Document Type : Research Paper
Author
Faculty of Physical Education and Sports Sciences, University of Kerbala, Iraq
Abstract
Keywords
INTRODUCTION
Researchers have focused on binary semiconductor compounds like SnS, SnSe, Cu2S, ZnS for advanced solar cell applications [1,2]. SnS semiconductor belonging to the IV-VI group, is engaged in the field of solar energy, exhibiting dual conductivity types (both p-type and n-type) in need of tin, sulfur content, and annealing temperature [3,4]. With a direct bandgap of (∼1.3) eV and an indirect bandgap ranging from 1.0 to 1.2 eV [5,6], SnS holds promise for efficient solar cells. Its orthorhombic crystal structure features each Sn atom surrounded by six S atoms, forming short and long SnS bonds with interatomic spacings of 2.7 Å and 3.4 Å, respectively. [7]. Several deposition techniques, among them chemical spray pyrolysis, have been employed to fabricate SnS films, both in their pristine form and with intentional doping [8The chemical spray pyrolysis method was chosen to deposit SnS films due to its simplicity and cost-effectiveness [9-10]. Critical parameters, such as base temperature, precursor ratios, solvent type, and spray rate, part in the process of deciding the physical properties of the films [11-12]. Among these factors, substrate temperature stands out as the most influential parameter.[13]. In found in this research, SnS and Ag-doped SnS films were produced via chemical spray pyrolysis, a cost-effective and straightforward technique, to investigate the physical properties of SnS:Ag.
MATERIALS AND METHODS
Reliable thin films of SnS and SnS:Ag were fabricated through a lab-built spray pyrolysis coating process. Precursors, consisting of 0.1 M SnCl2•2H2O and thiourea dissolved in deionized water, were utilized, and Ag doping levels (0%, 2%, 4%) were introduced by incorporating silver trichloride (AgCl3) in isopropyl alcohol. The optimized deposition conditions included a substrate temperature of 450°C, a spout-base distance of 29 cm, a 9-second spraying period followed by an 85-second interval, and a spray rate of 5 mL/min. The film thickness, determined through a weighing method, was measured at 340 ± 25 nm. UV-visible spectroscopy captured optical transmittance spectra within the range of 300–900 nm. Film structure was examined through XRD, and the surface was characterized using AFM.
Characterization
XRD
Fig. 1 presents the XRD spectra, indicating the characteristics of the deposited films. The broad reflectance of XRD peaks in the as-prepared thin films suggests a small particle size. Notably, peaks at 22.25°, 27.24°, 48.78°, and 67.75° correspond to (110), (021), (211), and (080) planes, respectively, consistent with SnS and in accordance with ICDD card no. (39-0354), highlighting a strong presence of the (021) plane. The heightened Bragg peaks suggest improved crystallinity, potentially attributed to the introduction of nucleating centers through doping [14,15]. Crystallite size calculations were conducted using Scherer’s equation [16,17].
The pure SnS sample displays a crystallite size of 12.98 nm, whereas the 2% Ag-doped SnS thin films show a particle size of 13.86 nm, and the 4% Ag-doped SnS thin film has a crystallite size of 14.87 nm. The diversity in crystallite size is likely linked to lattice dilatation [18].
Dislocation density (δ) and microstrain (ε) were determined using Eqs. 2 and 3 [19, 20]:
The value of δ was 4.52×1015 lines/m2 and ε was 23.31×10-4 for Ag content of 4 at%. (Table 1).
AFM Analysis
AFM was employed to assess the topography of both SnS and Ag-doped SnS. The 2-D images illustrate film surfaces, providing insights into particle size (Aps) and surface roughness (RMS). Fig. 3 presents AFM images of SnS analyzed over a 78nm x 78nm area. The films exhibit a uniform morphology with an irregular surface structure, featuring varying surface roughness and densely packed grains. The average particle size (Aps) is larger for films prepared without Ag doping. In Ag-doped films, Aps decreases to 52.36nm with a 2% doping concentration, and further increments to 4% lead to a decrease to 47.46nm. These findings align with references [21,22]. Fig. 3 and Table 2 present the AFM parameters.
Optical Analysis
Fig. 4 shows transmittance (T%) curves, indicating highly transparent films with an average value surpassing 70% in the visible range. With increasing Ag concentration, transmission decreases, and the absorption edge shifts towards higher wavelengths.[23].
To calculate the absorption coefficient (α) at film thickness (d) Eq.4. [24, 25] are used:
The values of α is offered in Fig. 5. These values are varied between 1×104cm-1and 4×104 cm-1.
The energy band gap (Eg) was determined using the derived relationship [26, 27].
The equation includes a proportionality constant represented by A. Fig. 6 depicts a decrease in bandgap (Eg) values from 1.46 eV to 1.36 eV for films doped with 2 at% Ag. A slight additional decrease is observed with higher doping, attributed to the band shrinkage effect resulting from an increase in carrier concentration.[28].
The extinction coefficient (k) was calculated using Eq. 6 [29- 30]:
The decrease in α with an increase in film doping content is associated with fluctuations in the crystallite size of the films [31].
In Fig. 7, the extinction coefficient (k) demonstrates a significant decline as the wavelength ranges from 450 nm to 600 nm. The low values of (k) suggest a film surface that is smooth and uniform [32].
The refractive index (n) was determined at various wavelengths using optical reflectivity (R) and the relationship given by Eq. 7 [33-34].
The variation of (n) is shown in Fig. 8. Due to Ag doping, a clear shift in maximum value of (n) to the higher wavelength region that can be easily noted [35].
CONCLUSION
SnS: Ag thin films were synthesized by spray pyrolysis, all films were polycrystalline also, the crystallinity was enhanced with doping ratio of 4 %. The band gap was seen in the area of (1.46-1.36) eV. Diffraction peaks were obtained. These peaks were confirming to orthorhombic SnS was observed with identified (021) plane. Average Particle size and surface roughness are studied by AFM and found to be decreased due to silver doping.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.
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