Synthesis of Sm-Doped CuO–SnO₂:FSprayed Thin Film: An Eco-Friendly Dual-Function Solution for the Buffer Layer and an Effective Photocatalyst for Ampicillin Degradation

Abstract

Synthesis and characterization of undoped and samarium-doped CuO–SnO₂:F thin films using the spray pyrolysis technique are presented. The effect of the samarium doping level on the physical properties of these films was thoroughly analyzed. X-ray diffraction patterns proved the successful synthesis of pure CuO–SnO₂:F thin films, free from detectable impurities. The smallest crystallite size was observed in 6% Sm-doped CuO–SnO₂:F thin films. The 6% Sm-doped CuO–SnO₂films demonstrated an increasedsurface area of 40.6 m²/g, highlighting improved textural properties, which was further validated by XPS analysis.The bandgap energy was found to increase from 1.90 eV for undoped CuO–SnO₂:F to 2.52 eV for 4% Sm-doped CuO–SnO₂:F, before decreasing to 2.03 eV for 6% Sm-doped CuO–SnO2:F thin films. Photoluminescence spectra revealed various emission peaks, suggesting a quenching effect. A numerical simulation of a new solar cell based on FTO/ZnO/Sm–CuO–SnO₂:F/X/Mo was carried out using Silvaco Atlas software, where X represented the absorber layer CIGS, CdTe, and CZTS. The results showed that the solar cell with CIGS as the absorber layer achieved the highest efficiency of 15.98. Additionally, the thin films demonstrated strong photocatalytic performance, with 6% Sm-doped CuO–SnO₂:F showing 86% degradation of ampicillin after two hours. This comprehensive investigation provided valuable insights into the synthesis, properties, and potential applications of Sm-doped CuO–SnO₂ thin films, particularly for solar energy and pharmaceutical applications.

1. Introduction

Global challenges, such as the depletion of fossil fuels and the increasing levels of environmental pollution, have intensified the demand for renewable, clean, and sustainable energy sources. Among these, solar energy stands out as a promising alternative.

The technological evolution of solar cells can be categorized into three main development stages: silicon, thin film, and organic solar cells [1,2,3]. Currently, thin film solar cells have achieved commercial maturity, with global production exceeding 30 GW (gigawatts). However, researchers face the ongoing challenge of producing solar cells that are both cost-effective and environmentally friendly, while also achieving high efficiency [4].

Among the various solar cell structures, those utilizing cadmium sulfide (CdS) as the buffer layer have garnered considerable attention due to their long-term reliability and consumer confidence [5,6]. Yet, research in this area has plateaued, primarily due to the toxic nature and environmental concerns associated with CdS [7,8]. As a result, attention is now focused on using cost-effective, non-toxic thin film materials. Materials such as In₂S₃ and CuO–SnO₂ thin films have emerged as promising candidates, owing to their suitable physical properties and low environmental impact [9,10,11]. In this study, we explore the optimization of CuO–SnO₂ thin films through the introduction of samarium, aiming to enhance device efficiency.

Our choice of samarium as adoping material was based on previous research conducted in our laboratory on titanium dioxide (TiO₂) thin film by Naffouti et al. [12]. The results showed that samarium doping demonstrated significant improvements in the physical properties of TiO₂, which led to a better performance in a variety of optoelectronic applications [12].

In addition to energy applications, photocatalysis is another process that uses light energy to purify and disinfect water by degrading organic pollutants such as antibiotics. While antibiotics are essential in modern medicine for treating bacterial infections and saving lives [13,14,15], their widespread use in both human and veterinary medicine has resulted in their unintended presence in water sources, creating significant risks to both human health and the environment [16]. Ampicillin, a commonly used broad-spectrum antibiotic, is frequently found in wastewater due to its extensive application. The presence of antibiotics in water systems creates a dual threat: it can promote the development of antibiotic-resistant bacteria, complicating treatment options and threatening public health, while also disrupting aquatic ecosystems and threatening biodiversity [17,18,19,20,21].

Addressing this challenge requires innovative methods for efficiently removing antibiotics from wastewater. Heterogeneous photocatalysis has emerged as a promising technology for the degradation of organic pollutants, including antibiotics, in water [22]. However, the resistance of certain antibiotics, such as ampicillin, to complete degradation remains a significant barrier [23].

This study investigates the potential of Sm-doped CuO–SnO₂-based thin films for solar energy and photocatalysis applications, providing a thorough characterization of their structure, morphology, and properties.

2. Experimental Details

2.1. Thin Film Synthesis and Characterization

Undoped and samarium (Sm)-doped CuO–SnO₂ mixed oxide thin films were successfully deposited onto glass substrates using the spray pyrolysis technique. All chemical precursors—including copper(II) chloride (CuCl₂), tin(IV) chloride (SnCl₄), samarium(III) chloride (SmCl₃), and methanol—were sourced from Sigma-Aldrich, with a purity exceeding 99%, ensuring the fabrication of high-quality thin films.

Glass substrates (2.5 × 2.5 cm²) were first cleaned in an ultrasonic bath for 15 min to remove surface contaminants. Two individual precursor solutions were then prepared: CuCl₂ was dissolved in distilled water to serve as the copper source, while SnCl₄ was dissolved in methanol to serveas the tin precursor. Both precursor concentrations were fixed at 0.2 mol·L⁻¹. To enhance the electrical conductivity of the resulting SnO₂films, 5 g of fluoride were added to the tin precursor solution.

The copper and tin precursor solutions were subsequently mixed at a molar ratio of [Cu]/[Cu + Sn] = 0.75. The resulting solution was subjected to vigorous magnetic stirring for 20 min to ensure complete homogenization, an essential step in achieving uniform film composition and optimal film quality.

The film deposition process was performed by spraying the prepared solution onto preheated glass substrates. A nozzle was positioned approximately 28 cm above the substrate, with a spray flow rate maintained at ~10 mL/min. The substrate temperature was held constant at 350 °C to facilitate efficient film adhesion and growth.

For the fabrication of Sm-doped CuO–SnO₂:F thin films, the same procedure was followed, with the addition of SmCl₃ to the precursor solution. Samarium was introduced in varying molar concentrations (0 to 6 mol%, in 2 mol% increments) to study the effect of dopant levels on the structural and functional properties of thin films.

To investigate the physical properties of Sm-doped CuO–SnO₂ thin films, several characterization techniques were employed. Structural analysis was conducted using X-ray diffraction (XRD) with a monochromatic diffractometer type X’pert PRO with CuKα radiations (1.5418 Å) and Raman spectroscopy using a Jobin Yvon T64000 spectrometer with a 488 nm argon laser excitation at room temperature. Surface morphology was analyzed with a Cari Zeiss GmbH T3447 scanning electron microscope (SEM). Additionally, transmission electron microscopy (TEM) was conducted using a JOEL 2010 device to further analyze the structural and morphological features of the thin films. X-ray photoelectron spectroscopy (XPS) was conducted usinga using a THERMO-VG ESCALAB 250 X-ray Photoelectron Spectroscopy (XPS) system (Thermo Fisher Scientific, Waltham, MA, USA). Specific surface areas(SBET) were calculated using a multipoint Brunauer–Emmett–Teller (BET) method, utilizing adsorption data within the relative pressure P/P0 range of 0.05–0.25. Optical properties were measured using a PerkinElmer Lambda 950 UV/Vis/NIR spectrophotometer in the wavelength range of 200 to 2000 nm at room temperature. Photoluminescence properties were studied with a PerkinElmer LS55 system using an excitation wavelength of 300 nm.

2.2. Numerical Model Description

The development of advanced solar cell architecture requires a comprehensive understanding of material behavior, which is often complex and expensive to investigate through experimental methods alone. Simulation tools such as Silvaco TCAD offer a practical and cost-effective alternative by enabling accurate modeling of semiconductor device performance under diverse operating conditions. In this study, Silvaco TCAD was employed to simulate the proposed solar cell structure using input parameters derived from experimentally measured electrical and optical properties.

The simulated device incorporated molybdenum and fluorine-doped tin oxide (FTO) as ohmic contacts, a Sm–CuO–SnO₂:F composite as the buffer layer, ZnO as the absorber, and material X as the optical window. Device performance was evaluated under standard AM 1.5 solar illumination at an intensity of 0.1 W/cm², utilizing advanced numerical techniques such as the Newton method. This simulation-driven approach facilitates efficient design optimization and performance evaluation, significantly accelerating the development of next-generation solar cell technologies.

2.3. Pharmaceutical Application (Antibiotic Degradation)

Investigation into the photocatalytic behaviors of undoped and Sm-doped CuO–SnO₂:F thin films started with their immersion in a beaker containing 50 mL ampicillin solution. Initially, thin films were allowed to sit in darkness for 30 min to establish an equilibrium between adsorption and desorption processes. Subsequently, they were exposed to sunlight to initiate the photocatalytic reaction. The setup allowed the study of thin films interacting with dye molecules in both dark and light, offering insights into their photocatalytic efficiency.

3. Results

3.1. Structural Properties

The crystallographic properties of undoped and Sm-doped CuO–SnO₂ mixed oxides thin films were assessed using X-ray diffraction (Figure 1) within the scan range (2θ) of 20° to 70°.

First, it is evidentthat the XRD patterns showed well-resolved peaks, indicating high-quality crystallites in all the samples. Additionally, all the thin layers displayed a polycrystalline nature, as shown by the presence of prominent CuO peaks (–111), (–202), (202), and (–113), which correspond to the monoclinic crystal structure of copper oxide (JCPDS No. 89-5895) [24,25,26]. These were accompanied by SnO₂ peaks at (110), (200), and (211), which are specificto the tetragonal structure (JCPDS No. 79-2205) [27].

Notably, all the diffraction peaks clearly corresponded to the monoclinic CuO and tetragonal SnO₂:F phases, with no additional peaks observed, confirming the successful synthesis of CuO–SnO₂:F compounds. The absence of solid solution phases suggests the formation of a CuO–SnO₂ nanocomposite structure, potentially forming p–n nanojunctions [28]. Moreover, the widening of the diffraction peaks, especially SnO₂ (110), indicates the presence of exceptionally fine crystallite sizes within the films, further shedding light on their structural characteristics.

The XRD data provided the average crystallite size D of the films using the Scherrer equation [29,30]:

$$\mathit{D}={\displaystyle \frac{\mathit{k}\mathit{\lambda }}{\mathit{\beta }\mathbf{c}\mathbf{o}\mathbf{s}\left(\mathit{\theta }\right)}}$$

(1)

where k is the constant (k = 0.9), λ is the wavelength of the X-ray beam (λ = 0.15406 nm), β is the width at half height of the maximum peak (FWHM) of the diffraction peak, 2θ is the angle of the diffraction line, and θ is the Bragg angle corresponding to the diffraction peak. To further investigate the crystal structure and defects, the micro-strain, ε, and dislocation density δ were calculated according to the following equations [31,32]:

$$\mathit{\epsilon }={\displaystyle \frac{\mathit{\beta }\mathbf{c}\mathbf{o}\mathbf{s}\left(\mathit{\theta }\right)}{4}}$$

(2)

$$\mathsf{\delta }={\displaystyle \frac{1}{{\mathit{D}}^2}}$$

(3)

Moreover, the lattice parameters were calculated using the MAUD software version 2.999.

The structural constants are represented in

Table 1. Structural (D and δ are the crystallite size and the dislocation density) parameters of undoped and Sm-doped CuO–SnO₂ coupled oxide thin films at different Sm doping levels.

Thin Layer	D (nm)	δ (×10−4 nm−2)	Lattice Parameters (A°)
CuO–SnO2:F	28	12.7	a = 4.683 b = 3.421 c = 5.146
2% Sm-doped CuO–SnO2:F	31	10.4	a = 4.685 b = 3.426 c = 5.147
4% Sm-doped CuO–SnO2:F	18	30.8	a = 4.686 b = 3.428 c = 5.149
6% Sm-doped CuO–SnO2:F	15	44.4	a = 4.689 b = 3.429 c = 5.151

, which shows that the smallest crystallite size was obtained for the 6% Sm-doped CuO-SnO₂:F thin film, indicating its potential for photocatalytic applications. Smaller crystallite sizes are beneficial as they reduce the recombination probability of electron–hole pairs; the shorter diffusion paths allow these charge carriers to reach the semiconductor surface more quickly, thereby enhancing photocatalytic activity [33]. Furthermore, Table 1 reveals that with increasing Sm doping, the dislocation density also increases. This increase in dislocation density can induce structural distortions, leading to a higher surface-to-volume ratio and the formation of internal electric fields. These fields facilitate charge separation and migration, reducing recombination rates and thus improving the overall efficiency of redox reactions on the catalyst surface. In addition, the lattice parameters were found to increase with higher Sm doping levels. This expansion was attributed to the substitution of Cu²⁺ ions (ionic radius ~0.73 Å) by larger Sm³⁺ ions (ionic radius ~1.08 Å), which introduced strain and distortion into the crystal lattice. As a result, the unit cell dimensions increased, confirming the successful incorporation of Sm³⁺ into the CuO lattice and its impact on the structural properties of the films [34].

Raman spectroscopy was employed to further analyze the structural and vibrational properties of the CuO–SnO₂ thin films before and after Sm doping. The spectra (Figure 2) revealed two distinct peaks centered around 310 cm⁻¹ and 350 cm⁻¹, which correspond to characteristic vibrational modes of CuO and SnO₂ phases. Additionally, a broad peak was observed around 1150 cm⁻¹, likely associated with multi-phonon scattering or defect-related vibrational modes.

Upon Sm doping, Raman peaks exhibited a noticeable red shift, indicating a slight expansion in the lattice parameters. This shift is typically associated with a reduction in vibrational frequency due to the substitution of smaller Cu²⁺ ions with larger Sm³⁺ ions, which leads to longer and slightly weaker bonds within the lattice. This interpretation is consistent with the XRD results, which showed an increase in lattice constants upon Sm incorporation. Moreover, the intensity and sharpness of the Raman peaks improved significantly in the doped sample, suggesting enhanced crystallinity. The vibrational changes observed upon doping further support the idea of improved structural order and reduced disorder, in line with the formation of a more uniform and well-crystallized film structure [35].

3.2. Morphological Properties

The surface morphology of undoped and 6% Sm-doped CuO–SnO₂:F thin films was thoroughly examined using scanning electron microscopy (SEM) (Figure 3a,b). The observation revealed distinctive rounded and spherical shapes in both undoped and Sm-doped samples. Moreover, the porosity of the films was significantly increased after doping. This enhanced porosity is beneficial in various applications, particularly in catalysis, as it provides a larger surface area for catalytic reactions. The increased porosity improves the accessibility of reactants to active sites, which may lead to better catalytic performance and efficiency [36,37].

The TEM results presented in Figure 3c,d confirmed the morphological features previously observed in the SEM analysis, revealing well-defined and uniformly distributed nanoparticles. The micrographs clearly showed the grain-like structure and compact arrangement consistent with the surface texture observed in the SEM images. Moreover, the crystallite sizes determined from the TEM images were approximately 25 nm and 17 nm for the undoped and Sm-doped CuO–SnO₂:F thin films, respectively. These values closely match those estimated from the XRD data using the Scherrer equation, further validating the consistency between structural and morphological characterization.

Elemental mapping analysis was conducted to examine the spatial distribution of elements within the CuO–SnO₂ thin films both before and after Smdoping (6%) (Figure 3e,f). In the undoped samples, the mapping clearly confirmed the uniform presence of Cu, Sn, and O across the entire substrate surface, indicating a well-distributed film composition. After Sm doping, the mapping results additionally revealed the successful incorporation of Sm, with a homogenous distribution similar to the primary elements. The even coverage of Cu, Sn, O, and Sm across the substrate confirms not only the uniform deposition of the film but also suggests strong interaction and integration of Sm within the CuO–SnO₂ matrix. This elemental interaction is likely to play a role in modifying the structural and electronic properties of the film, as supported by the enhanced crystallinity and spectral shifts observed in complementary analyses.

Energy-dispersive X-ray spectroscopy (EDS) analysis was carried out to confirm the elemental composition of the undoped and 6% Sm-doped CuO–SnO₂ thin films. For the undoped film, the EDS spectrum (Figure 3g,h) confirmed the presence of copper (Cu), tin (Sn), and oxygen (O) as expected. In the Sm-doped sample, the EDS spectrum (Figure 3c) revealed the additional presence of samarium (Sm), confirming the successful incorporation of Sm into the CuO–SnO₂ matrix. The detection of samarium indicates that the doping process was effective. Furthermore, minor peaks corresponding to elements such as silicon (Si) and sodium (Na) were observed, likely originating from the glass substrate used during the film deposition process [38].

3.3. BET Surface Area

The Brunauer–Emmett–Teller (BET) surface area is a critical parameter that directly impacts the performance of materials in applications such as catalysis, adsorption, and energy storage. BET analysis is commonly used to determine the specific surface area of porous and nanostructured materials by measuring nitrogen adsorption–desorption isotherms. A higher surface area typically correlates with a higher number of active sites available for chemical reactions, which is particularly important in photocatalysis.

In our previous study, the BET surface area of the undoped CuO–SnO₂ thin film was found to be 13.4 m²/g [25]. In the current work, the BET surface area of the 6% Sm-doped CuO–SnO₂ thin film, shown in Figure 4, was significantly higher at 40.6 m²/g. This substantial increase can be attributed to structural changes induced by Sm doping, such as smaller grain sizes, enhanced porosity, or modified crystallite boundaries—all contributing to a more developed surface morphology [39].

The increase in surface area provides a larger number of active sites for photocatalytic reactions, leading to improved degradation of pollutants. Moreover, a higher surface area enhances light absorption and facilitates more effective interaction between the photocatalyst and the reactant molecules in the solution. Consequently, the photocatalytic efficiency of the Sm-doped CuO–SnO₂ thin film is significantly improved compared to the undoped version [40].

3.4. XPS Studies

To investigate the effects of Sm doping on the CuO–SnO₂ thin films, X-ray photoelectron spectroscopy (XPS) analysis was performed on both the undoped and 6% Sm-doped samples. In both cases (Figure 5), characteristic peaks for Sn 3d (at 486 and 495 eV), Cu 2p (at 934 and 944 eV), and O 1s (at 529 eV) were clearly observed, confirming the presence of the main constituent elements. In the Sm-doped sample (6%), an additional peak corresponding to Sm 3d appeared around 1083 eV, which was not present in the undoped film—confirming the successful incorporation of Sm into the lattice [41].

Furthermore, the O 1s peak in the doped film showed broadening and asymmetry at higher binding energies, indicating the formation of oxygen vacancies or other Sm-induced defect states. These defects can create localized band tail states near the conduction or valence bands, which are known to enhance charge separation and prolong carrier lifetimes—beneficial for applications such as photocatalysis and optoelectronics [42].

3.5. Optical Properties

Designing and evaluating optoelectronic devices require an understanding of the optical properties of materials. In this study, we analyzed the optical transmission and reflection spectra of CuO–SnO₂:F coupled oxide thin films across a broad wavelength range of 250–2000 nm to assess the impact of samarium (Sm) incorporation on optical performance and material quality. Figure 6a presents the optical transmission behavior of the undoped and Sm-doped CuO–SnO₂:F thin films at various Sm doping levels. The undoped films exhibited high absorbance, particularly in the UV and visible regions (250–600 nm), where transmission approached zero percent, indicating strong light absorption. As the Sm doping level increased, the intrinsic absorption shifted to blue. The transmission reached a peak of 4% before shifting back to higher wavelengths at the 6% doping level, at which point the behavior resembled that of the undoped CuO–SnO₂:F thin films.

The initial shift in intrinsic absorption towards lower wavelengths with increasing Sm doping can be attributed to the Burstein–Moss effect. In this case, the addition of Sm ions increases the free carrier concentration, pushing the Fermi level deeper into the conduction band and effectively widening the optical bandgap [43]. This results in a blue shift, as higher-energy photons are required for electron excitation. However, at the 6% Sm doping level, the absorption shifted back to higher wavelengths. This shift was due to the higher dislocation density, which often leads to greater structural disorder, as reflected in the structural parameters summarized in Table 1. From this, we deduced that the highest dislocation density occurred at the 6% Sm doping level. Excessive Sm incorporation introduced defects, band tail states, impurity scattering, and localized energy states near the conduction band, which reduced the bandgap and enabled the absorption of lower-energy photons, causing a red shift.

Additionally, a notable increase in transmittance was observed with rising Sm concentration, particularly in the visible region, where transmission reached a maximum of 70% for the optimal doping level. However, at the 6% Sm doping level, transmittance started to decrease, dropping to 30%. This suggests that excessive doping may induce structural or electronic changes that negatively impact optical transparency.

Figure 6b shows reflection spectra in which all the thin films display a similar pattern: decreased reflection in the UV region and a plateau in the visible range. This indicates consistent reflection properties across various samarium doping levels, particularly within the visible spectrum.

The absorbance spectra (Figure 6c) of the undoped and samarium (Sm)-doped CuO–SnO₂ thin films revealed an interesting trend: absorbance decreased up to the doping level of 4%, after which it began to increase again at 6%. This behavior suggests that Sm doping initially reduces the material’s absorption of incident light, possibly due to enhanced transparency or a reduction in light scattering effects caused by the doping process. However, beyond the 4% doping level, there is a reversal in this trend, where absorbance starts to increase. This increase could be attributed to various causes, such as modifications in the electronic structure or the introduction of doping-induced defects within the material.

We calculated the energy bandgap by analyzing the transmittance and reflection spectra using the Tauc formula, which is expressed by the following equation [44,45]:

$$\mathit{\alpha }\mathit{h}\mathit{v}={\mathit{A}\left(\mathit{h}\mathit{v}-\mathit{E}\mathit{g}\right)}^{\mathit{n}}$$

(4)

where A is the constant related to the material, h is Planck’s constant, n corresponds to ½ for direct transitions and 2 for indirect transitions, and α was calculated using the following equation [46,47]:

$$\mathit{\alpha }={\displaystyle \frac{\mathbf{1}}{\mathit{e}}}\mathbf{ln}{\displaystyle \frac{{(\mathbf{1}-\mathit{R})}^2}{\mathit{T}}}$$

(5)

The optical bandgap values are provided in

Table 2. Optical bandgaps of the undoped and Sm-doped CuO–SnO₂ coupled oxide thin films at different Sm doping levels.

Thin Layers	Gap Energy (eV)
CuO–SnO2:F	1.91
2% Sm-doped CuO–SnO2:F	2.34
4% Sm-doped CuO–SnO2:F	2.52
6% Sm-doped CuO–SnO2:F	2.04

.

As shown in Figure 6d, the band gap increases rapidly with the increase in Sm incorporation, reaching a maximum value of 2.52 eV for thin films fabricated with a doping level of 4%. However, beyond this point, the band gap begins to decrease, reaching 2.04 eV at a doping level equal to 6%, which makes it suitable for use as the buffer layer in solar cell structures [12].

Finally, to complete our optical analysis, we examined the optical parameters of the thin films, including the refractive index n (λ), the extinction coefficient k (λ), the real (ε_r) and imaginary (ε_i) parts of the dielectric constant (ε), and the volume energy loss function (VELF). All these parameters were determined using the following equations [48,49]:

n^∗ = n + ik

(6)

$$\mathit{n}=\left({\displaystyle \frac{\mathbf{1}+\mathit{R}}{\mathbf{1}-\mathit{R}}}\right)+\sqrt{{\displaystyle \frac{\mathbf{4}\mathit{R}}{{\left(\mathbf{1}-\mathit{R}\right)}^{\mathbf{2}}}}-{\mathit{k}}^{\mathbf{2}}}$$

(7)

ε = ε_r + iε_i

(8)

ε_r = n² − k²

(9)

ε_i = 2nk

(10)

$$\mathit{V}\mathit{E}\mathit{L}\mathit{F}=-\mathit{I}\mathit{m}\left({\displaystyle \frac{\mathbf{1}}{{\mathbf{ԑ}}_{\mathit{r}}+{\mathit{i}\mathbf{ԑ}}_{\mathit{i}}}}\right)={\displaystyle \frac{{\mathbf{ԑ}}_{\mathit{i}}}{\left({{\mathbf{ԑ}}_{\mathit{r}}}^{\mathbf{2}}+{{\mathbf{ԑ}}_{\mathit{i}}}^{\mathbf{2}}\right)}}$$

(11)

In the investigation of optical parameters of the undoped and Sm-doped CuO–SnO₂:F thin films, doping with samarium resulted in a stable refractive index around 1.6, contrasting with the higher value of 1.8 observed in the undoped thin films (Figure 6e). This consistency in refractive index suggests that introducing Sm ions does not significantly alter the material’s optical transparency or its ability to propagate light. Moreover, the extinction coefficient (k) decreased and maintained near-zero values in the infrared (IR) region for the Sm-doped films (Figure 6f), indicating reduced light absorption and contributing to enhanced transparency. Additionally, both the real and imaginary parts of the dielectric constant followed similar trends to the refractive index and extinction coefficient, respectively (Figure 6g,h).

The volume energy loss function (VELF), which quantifies energy dissipation mechanisms within the material, is crucial for analyzing optical and electronic properties. Figure 6i shows that the VELF reached its minimum at the 6% Sm doping level in the CuO–SnO₂ thin films, indicating reduced plasmon energy losses and lower electronic damping. This is highly beneficial for optoelectronic applications, as a lower VELF enhances charge carrier mobility, dielectric stability, and overall energy efficiency, making 6% Sm-doped CuO–SnO₂ an excellent candidate as a green buffer layer for high-performance solar cells.

Figure 7 presents the photoluminescence spectra, providing an important understanding of the emission behavior of these films. PL spectra of the undoped and Sm-doped CuO-SnO₂:F revealed three distinct peaks that provide insights into the electronic and defect states within the material.

The observed peaks at 395 nm, 410 nm, and 490 nm can be attributed to different phenomena. The peak at 395 nm is likely due to the near-band-edge emission, which is associated with the recombination of excitons (electron–hole pairs) close to the conduction band edge of SnO₂. The 410 nm peak may result from shallow defect states or donor–acceptor pair recombination, indicating the presence of defects in the crystalline film, specifically missing or extra oxygen atoms within the SnO₂ matrix. Finally, the 490 nm peak is generally attributed to deep-level defect states, which could be associated with complex defect centers or impurities within the CuO-SnO₂ mixed oxide [50]. The decrease in photoluminescence peak intensity after doping with samarium can be attributed to several factors. Samarium ions introduce non-radiative recombination centers in the host material. These centers capture charge carriers (electrons and holes) that would otherwise recombine radiatively to emit light, thus reducing the overall PL intensity. Additionally, samarium ions increase scattering of the emitted photons, further diminishing the observed PL signal. Doping can also alter the crystal structure or introduce defects, both of which can adversely affect the material’s electronic properties and reduce the efficiency of the radiative recombination processes that are responsible for photoluminescence [51].

3.6. Solar Cell Simulation

Solar cells offer a renewable alternative to fossil fuels. Among the conventional absorber layers used in thin-film solar cells, materials such as Cu (In,Ga)Se₂ (CIGS), CdTe, and Cu₂ZnSnS₄ (CZTS) have gained significant attention due to their high efficiency and stability [52,53,54,55,56]. However, the CdS buffer layer in many of these solar cells raises environmental concerns due to the toxicity of cadmium. To address this issue, researchers have been exploring alternative buffer layers that maintain high efficiency while being environmentally friendly. Building on our previous studies, we explored the replacement of CdS with CuO–SnO₂:F thin films, achieving an efficiency of 15.31% [25]. In this work, we further enhanced the performance by doping CuO–SnO₂:F with 6% samarium (Sm), achieving key parameters of 0.72 V open-circuit voltage, 30 mA/cm² short-circuit current, and an 86% fill factor, which resulted in an improved efficiency of 15.98%, as shown in

Table 3. Simulation parameters of Cu (In,Ga)Se₂ (CIGS)-, CdTe-, and Cu₂ZnSnS₄ (CZTS)-based solar cell absorber layers.

Absorber Layer	Voc (V)	Jsc (mA)	FF (%)	Efficiency (%)
CdTe	0.28	12.66	70	9.36
CZTS	1.39	12.72	72	13.22
CIGS	0.72	30	86	15.98

. The structure and the mesh of our simulated solar cell are illustrated in Figure 6a.

The increase in solar cell efficiency from 15.31% to 15.98% after 6% Sm doping of CuO–SnO₂:F is attributed to several key factors. Samarium doping enhances electrical conductivity by increasing charge carrier concentration, reducing series resistance, and improving electron transport [25]. It also optimizes the band alignment between the buffer and absorber layers, facilitating better charge separation and minimizing recombination losses. Additionally, Sm acts as a defect passivator, reducing trap states and extending carrier lifetimes, leading to higher open-circuit voltage (0.72 V) and fill factor (86%). The slight change in optical properties ensures high transparency, allowing more photons to reach the absorber layer, which maintains a strong short-circuit current density (30 mA/cm²) [57].

In addition to integrating Sm-doped CuO–SnO₂:F as a Cd-free buffer layer in CIGS-based solar cells, we also evaluated its performance with other widely used absorber materials, namely CdTe and CZTS, to assess its compatibility and efficiency across different photovoltaic technologies, as shown in Figure 8b. The results are particularly promising, demonstrating that this buffer layer effectively supports multiple absorber layers while maintaining competitive efficiency levels.

For the CdTe-based solar cells, the recorded parameters included an open-circuit voltage (Voc) of 0.28 V, a short-circuit current density (Jsc) of 12.66 mA/cm², a fill factor (FF) of 70%, and an overall efficiency of 9.36%. While the Voc was relatively low, these results indicate that the buffer layer still facilitated efficient charge transport, and the moderate FF suggests controlled recombination losses.

On the other hand, the CZTS-based solar cells showed significantly better results, with a Voc of 1.39 V, aJsc of 12.72 mA/cm², an FF of 72%, and an improved efficiency of 13.22%. The notably higher Voc in CZTS indicates excellent band alignment between the Sm-doped CuO–SnO₂:F buffer layer and the CZTS absorber, effectively reducing interface recombination and enhancing charge separation [58]. Additionally, the FF values of 70% for CdTe and 72% for CZTS indicate that this buffer layer contributes to stable charge extraction with minimal resistive losses.

These findings are particularly significant as they confirm that Sm-doped CuO–SnO₂:F is not only an effective substitute for the toxic CdS, but also a highly adaptable buffer layer that can work efficiently with multiple absorber materials. This enables the creation of eco-friendly, high-performance thin-film solar cells [59].

3.7. Antibiotic Degradation: Ampicillin

Ampicillin (AMP), a commonly used antibiotic, persists in aquatic ecosystems and can have toxic effects when released into water [60,61]. Consequently, there has been a growing interest in developing advanced methods to effectively degrade such pharmaceutical pollutants. Among these methods, photocatalysis powered by solar light and using metal oxide thin films is the most promising due to its lowcost and eco-friendly nature to combat water contamination [62]. These materials are known for their chemical stability and ability to generate highly reactive species that can break down organic contaminants [63].

CuO–SnO₂-based photocatalysts have demonstrated potential in the degradation of organic pollutants [24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64], but they have not been explored for ampicillin degradation in previous studies. Notably, this work represents the first attempt to investigate the effects of doping CuO–SnO₂:F with samarium (Sm), a rare-earth element, to improve its photocatalytic performance for the degradation of ampicillin and achieve higher degradation efficiencies. Samarium can enhance the material’s photocatalytic activity in several ways. First, Sm ions can introduce new energy levels within the bandgap of the composite, which can act as electron trap sites, reducing the recombination of charge carriers (electrons and holes). By minimizing recombination, more charge carriers remain available to participate in the degradation process. Additionally, Sm doping can improve the absorption of light, especially in the visible region, due to the presence of f-electrons in Sm ions, which help absorb a broader spectrum of light [65]. This allows the photocatalyst to be activated more efficiently under visible light, which is more abundant and practical for real-world applications.

The enhancement in photocatalytic activity is clearly reflected in the experimental results presented in Figure 9a. As shown in Figure 9b, the absorption spectra of ampicillin showed a rapid decline in the intensity peak, indicating significant degradation of the antibiotic over time. The photocatalytic efficiency is quantified by the ratio of the initial concentration (C₀) to the final concentration (C) of ampicillin after a certain time using the following equation [66]:

$$\mathbf{e}\mathbf{f}\mathbf{f}\mathbf{e}\mathbf{c}\mathbf{i}\mathbf{e}\mathbf{n}\mathbf{t}\mathbf{y}={\displaystyle \frac{{\mathit{C}}_{\mathbf{0}}-\mathit{C}}{{\mathit{C}}_{\mathbf{0}}}}\mathbf{\ast }\mathbf{100}$$

(12)

Notably, the 6% Sm-doped CuO-SnO₂:F thin films exhibited remarkable performance, achieving 86% degradation efficiency after 2 h of sunlight irradiation (Figure 9b). This high level of efficiency can be credited to the effects of Sm doping, which not only enhanced the material’s light absorption, but also improved charge carrier separation, resulting in a more efficient degradation of ampicillin. Additionally, increased porosity provided more active sites for catalytic reactions, as shown in Figure 9.

We compared the efficiency of our mixed oxide thin film with earlier works demonstrating the highest photocatalytic efficiency for ampicillin degradation, as shown in

Table 4. Ampicillin degradation efficiency of various metaloxides.

Metal Oxide	Deposition Method	Degradation Efficiency in 2 Hunder Sunlight Irradiation	Reference
ZnO	Hydrothermal synthesis	41%	[67]
ZnO/ZnWO4 nanocomposite	Activated carbon-supported method	83%	[68]
8% bismuth-dopedCuO–ZnO	Spray pyrolysis	77%	[26]
6% Sm-doped CuO–SnO2	Spray pyrolysis	86%	Our work

.

A comparative analysis of various metal oxide-based photocatalysts reveals significant differences in degradation efficiency under sunlight irradiation over a 2 h period, depending on the material composition and deposition techniques. Pure ZnO, synthesized via hydrothermal methods, demonstrated a relatively modest degradation efficiency of 41% [67]. In contrast, a ZnO/ZnWO₄ nanocomposite prepared using an activated carbon-supported method significantly improved the efficiency to 83% [68], highlighting the positive impact of composite formation and support materials. Similarly, an 8% bismuth-doped CuO–ZnO film fabricated via spray pyrolysis achieved a degradation efficiency of 77% [26], emphasizing the role of dopants in enhancing photocatalytic performance. Notably, our study on 6% samarium-doped CuO–SnO₂, also synthesized using spray pyrolysis, achieved the highest degradation efficiency of 86%, demonstrating the synergistic effects of Sm doping and the CuO–SnO₂ heterostructure in boosting photocatalytic activity under sunlight.

The degradation of ampicillin using the undoped and Sm-doped CuO-SnO₂ photocatalysts followed the same mechanism facilitated by photocatalytic reactions (Figure 9c). Upon irradiation with light (hv), the CuO–SnO₂-based photocatalyst absorbs energy, generating electron–hole pairs in the CuO and SnO₂ components. This process is represented by the following equation [69]:

CuO-SnO₂ + hv →CuO(e⁻ + h⁺)/SnO₂

(13)

Here, the photogenerated electrons (e⁻) and holes (h⁺) are separated, with the electrons migrating to the SnO₂ component and the holes remaining in the CuO component. The subsequent reactions occur as follows:

CuO (e⁻ + h⁺)/SnO₂ → CuO (h⁺)/SnO₂(e⁻)

(14)

The electrons on SnO₂ reduce oxygen (O₂) to form superoxide radicals (O₂⁻), while the holes on CuO oxidize water molecules to form hydroxyl radicals (OH^•):

SnO₂(e⁻) + O₂ → O₂⁻ + SnO₂

(15)

CuO (h⁺) + H₂O → h⁺ + OH⁻ + CuO + H₂

(16)

These reactive species, O₂⁻ and OH^•, are highly reactive and play a significant role in breaking down ampicillin. Superoxide radicals (O₂⁻) react with ampicillin to produce byproducts such as ammonia (NH₃), sulfate ions (SO₄²⁻), carbon dioxide (CO₂), and water (H₂O):

O₂⁻ + AMP → Products (NH₃ + SO₄²⁻ + CO₂ + H₂O)

(17)

Similarly, the holes (h⁺) also interact with ampicillin, further breaking it down into similar byproducts:

h⁺ + AMP → Products (NH₃ + SO₄²⁻ + CO₂ + H₂O)

(18)

Thus, the photocatalytic degradation of ampicillin involves the generation of holes and reactive oxygen species that attack ampicillin molecules, leading to their complete mineralization into harmless byproducts such as ammonia, sulfate, carbon dioxide, and water. This mechanism describes how the CuO–SnO₂:F composite enhances the photocatalytic process, making it effective for environmental remediation. As a result, the incorporation of rare-earth ions such asSmions into the mixed oxide structure is a powerful strategy for optimizing photocatalytic materials, improving their efficiency and sustainability for environmental remediation. This approach is particularly valuable for water treatment processes targeting persistent pollutants such as ampicillin, offering a promising solution for the degradation of pharmaceutical contaminants in aquatic ecosystems.

To assess the role of various reactive species in the catalytic reaction, we performed a series of scavenger tests using isopropanol, ascorbic acid, Na₂SO₄, and EDTA. These scavengers were used to selectively quench hydroxyl radicals (HO•), superoxide radicals (O₂⁻), hydrated electrons (e⁻), and photogenerated holes (h⁺), respectively. The experimental results (Figure 9d) showed a significant decrease in degradation rates with the addition of each scavenger, indicating the active involvement of these species in the photocatalytic degradation process. This confirms that multiple radical-mediated pathways are operating, supporting the proposed photocatalytic mechanism and highlighting the complex interactions of reactive intermediates during the reaction.

4. Conclusions

In conclusion, this study successfully fabricated undoped and samarium-doped CuO–SnO₂:F thin films using the spray pyrolysis technique, advancing the field significantly. The detailed investigation of samarium doping levels revealed notable enhancements in the physical properties of the thin films. Numerical simulation of a novel solar cell structure incorporating 6% Sm–CuO–SnO₂ thin films as a green buffer layer demonstrated the highest efficiency of 15.98% when paired with a CIGS absorber layer. Additionally, the 6% Sm–CuO–SnO₂ thin films exhibited a high degradation efficiency of 86% for ampicillin after two hours of exposure, highlighting their dual potential functions in both environmental remediation and solar cell applications.