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Article

Substrate-Dependent Characteristics of CuSbS2 Solar Absorber Layers Grown by Spray Pyrolysis

1
Department of Physics, Faculty of Basic Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran 14115-175, Iran
2
Department of Mathematical, Physical and Computer Science, University of Parma, Parco Area delle Scienze 7/A, 43124 Parma, Italy
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(6), 683; https://doi.org/10.3390/coatings15060683
Submission received: 20 April 2025 / Revised: 3 June 2025 / Accepted: 4 June 2025 / Published: 6 June 2025

Abstract

Copper antimony sulfide (CuSbS2) is an affordable and eco-friendly solar absorber with an optimal bandgap and high absorption coefficient, and it stands out as a promising candidate for thin-film solar cells. This study investigates the effects of indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and glass substrates on the microstructural, morphological, and optical properties of CuSbS2 (CAS) layers synthesized via spray pyrolysis. X-ray Diffraction (XRD) and Raman spectroscopy analyses revealed that CAS phases formed on ITO and FTO substrates exhibited a phase composition without additional copper phases. However, the CAS layer on glass contained a copper sulfide (CuS) phase, which can be detrimental for solar cell applications. Furthermore, the influences of the substrate morphology and contact angle on the growth mechanisms of CAS layers was examined, highlighting the relationship between the substrate micromorphology and the resultant film characteristics. Advanced image processing techniques applied to Atomic Force Microscopy (AFM) images of the substrate surfaces facilitated a comprehensive comparison with the surface characteristics of the CAS films grown on those substrates. Field Emission Scanning Electron Microscopy (FESEM) indicated that CAS layers on ITO possessed larger grains than FTO, whereas those on FTO exhibited lower roughness with a more uniform grain distribution. Notably, the optical properties of the CAS layers correlated strongly with their microstructural and morphological characteristics. This work highlights the critical influence of substrate choice on the growth and characteristics of CAS layers through a comparative analysis.

Graphical Abstract

1. Introduction

Clean and affordable energy needs have become one of the most essential requirements for sustainable development [1,2,3]. Thin-film solar cells offer significant advantages in meeting this demand [4,5,6]. Among different materials and compounds used in thin-film solar cells, gallium arsenide (GaAs), copper indium gallium selenide (CIGS), and cadmium telluride (CdTe) are recognized for their high efficiency [7]. However, these materials face certain challenges, including the scarcity and cost of raw materials, as well as the presence of toxic elements [8,9,10]. Consequently, to mitigate these drawbacks, research on alternative absorbers is currently underway [11]. Among the absorber materials under investigation, copper antimony sulfide or CuSbS2 (CAS) stands out due to its eco-friendliness, abundance, and cost-effective fabrication process [12]. In addition, this lead-free and inorganic p-type compound [13] has a suitable direct band gap of approximately 1.5 eV and an absorption coefficient higher than 105 cm−1 in the visible light range of the solar spectrum [14]. These properties make CAS an interesting candidate for use as an absorber in thin-film solar cells [15,16]. In addition, a recent study showed that copper antimony selenide (CuSbSe2), which is similar to CAS, exhibits delocalized free carriers that improve mobility and diffusion lengths, as demonstrated by terahertz spectroscopy and temperature-dependent mobility studies. Its favorable layered structure, strong quasibonding interactions, and balanced dielectric properties minimize carrier localization, making it a stable and efficient material for photovoltaic applications [17]. CAS possesses an orthorhombic crystal structure and exhibits Raman active modes. Notably, within the CAS crystal structure, two designated sites for anions are occupied by two S atoms. All the atoms are located in 4c sites and each atom contributes to the gamma point for the Pnma space group [18]. There are several methods for the fabrication of CAS layers, such as chemical bath [19], spin coating [20], electrodeposition [21], chemical vapor deposition [22], thermal evaporation [23], sputtering [24], and spray pyrolysis [25]. The spray pyrolysis method provides the benefit of growing CAS layers on scalable substrates without needing vacuum technology. For this reason, this technique has become one of the most cost-effective methods for synthesizing CAS layers, enabling large-scale production [26]. Obtaining pure CAS is quite challenging because it tends to form additional binary and ternary phases like CuS, Cu2S, Sb2S3, Cu3SbS4, Cu3SbS3, and Cu12Sb4S13 [27]. Some of these extra phases that contain copper are detrimental to solar cells. To avoid this, stoichiometric or copper-poor layers are preferred in solar cell applications [28]. CAS thin-film solar cells can be classified into two configurations: substrate and superstrate. In the superstrate configuration, the CAS layer is deposited on top of a sequence of transparent layers, typically consisting of glass covered with a transparent conductive oxide (TCO), followed by an electron transport layer (ETL). This arrangement allows light to pass through the glass and these layers before reaching the CAS film [29,30]. In the substrate configuration, the CAS layer is deposited onto a substrate, such as glass coated with a thin molybdenum (Mo) layer or conductive oxides, which provides mechanical support and influences the film’s properties [24,31]. When Mo is used as the back contact, it renders the substrate opaque, requiring light to enter from the opposite side of the solar cell. Alternatively, the use of a transparent back contact enables the development of bifacial solar cells, allowing light absorption from both sides [32,33].
The highest efficiency achieved for CAS in the substrate configuration was reported by Banu et al., which was approximately 3.22% [34]. In substrate configuration, back contacts like Mo [35], fluorine-doped SnO2 (FTO) [20], and indium tin oxide (ITO) [36] can be used. The interfacial properties of the back contact and CAS layer are critical, as they can significantly influence the transport of electron/hole carriers [37]. Among the back contacts used for the substrate configuration in CAS solar cells, Mo has been used more frequently [31,34,35,38,39,40,41,42] than FTO [20] and ITO. Indeed, Mo possesses high conductivity and chemical/mechanical stability, making it an attractive material to be used as a back contact [43]. One reason for the low efficiency of CAS solar cells that use Mo as back contacts can be attributed to the formation of voids and pinholes in the CAS layers [44]. Furthermore, it has been reported that the CAS-Mo interface can create a nearly Schottky contact due to the larger work function of CAS compared to Mo [37,40]. Welch suggested that MoOx with a deep work function of 6.6 eV can better collect the majority carriers rather than Mo [40]. There are also some issues related to the adhesion of the CAS to Mo, which necessitate the deposition on Mo-chalcogenide layers. Therefore, it is worth investigating other back contacts for CAS-based solar cells in substrate configurations, such as FTO and ITO, which have been less frequently studied. In this study, the microstructural and morphological properties of the CAS layers grown by spray pyrolysis on different substrates (which can also serve as back contacts) are discussed using X-ray Diffraction (XRD), Raman spectroscopy, and Field Emission Scanning Electron Microscopy (FESEM) analyses. Technical image processing is employed for the in-depth evaluation of surface statistics. Technical image processing is a highly effective method for surface characterization and permits us to gain valuable insights that are not accessible through traditional surface analysis methods [45,46]. This technique, in particular, enables the exploration of FESEM images, which can provide a better understanding of the correlation between surface statistics and the physical properties of the material [47]. Moreover, the surface characteristics of each substrate were evaluated by an Atomic Force Microscopy (AFM) analysis and contact angle measurement to understand the relationship between the micromorphology of the substrates and the quality of the CAS thin films grown via the spray pyrolysis technique.
This study establishes the relationship between the micro-morphological and microstructural characteristics of substrates and CAS layers grown using the spray pyrolysis technique, underscoring the importance of optimizing substrate properties for achieving the desired performance. The findings have the potential to open new avenues for employing innovative back contact materials in the fabrication of CAS solar cells, a topic of significant relevance to both academia and industry. A key novelty of this work lies in the comparative analysis of CAS layers grown on different substrates, demonstrating how substrate selection affects growth patterns, microstructure, and micro-morphology. This investigation highlights the pivotal role of the substrate in defining the structural and functional properties of CAS layers, which are crucial for the development of high-performance, substrate-configured CAS-based thin-film solar cells.

2. Experimental Details

The growth process was performed using the optimized growth conditions reported in our previous study [48]. In this study, we investigated the effect of using different substrates, including glass, FTO- and ITO-covered glass, on the microstructural and micromorphological properties of CAS layers, which are named CAS-glass, CAS-FTO, and CAS-ITO, respectively. The FTO and ITO substrates were obtained from commercial sources (manufactured by Nano Gostar Sepahan (NGS), Esfahan, Iran), with thicknesses of approximately 350 nm and 200 nm, respectively.
The substrate temperature was maintained at 310 °C, while the solution was sprayed at a rate of 17.5 mL/min. The separation between the nozzle and the substrate was measured at 30 cm. Compressed air, which was filtered and kept at a pressure of 3 kg/cm2, was employed to atomize the experimental solution. The glass substrate was the slide for an ordinary microscope. The analyses performed in this study to characterize the microstructural and crystallographic properties of layers included Raman spectroscopy (Nicolet Almega Raman spectrometer (λex = 532 nm), Thermo Scientific, Waltham, MA, USA), and XRD (X’Pert MPD X-ray diffractometer with Cu kα radiation (λ = 0.15406 nm), Philips, Amsterdam, The Netherlands). The micromorphology of grown CAS layers was evaluated by FESEM (S-4160, HITACHI, Tokyo, Japan). Technical image processing was employed to perform a comprehensive analysis of the FESEM images of CAS layers deposited on various substrates, employing the open-source Gwyddion 2.63 software [49]. This study focused on evaluating the average grain size distribution and micro-morphological properties of the CAS layer surfaces using the texture aspect ratio (Str) and fractal dimension (FD) as key parameters. Following the methodologies outlined in Ref. [46], approximate grain sizes and their distributions were estimated by considering the grain cross-sectional areas as disks. Specifically, an equivalent disk diameter was calculated for 20 distinct grains, and the resulting data were visualized using box-and-whisker plots to assess the grain size distribution for each sample. While this method does not provide precise grain size measurements, it effectively captures the variation in average grain diameters, enabling a quantitative comparison across the sample.
To investigate the relationship between the micromorphology of the substrate and CAS layer, an AFM analysis (Veeco, CPII, Plainview, NY, USA) was performed on glass, ITO, and FTO Additionally, the surface properties of the substrates were evaluated through contact angle measurements (GAS 20, Jikan, Tehran, Iran).
The optical absorption spectra of the CAS layers grown on each substrate were analyzed using a UV-Vis-IR spectrophotometer (UV–Vis–NIR T80+, PG Instruments, Wibtoft, England), and the bandgap energies were determined using the Tauc equation [50]. This comprehensive approach provided valuable insights into the physical and optical characteristics of the substrates and their influences on the CAS layers.

3. Results and Discussion

3.1. XRD Analysis

The XRD patterns of the CAS layers grown on different substrates are shown in Figure 1. The results reveal that the CAS layers exhibit distinct crystal planes corresponding to an orthorhombic structure, as indexed in the standard diffraction file (JCPDS 44-1417).
The XRD pattern of the film grown on the glass substrate shows the presence of the copper sulfide (CuS) crystalline phase (JCPDS 06-0464) alongside Sb2S3 (JCPDS 42-1393). However, the XRD patterns of the layers grown on ITO and FTO substrates exhibit the CAS phase, with some peaks associated with Sb2S3, and no detectable presence of the CuS phase. An important distinction lies in the electrical behavior of CuS and Sb2S3 materials within photovoltaic devices. CuS is known for its high conductivity, which can be problematic if it accumulates at the grain boundaries of the absorber layer, potentially leading to short-circuiting of the p-n junction. In contrast, Sb2S3 has a much lower conductivity, making its presence far less concerning in this context, as it does not pose the same risk of electrical failure. This findings suggests that ITO and FTO substrates serve as more suitable platforms for achieving the desired CAS composition without copper-rich phases, a critical aspect for optimizing photovoltaic performance [28]. A similar observation was reported by Ashfagh et al., who studied the impacts of different substrates on the fabrication of the copper zinc tin sulfide (CZTS) layers [51]. It was observed that the deposition of CZTS on ITO resulted in a reduction of secondary phases compared to soda lime glass [51]. The full width at half maximum (FWHM) of the peak corresponding to the (111) plane, along with the crystal size calculated from the Scherrer equation [52] for all CAS samples are presented in the Table 1.
Based on the values presented in Table 1, the crystal size of the CAS layer grown on glass is smaller than on other samples.

3.2. Raman Spectroscopy

Theoretical studies indicate that the atoms in the unit cell of CAS occupy 4c Wyckoff sites [53]. Consequently, each atom contributes to the gamma point for the Pnma space group. With a total of 16 atoms in the unit cell, this results in 48 allowed vibrational modes at the gamma point. After excluding the three acoustic modes, the 45 optical modes are presented as follows, according to Formula (1):
8 A g x 2 , y 2 , z 2 + 4 A u + 4 B 1 g x y + 8 B 2 g x z + 4 B 3 g y z + 7 B 1 u x + 3 B 2 u y + 7 B 3 u z
where the Raman active modes are Ag, B1g, B2g, and B3g. Therefore, 24 Raman active modes are obtained. However, according to experimental studies, only 19 peaks are recognized [54].
The most intense peak corresponding to the symmetric Sb–S stretching mode in the SbS3 pyramid is reported to occur in the range of 331–336 cm−1 [8,24,55,56,57,58,59]. The Raman spectra of the CAS layers deposited on different substrates are shown in Figure 2.
These spectra are depicted in the range of 150 to 500 cm−1, which is the typical range where CAS active Raman modes have been experimentally reported [34,48,56]. Figure 2 illustrates that the main peaks of 335 or 331 cm−1 corresponding to CAS can be observed in all samples [24,60]. This can confirm the XRD result that shows the formation of CAS on all substrates.

3.3. FESEM Microscopy and Technical Image Processing

To examine the micro-morphological properties of the CAS layers, an FESEM analysis was performed, as shown in Figure 3a–c.
Through the initial inspection of Figure 3, an obvious difference in the micromorphology of CAS layers on different substrates is observed. Figure 3a–c illustrate that the CAS layer on glass has a pseudo-hexagonal shape, while on FTO, it appears as relatively flat grains, and on ITO, semispherical grains are formed. It has to be noted that in lower-magnification FESEM images, such uniformity in shape may not be preserved. The surface micromorphology provided in the FESEM images (Figure 3a–c) was studied more precisely using technical image processing in Figure 3d,e. Using masks and filters, the main background grains and pores were separated, as illustrated in the Supplementary Materials (Figure S1).
After the identification of the grain boundaries (Figure S1), the equivalent disk radius (EDR) was calculated for each sample, with the corresponding size distribution provided in the Supplementary Materials (Figure S2). Figure S2 shows that the distribution of CAS grain size on FTO was more uniform compared to other samples. In addition, the mean values of the EDR decreased as the substrate changed from glass (0.901 μm) to ITO (0.831 μm) and further to FTO (0.188 μm). The grain size of the CAS layers grown on glass was larger than those on ITO and FTO. A similar observation was reported by Park et al., who compared the growth of CIGS layers on different substrates. They found that the CIGS layer formed on glass exhibited larger crystal grains than those grown on FTO substrates [61].
To analyze the roughness of CAS layers grown on different substrates, the FESEM images were evaluated. As shown in Figure 3d, the sample grown on FTO exhibited a lower RMS roughness compared to the layer grown on ITO. This variation can be attributed to the distinct physical and chemical properties of the substrates, which will be discussed in detail in the next section. Additionally, Figure 3e illustrates that the fractal dimension was higher for the sample grown on the FTO substrate. A greater FD indicates a higher degree of self-similarity in the surface structure [62]. This suggests that the CAS layer deposited on FTO exhibits a more complex and self-similar surface morphology compared to the layers grown on the other substrates.
The texture aspect ratio (Str) is a parameter that quantifies the isotropy of a surface [63], offering insights into the uniformity of the surface texture and the degree of lay (directionality of texture) across different directions [64]. It has to be noted that the Str values range between zero and one, in which values closer to zero signify an anisotropic surface and values approaching one represent an isotropic surface [63,64]. In this study, the sample grown on the ITO substrate exhibited the lowest Str value, indicating a more anisotropic surface micromorphology compared to the other samples. This finding highlights the influence of the substrate properties on the directional characteristics of the CAS layer’s surface texture.

3.4. AFM Analysis and Contact Angle Measurements of the Substrates

To investigate the influence of substrate micromorphology on CAS layers, AFM and contact angle measurements were performed on bare glass, FTO-coated glass, and ITO-coated glass substrates (Figure 4).
These analyses provide valuable insights into the roles of substrate surface properties, such as roughness and wettability, in determining the morphology and growth dynamics of CAS layers across different substrates. The measured surface roughness (Rq) and contact angles are summarized in Table 2.
The interaction between water droplets and a surface is influenced by both the surface’s microstructure and its chemical composition [65]. For FTO, studies generally report water contact angles below 90°, indicating its hydrophilic nature [66,67,68,69]. Our results (Table 2) corroborate these findings, with a contact angle of 65° confirming the hydrophilic properties of FTO. Conversely, ITO demonstrates more variable behavior. While some studies classify ITO as hydrophilic, with contact angles below 90° [70,71], others report hydrophobic characteristics, with contact angles exceeding 90° [72,73]. In our study, the measured contact angle for ITO was 110°, indicating a predominantly hydrophobic surface. These differences in substrate surface properties, including roughness and wettability, are expected to play critical roles in the morphology and growth dynamics of the CAS layers.
Arya et al. observed that a commercially sourced indium tin oxide (ITO) surface exhibited hydrophobic properties. Remarkably, laser structuring further enhances its hydrophobicity, as evidenced by an increase in the contact angle [74]. Similarly, in our study, ITO surfaces displayed hydrophobic behavior with a contact angle of 110°. This hydrophobicity may also be influenced by the substrate’s relatively high surface roughness, as suggested by the Wenzel model [75], which correlates increased roughness with amplified surface wettability characteristics. In contrast, the glass substrate exhibited a contact angle of 30°, classifying it as hydrophilic. Since contact angle measurements can be used to estimate surface energy [76,77], and the substrate surface energy significantly impacts the nucleation process and micromorphology of thin films [78], there is likely a relationship between the varying contact angles observed (Table 2) and the different micro-morphologies of the CAS layers shown in Figure 3a–c. These findings underscore the critical roles of the substrate surface properties, including wettability and roughness, in influencing the growth dynamics and final morphology of the CAS layers.
In the Supplementary Materials (Figure S3), it is demonstrated that the ITO substrate, which exhibited the highest contact angle (indicating the lowest wettability), led to the formation of 3D spherical structures. Conversely, the lower contact angles (higher wettability) observed on glass and FTO promoted the growth of uniform, planar films with good adhesion to the substrate, aligning with the findings of Gebremichael et al. [79]. However, it is important to note that the contact angle measurement alone cannot fully explain the nucleation process. The behavior of a water droplet on the surface is influenced by micrometer-scale substrate features, and the nucleation dynamics are governed by nanometer-scale surface characteristics [80], underscoring the complexity of the relationship between wettability and film morphology.
These findings, supported by the AFM analysis, provide valuable insights into the growth mechanism of CAS at both the macro and micro-scales. AFM images and the RMS roughness of the substrates presented in Figure 4a–c and Table 2 reveal that ITO exhibited the highest roughness compared to FTO and glass. This observation aligns with the earlier observation that the roughness of CAS-ITO is greater than that of CAS-FTO (Figure 3d). Specifically, the roughness ratio of the CAS layer (Figure 3d) to the substrate (Table 2) is almost two for ITO and FTO, indicating that the CAS layers grown on both substrates duplicate the substrate’s surface roughness. Conversely, the bare glass substrate exhibited the lowest roughness, whereas the CAS layer on glass had a roughness that was over two orders of magnitude greater than the bare glass.
This observation may be related to the different growth mechanisms of the CAS layer on crystalline substrates, such as ITO and FTO, and the amorphous glass substrate, as discussed earlier in Section 3.1. Figure S3 also highlights the poor adhesion of the CAS layer on glass compared to other samples (see the Supplementary Materials).
Additionally, a fractal dimension analysis was performed using the cube-counting method [47] to further study the AFM images of the substrates presented in Figure 5. The analysis revealed that the FD was highest for glass, followed by FTO, and lowest for ITO. A higher FD of the FTO surface suggests a more self-similar structure compared to the crystalline ITO substrates, which is in line with the higher self-similarity of the CAS layer on FTO, as discussed before.
Furthermore, the fractal structure can promote a higher number of nucleation sites, likely due to its ability to create a high local concentration of molecules that facilitates nucleation [81]. The higher fractal dimension observed for glass compared to FTO, and for FTO compared to ITO, indicates increased complexity and suggests the presence of more potential nucleation sites. Consequently, the higher number of nucleation sites on FTO compared to ITO contributes to a higher nucleation density, influencing the growth dynamics and morphology of the CAS layers.
However, glass is an exception; it has the highest FD, yet it leads to the lowest fractal dimension of the CAS layer grown on it. This anomaly is likely attributed to its amorphous structure, which results in weaker adhesion (Figure S3) to the CAS layer compared to crystalline substrates. Overall, the FTO substrate provides a more favorable foundation for the formation of uniform, planar grains, resulting in a smoother CAS layer with strong adhesion to the substrate. While smaller grain sizes may not be optimal for solar cell performance, the FTO substrate’s ability to regulate the CAS layer’s roughness and promote an improved micromorphology represents a significant advantage. This characteristic positions FTO as a promising candidate for further exploration in future studies employing the spray pyrolysis method.

3.5. UV-Vis Spectroscopy

UV-Vis measurements were carried out to evaluate the spectral absorbance of CAS layers deposited on different substrates and to estimate their optical bandgap using the Tauc method (Figure 6). Figure 6a presents the absorbance spectra, showing onsets between 350 and 400 nm for all layers. As depicted in Figure 6a, there is a broad absorbance from around 350 to 800 nm, as reported for the CAS layer in the literature [8,82,83,84]. In addition, the direct band gap was determined using the Tauc method by plotting (αhν)2 against hν, where α is the absorption coefficient and hν represents the photon energy (E) (Figure 6b). The absorption coefficient, α, was calculated using Equation (2) [85]. In Equation (2), T represents the transmittance and was measured by the spectrometer, while R denotes reflectance (R = 1 − T − A) [86]. The reflectance spectra are presented in the Supplementary Materials (Figure S4). Additionally, d refers to the thickness of the layers, determined from the FESEM cross-section and presented in the Supplementary Materials.
α = 1 d L n T 1 R
The measured bandgaps for the CAS layers deposited on the glass, ITO, and FTO substrates were 1.66, 1.26, and 1.19 eV, respectively, all within the range reported in the literature for CAS [87].
It was shown in this study that on glass substrates, the formation of CuS phases is observed by XRD analysis, which may suggest a depletion of copper in the remaining CAS phase, making it copper-poor. In contrast, other substrates (ITO and FTO) allow the formation of the CAS phase without an additional phase of CuS, indicating a stoichiometric or slightly copper-rich CAS composition. Chalapathi et al. [88] suggested that the Cu/Sb ratio significantly affects the bandgaps, as a higher Cu/Sb ratio generally reduces the band gap and a lower ratio increases it. Therefore, in this study, the copper-rich CAS phase may result in lower band gaps (samples fabricated on ITO and FTO) in comparison to copper-poor CAS layers (on the glass substrate).

4. Conclusions

CuSbS2 (CAS) films deposited via spray pyrolysis are emerging as a promising absorber material in thin-film solar cells. Their suitable bandgap for solar absorption, along with the availability and environmental friendliness of their constituent elements, make CAS an attractive option for photovoltaic applications. This study investigates how different substrate choices influence the physical properties of CAS layers, offering insights that could enhance their performance in solar energy technologies.
XRD and Raman spectroscopy confirm the presence of CAS phases in all samples grown under the same conditions on ITO, FTO, and glass substrates. However, the glass substrates also show a higher tendency to form a secondary copper-rich phase (CuS), which is not observed on ITO and FTO substrates.
Examining the AFM images provided valuable insights into how substrate surface characteristics influence the micromorphology of the fabricated films. The CAS layers grown on FTO and ITO exhibited a roughness approximately twice that of their respective substrates, whereas on glass, the increase was significantly greater by two orders of magnitude. These findings emphasize the roles of substrate properties in film formation and suggest that ITO and FTO may be more suitable for further research.
The CAS layer grown on the ITO substrate exhibited a larger grain size than the layer on FTO. On the other hand, the technical imaging analysis of FESEM images indicated that films on FTO had a lower roughness, a more uniform grain size distribution, greater self-similarity, and a more isotropic surface texture. The correlation between the micro-morphological and microstructural characteristics of the substrates and CAS layers highlights the need to optimize the substrate properties to achieve the desired performance.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/coatings15060683/s1: Figure S1. For the CAS layer on glass. Figure S2. Distribution of the EDR of CAS grains on each substrate in a box and whisker plot. Figure S3. FESEM cross-sectional images of CAS layers on (a) glass, (b) ITO, (c) FTO, and (d) glass at low magnification. Figure S4. The reflectance spectra of the CAS-ITO, CAS-FTO, and CAS-glass samples.

Author Contributions

S.S.: conceptualization, sample preparation, analysis, data processing, writing an original draft. E.I.: supervision, resources, validation, funding acquisition, writing, review and editing. P.R.K.: writing, review and editing, data processing. S.P.: writing, review and editing. G.F.: writing, review and editing. A.P.: writing, review and editing, A.B.: conceptualization, validation, writing, review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicablePlease add the corresponding content of this part.

Data Availability Statement

The data will be provided upon request.

Acknowledgments

This research was carried out with the financial and technical support of Tarbiat Modares University. Additional technical assistance was provided by the ThiFiLab at the University of Parma. Authors would like to acknowledge Mohammad Kashif at University of Parma for providing further assistance in XRD analysis.

Conflicts of Interest

The authors confirm that they do not have any identifiable financial conflicts of interest or personal affiliations that might have the potential to affect the findings presented in this paper.

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Figure 1. XRD patterns of CAS thin films synthesized on glass, FTO, and ITO substrates, highlighting the orthorhombic structure and indexed crystal planes (JCPDS 44-1417).
Figure 1. XRD patterns of CAS thin films synthesized on glass, FTO, and ITO substrates, highlighting the orthorhombic structure and indexed crystal planes (JCPDS 44-1417).
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Figure 2. Raman spectra of CAS samples synthesized on different substrates of glass, FTO, and ITO.
Figure 2. Raman spectra of CAS samples synthesized on different substrates of glass, FTO, and ITO.
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Figure 3. FESEM images of CAS samples synthetized on different substrates: (a) glass, (b) FTO, and (c) ITO. (d) Surface RMS roughness values of the samples. (e) Left Y-axis, fractal dimension (FD); right Y-axis, texture aspect ratio (Str).
Figure 3. FESEM images of CAS samples synthetized on different substrates: (a) glass, (b) FTO, and (c) ITO. (d) Surface RMS roughness values of the samples. (e) Left Y-axis, fractal dimension (FD); right Y-axis, texture aspect ratio (Str).
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Figure 4. AFM images of (a) glass, (b) FTO, and (c) ITO substrates; Contact angle analysis of (d) glass, (e) FTO, and (f) ITO substrates.
Figure 4. AFM images of (a) glass, (b) FTO, and (c) ITO substrates; Contact angle analysis of (d) glass, (e) FTO, and (f) ITO substrates.
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Figure 5. Fractal dimension analysis of the CAS layer grown on different substrates: glass, FTO, and ITO.
Figure 5. Fractal dimension analysis of the CAS layer grown on different substrates: glass, FTO, and ITO.
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Figure 6. (a) Absorbance spectra and (b) (αhν)2 as a function of E for all the CAS layers deposited on glass, FTO, and ITO.
Figure 6. (a) Absorbance spectra and (b) (αhν)2 as a function of E for all the CAS layers deposited on glass, FTO, and ITO.
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Table 1. FWHM of the XRD peak related to the (111) plane and corresponding crystal size of CAS samples deposited on different substrates (FTO, ITO, and glass).
Table 1. FWHM of the XRD peak related to the (111) plane and corresponding crystal size of CAS samples deposited on different substrates (FTO, ITO, and glass).
SamplesFWHM (Degrees)Crystal Size (nm)
CAS-FTO0.1530
CAS-ITO0.2721
CAS-glass0.4016
Table 2. Surface roughness and contact angles of the FTO, ITO, and glass substrates. Rq is the root mean squared roughness.
Table 2. Surface roughness and contact angles of the FTO, ITO, and glass substrates. Rq is the root mean squared roughness.
SampleRq (nm)Contact Angle (°)
FTO2065 ± 5
ITO39110 ± 5
Glass0.1730 ± 5
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Shapouri, S.; Irani, E.; Rajabi Kalvani, P.; Pasini, S.; Foti, G.; Parisini, A.; Bosio, A. Substrate-Dependent Characteristics of CuSbS2 Solar Absorber Layers Grown by Spray Pyrolysis. Coatings 2025, 15, 683. https://doi.org/10.3390/coatings15060683

AMA Style

Shapouri S, Irani E, Rajabi Kalvani P, Pasini S, Foti G, Parisini A, Bosio A. Substrate-Dependent Characteristics of CuSbS2 Solar Absorber Layers Grown by Spray Pyrolysis. Coatings. 2025; 15(6):683. https://doi.org/10.3390/coatings15060683

Chicago/Turabian Style

Shapouri, Samaneh, Elnaz Irani, Payam Rajabi Kalvani, Stefano Pasini, Gianluca Foti, Antonella Parisini, and Alessio Bosio. 2025. "Substrate-Dependent Characteristics of CuSbS2 Solar Absorber Layers Grown by Spray Pyrolysis" Coatings 15, no. 6: 683. https://doi.org/10.3390/coatings15060683

APA Style

Shapouri, S., Irani, E., Rajabi Kalvani, P., Pasini, S., Foti, G., Parisini, A., & Bosio, A. (2025). Substrate-Dependent Characteristics of CuSbS2 Solar Absorber Layers Grown by Spray Pyrolysis. Coatings, 15(6), 683. https://doi.org/10.3390/coatings15060683

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