2 binary compounds for a clean and economical power source.


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Various approaches including physical and chemical techniques have been explored to deposit 53 both SnS and SnS2 thin films. Among these techniques, the chemical solution process (CSP) or 54 chemical bath deposition (CBD) offers more inexpensive and flexible depositions. The preparation of 55 single-phase SnS and SnS2 films by CSP depends on the release and reaction rate of sulfur ions in the 56 reaction solution, which can be controlled by the selection of a suitable sulfur precursor at a 57 concentration, which affects not only the formation of the phase but also the growth rate, thickness, 58 and other physical properties of the product films. Therefore, optimizing the concentration of the 59 sulfur precursor is highly preferable to obtain high-quality SnS and SnS2 films for the desired applications. Previous research suggests that thioacetamide (TA) and sodium thiosulfate have been mainly used as sulfur precursors since 1987 ( Figure 1). Of these, thioacetamide is more preferable 62 because it works under both acidic and alkaline bath conditions.

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Therefore, a set of SnS thin films were deposited by CSP using varying thioacetamide

Experimental Section
The deposition procedure for SnS and SnS 2 films by CSP is schematically presented in Figure 2, and the process has been described in detail in our previous reports [35,36]. Analytical-grade tin chloride (SnCl 2 .2H 2 O, 0.1 M) and tartaric acid (C 4 H 6 O 6 , 1.2 M) were used as a tin source and complexing agent, respectively, and 10 mL of thioacetamide (C 2 H 5 NS, 0.50-0.75 M) was used as the source material for the S ions. The as-prepared thin films were cleaned using deionized (DI) water and dried in a vacuum oven. By the following equations, the kinetics of SnS and SnS 2 films can be understood.

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The compositional, structural, optical, and electrical characteristics of the as-deposited films  In an aqueous tin chloride solution, the complexation of tartaric acid is The free Sn 2+ ions are slowly released by the dissociation of tin tartrate ions (Sn(C 4 H 6 O 6 ) 2+ ) in a controlled way, as below: The hydrolysis of thioacetamide can produce S 2− ions by When the thioacetamide concentration is changed, multiphase or other single-phase films can be formed by the following reactions: The compositional, structural, optical, and electrical characteristics of the as-deposited films were analyzed using an X-ray photoelectron spectrometer (Waltham, MA, USA) (XPS: A VG Multilab 2000; Al Kα radiation = 1486.6 eV), X-ray diffractometer (Almelo, Overijssel, Netherlands) (XRD: PANalytical X'Pert PRO MPD, Malvern Panalytical with Cu Kα radiation, λ = 1.5406 Å), UV-Vis-NIR spectrometer (Santa Clara, CA, USA) (Aglient), and Hall measurement system (Anyang-city, Gyeonggi-do, South Korea) (ECOPIA HMS-3000VER), respectively.

Results and Discussion
All the deposited tin sulfide films were confirmed to be satisfactorily adherent. The films appeared in dark-brown and orange shades at TA concentration ranges of 0.50−0.65 M and 0.70−0.75 M, respectively. The effect of TA concentration on the elemental composition and chemical states of elements was investigated by XPS. The XPS survey spectrum indicated that the deposited films were mainly composed of the Sn 3d doublet along with the S 2p peaks, representing the existence of the elements Sn and S. A high-resolution scan of the Sn 3d and S 2p peaks is shown in Figure 3a,b, respectively.
The Sn 3d doublet in the films grown with lower TA concentrations (0.5-0.6 M) showed two strong peaks at about 485.6 and 494.1 eV that were related to Sn 3d 5/2 and Sn 3d 3/2 , respectively, which are the characteristics of Sn 2+ in SnS. The Sn 3d 5/2 and Sn 3d 3/2 peaks were observed to expand at a TA concentration of 0.65 M, and the deconvolution of these peaks reveals the existence of two humps in each peak, linked to the elevated intensity for Sn 2+ and low intensity for Sn 4+ , indicating the coexistence of secondary Sn 2 S 3 along with SnS. As the TA concentration was increased further (0.7 M), the spectra showed the binding energies for Sn 3d 5/2 and Sn 3d 3/2 as 486.7 (487.2) and 494.9 (495.1) eV, respectively [37,38], revealing the presence of Sn 4+ produced in the SnS 2 phase. Beyond this concentration (0.75 M), the deconvolution of the broadened Sn 3d doublets suggests the presence of low intense Sn 2+ and high intense Sn 4+ , confirming the formation of the Sn 2 S 3 secondary phase along with SnS 2 . The high-resolution XPS spectrum of S 2p for the as-prepared films exhibited single peaks related to S 2− -Sn 2+   In general, the structural properties of the thin films were observed to be highly influenced by the compositional variations in the film. In this study, the structural behavior of SnS films grown using different TA concentrations was analyzed using XRD. The diffraction patterns (Figure 4) confirmed the polycrystalline nature of the as-prepared films; some of these showed various binary phases, depending upon the TA concentration: the films deposited in the TA concentration range of 0.5−0.6 M exhibited a strong peak at 2θ = 31.6 • , which is related to the (111) plane, along with other minor peaks at around 2θ = 27.1 • , 38.8 • , and 51.1 • , which were assigned to the diffraction from the (101), (131), and (112) planes of orthorhombic SnS (JCPDS Card No. 39-0354) [39], respectively. Moreover, the crystalline state improved with an increase in the TA concentration. As the TA concentration was increased to 0.55 M, the film exhibited additional planes of (130) and (260) related to the Sn 2 S 3 binary phase (JCPDS Card No. 14-0619) [40]; as the TA concentration was further increased to 0.7 M, the film exhibited different diffraction planes of hexagonal SnS 2 (JCPDS Card No. 23-0677) [41] with (001) reflection at 15.02 • as the preferred orientation; and beyond this TA concentration (0.75 M), the film additionally exhibited (130), (260), and (540) planes of the secondary Sn 2 S 3 phase along with SnS 2 . Therefore, the films deposited under TA concentrations of 0.6 and 0.7 M exhibited good crystalline and pure orthorhombic SnS (111) and hexagonal SnS 2 (001) phases, respectively. Thus, these results are observed to agree well with the XPS observations.  The average crystallite size, D, of the deposited films was estimated using the Debye-Scherrer formula [42] using the strong (1 1 1) and (0 0 1) peaks of the deposited films with a TA concentration in the range of 0.5-0.65 M and 0.7-0.75 M, respectively. It was found that the crystal size varied from 15 to 29 nm with the change of TA concentration from 0.5 to 0.65 M. In addition, a D of 26 nm was obtained for the film deposited at a concentration of 0.7 M, and it was decreased to 24 nm for a further increase in TA concentration to 0.75 M. Therefore, the D of the SnS (TA = 0.6 M) and SnS 2 (0.7 M) films was obtained as 29 nm and 26 nm, respectively. The surface morphology and surface average grain size of both SnS (TA = 0.6 M) and SnS 2 (0.7 M) films were obtained from SEM images ( Figure 5). Both the films showed compact morphology without pin-holes and cracks. The SnS film showed almost elliptical-shaped grains with an average grain size of~295 nm, whereas the SnS 2 film showed shuttle-shaped grains of nearly 272 nm in size.  The optical studies of the films deposited at different TA concentrations were carried out using the UV-Vis-NIR photo spectrometer. The films deposited at TA concentrations in the range of 0.5-0.6 and 0.7 M exhibited a sharp absorption edge in the absorption spectra, whereas at TA concentrations of 0.65 and 0.75 M, the films exhibited a nonlinear fall in the absorption edge, indicating the potential presence of the secondary phase of Sn 2 S 3 along with either the SnS or SnS 2 phase. The band gap of the prepared films was estimated from the Tauc plots ((αhν) 2 vs. hν) [43], as shown in Figure 6. The films deposited at concentrations of 0.6 and 0.7 M are observed to exhibit band gaps of 1.28 and 2.92 eV, respectively, corresponding to the energy gaps of the SnS and SnS 2 phases. Furthermore, the films prepared at a 0.65 M concentration exhibited two different values of band gap-specifically, ∼0.94 and ∼1.34 eV-that are attributed to the presence of the Sn 2 S 3 secondary phase along with SnS; on the other hand, at a concentration of 0.75 M, the films exhibited band gaps of ∼1.11 and ∼2.53 eV, corresponding to the presence of the Sn 2 S 3 secondary phase and SnS 2 phase, respectively. Figure 7 shows the variation of the refractive index (n) and extinction coefficient (k) as a function of the TA concentration. The evaluated n value is observed to first increase and then decrease with increasing TA concentrations up to 2 M, whereas the calculated k value exhibits a reverse variation trend. The films grown at TA concentrations of 0.6 and 0.7 M exhibit n values of 3.24 and 2.63 and k values of 0.13 and 0.61, respectively. The obtained n and k values in the present study are in good agreement with the reported values [2,44].
Electrical parameters such as conductivity type, resistivity, mobility, and carrier density of the films deposited at various TA concentrations were determined using Hall effect studies.

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Electrical parameters such as conductivity type, resistivity, mobility, and carrier density of the

Conclusions
In this study, SnS thin films were prepared using chemical solution process by varying the concentration of TA (0.5-0.75 M), while the other growth parameters were maintained constant. The films prepared at TA concentrations of 0.6 and 0.7 M exhibited Sn/S atomic ratios of 0.98 and 0.51, confirming the formation of single SnS and SnS 2 phases, respectively. The XRD studies showed that all deposited films were polycrystalline in nature and exhibited various binary phases depending upon the TA concentration. The films deposited at a TA concentration of 0.6 M exhibited a pure orthorhombic SnS phase, whereas those at 0.7 M concentration exhibited a completely hexagonal SnS 2 phase. In addition, they showed the direct optical band gap of 1.28 and 2.92 eV with p-and n-type conductivities, which confirmed the suitability of these films not only for application as solar absorbers and buffers, respectively, but also for other applications.