Increasing the Photovoltaic Efficiency of Semiconductor (Cu_1−xAg_x)₂ZnSnS₄ Thin Films through Ag Content Modification

Abstract

The research referred to in this study examines the morphological, structural, and optical characteristics of kesterite (Cu_1−xAg_x)₂ZnSnS₄ (CAZTS) thin films, which are produced using a process known as thermal evaporation (TE). The study’s main goal was to determine how different Ag contents affect the characteristics of CAZTS systems. X-ray diffraction (XRD) and Raman spectroscopy were used to confirm the crystal structure of the CAZTS thin films. Using a mathematical model of spectroscopic ellipsometry, the refractive index (n) represented the real part of the complex thin films, the extinction coefficient (k) portrayed the imaginary part, and the energy bandgap of the fabricated thin films was calculated. The energy bandgap is a crucial parameter for solar cell applications, as it determines the wavelength of light that the material can absorb. The energy bandgap was found to decrease from 1.74 eV to 1.55 eV with the increasing Ag content. The ITO/n-CdS/p-CAZTS/Mo heterojunction was well constructed, and the primary photovoltaic characteristics of the n-CdS/p-CAZTS junctions were examined for use in solar cells. Different Ag contents of the CAZTS layers were used to determine the dark and illumination (current–voltage) characteristics of the heterojunctions. The study’s findings collectively point to CAZTS thin layers as potential absorber materials for solar cell applications.

1. Introduction

The growth of renewable energy sources, such as solar energy, is essential for lowering our reliance on fossil fuels and reducing the effects of climate change. While there are still many obstacles to overcome in the development of PV technology, scientists working in the fields of materials science and nanotechnology are making great strides in the direction of solar cells that are highly efficient, inexpensive, and made of abundant natural materials [1]. Since many years ago, manufactured photovoltaic (PV) samples made of copper indium gallium selenide (CIGS) and cadmium telluride (CdTe) have been employed for commercial applications. They are renowned for having conversion efficiencies that are quite high in comparison to those of other thin-film PV technologies, which can reach up to 20% for CdTe and 22% for CIGS. These materials do, however, have a number of serious disadvantages that prevent their broad usage in the solar industry. The toxicity of these materials is one of the main problems. Cost is a further problem with CdTe and CIGS. Even though they are less expensive to create than conventional silicon-based PV systems, they are still rather expensive.

Thin-film PV cells must be produced using complicated manufacturing procedures, which raises the cost of raw materials. Notwithstanding these disadvantages, standard silicon-based PV technologies have a number of benefits over CdTe and CIGS. They are appropriate for applications where weight and flexibility are key considerations, such as building-integrated PV systems, because they are lightweight and flexible. Additionally, they perform better in low-light circumstances and have a faster energy payback period, which means they can produce more energy overall than was needed to manufacture them. There has been an increased interest recently in creating less harmful and cheaper thin-film PV alternatives. Perovskite-based PV technologies are one approach that has shown promise in lab tests and is moving quickly toward industrialization. In comparison to CdTe and CIGS, perovskite-based PV cells have the potential to be more cost-effective, more efficient, less poisonous, and simpler to make. However, further studies are required before they can be widely commercialized in order to overcome the stability and durability difficulties [2,3]. Silicon thin-film solar cells are typically fabricated using amorphous silicon (a-Si) or microcrystalline silicon (μc-Si). They offer advantages such as lower material usage compared to traditional crystalline silicon and can be deposited on flexible substrates, enabling lightweight and potentially lower-cost solar panels. However, they suffer from lower efficiency compared to crystalline silicon solar cells. Cadmium telluride thin-film solar cells are known for their high absorption coefficient, which allows for a relatively thin layer of semiconductor material. This contributes to lower production costs and potentially higher efficiency compared to silicon thin films. However, concerns about the toxicity of cadmium and challenges in recycling the material after the end of the panel’s life cycle remain significant drawbacks. CAZTS thin-film solar cells represent a newer class of materials that offer several potential advantages over Si and CdTe thin films. CAZTS is composed of earth-abundant elements, including copper, zinc, tin, and sulfur, with silver potentially replacing some of the copper to optimize its properties. This reduces concerns about resource depletion and environmental impact compared to CdTe. CAZTS has a tunable bandgap, which means it can be engineered to absorb a broader range of sunlight, potentially improving its efficiency compared to both Si and CdTe. Initial research suggests that CAZTS could achieve efficiencies comparable to or higher than CdTe thin films, making it a promising candidate for next-generation thin-film solar cells. CAZTS has shown promising stability under various conditions, indicating potential longevity and reduced susceptibility to degradation compared to Si and CdTe. Similar to other thin-film technologies, CAZTS can be deposited on flexible substrates, opening possibilities for lightweight and flexible solar panel applications [4,5]. Since the spin coating is quick and inexpensive, it is a common technique for depositing CZTS thin films. The procedure entails coating a substrate with a thin layer of a precursor solution before rapidly spinning the substrate to equally disperse the solution. The precursor is then heated to form a solid CZTS coating on the substrate [6]. Another popular method for depositing CZTS thin films is sputtering. High-energy ions are used to bombard a target material in this process, causing atoms to be expelled and deposited onto a substrate. The composition and qualities of the CZTS film can be controlled by sputtering with a variety of gases, such as argon [7,8]. In thermal evaporation, the precursor substance is heated in a vacuum chamber until it vaporizes, and then allowed to deposit onto a substrate, where it condenses into a solid film. High-quality CZTS films with good crystallinity and purity can be produced using this technique [9,10]. Pulsed laser deposition (PLD) is a process that ablates a target material with a high-energy laser before depositing it onto a substrate. With perfect control over the deposition rate and composition, this technique may make CZTS films that are very uniform, dense, and of the highest quality [11,12]. In order to generate a CZTS film, a preliminary film of electron-beam-evaporated precursors is heated in the presence of sulfur gas after being deposited onto a substrate by electron beam evaporation. Using this technique, superior CAZTS films with good regularity and crystallinity may be created [13,14]. In spray pyrolysis, a precursor solution is sprayed onto a hot substrate, which causes it to break down and produce a CZTS film. Although this technique is rather straightforward and can make large-area CZTS films, it can be difficult to regulate the film’s thickness and homogeneity [15,16].

The PCE for pure CZTS solar cells is currently recorded at 9.2%, whereas the PCE for pure CZTSe solar cells is reported at 11.6%. Mixed CZTS/CZTSe solar cells have a PCE that is marginally higher, at 12.6%. These efficiencies, however, are still below those of CIGS and CdTe solar cells, which have shown efficiencies of up to 21.7%. Notwithstanding their lower efficiency, CZTS-based solar cells continue to spark a lot of research interest because of their inexpensive, non-toxic, and readily available constituent materials. Researchers are investigating a number of methods to increase the stability and efficiency of CZTS-based solar cells, including optimizing deposition techniques, engineering the band structure, and creating new device topologies [17,18].

Highly effective (Cu_1−xAg_x)₂ZnSn(S,Se)₄ solar cells were studied by Xue Yu and colleagues for their use in flexible Mo foil [19]. The efficiency of Cu₂ZnSn(S,Se)₄, encoded as CZTSSe junction, was significantly increased by cation substitution. The bandgap of CAZTSSe thin layers was changed in this study by doping with Ag_x (where x is the content of Ag, ranging from 0 to 0.5). Additionally, it was discovered that Ag doping could clearly raise the CAZTSSe absorber’s average grain size from 0.4 to 1.1 mm. Furthermore, the open-circuit voltage (Voc) steadily dropped as a result. An increase in power conversion efficiency (PCE) from 4.34% to 6.24% was achieved.

The current investigation utilized thermal evaporation to create (Cu_1−xAg_x)₂ZnSnS₄ (CAZTS) thin films on pre-cleaned glasses to examine their structural and optical characteristics at various Ag concentrations (x ranging from 0 to 0.5). The study aimed to investigate the effects of Ag content on the structural, morphological, optical, and electrical characteristics of these extremely thin CAZTS layers. Additionally, spectroscopic ellipsometry was used for the first time to estimate the optical properties and the energy bandgap, of the CAZTS films. Furthermore, the study assembled an ITO/n-CdS/p-CAZTS/Mo heterojunction, and the current density (J) and voltage (V) characteristics of these manufactured heterojunctions were analyzed for various CAZTS layers.

An important strategy of interface engineering should be considered during assembly of the heterojunction solar cells. Interface engineering is a significant strategy to enhance the photovoltaic performance of (Cu_1−xAg_x)₂ZnSnS₄ (CAZTS) thin-film solar cells [20,21]. Therefore, it is important to consider several aspects. First, the choice of buffer layers plays a critical role in enhancing device performance by optimizing the interface between the CAZTS absorber layer and other functional layers (such as electrodes or window layers). Common buffer layers include CdS, Zn(O,S), or more advanced materials, such as CZTS. Second, to minimize interface recombination and enhance the carrier lifetime, surface passivation techniques are employed. This can involve treatments with chemicals or deposition of thin passivating layers to reduce defect states at the surface of the CAZTS film. Third, engineering interfaces to facilitate efficient charge carrier extraction is crucial. This can be achieved through the use of selective contacts, such as transparent conductive oxides (TCOs), e.g., indium tin oxide (ITO) or aluminum-doped zinc oxide (AZO), which are tailored to allow efficient extraction of either electrons or holes from the CAZTS layer. Fourth, the choice of deposition techniques for absorbing layers of CAZTS thin films, the buffer layer, and contacts influences the interface properties and device performance. Optimization of the deposition parameters can improve the film quality and interface characteristics. Therefore, the effective engineering of interfaces in (Cu_1−xAg_x)₂ZnSnS₄ thin films is very important to improve the photovoltaic device performance, advancing toward more efficient and stable solar energy conversion technologies.

2. Experimental Techniques

2.1. Chemicals

Silver nitrate Ag(NO₃), as a source for Ag, copper (II) nitrate Cu(NO₃)₂, for obtaining Cu, zinc acetate Zn(CH₃COO)₂, for formatting the Zn atoms, stannous chloride encoded as (SnCl₂), citric acid in (C₆H₈O₇) form, deionized water, sulfide amine solution, and pure alcohol were the components used to synthesize the CAZTS thin layers.

2.2. Constructing CAZTS Layers

The silver, copper, zinc, and antimony precursors mentioned in the Chemicals Section were used to prepare an aqueous solution for the synthesis of CAZTS thin-film samples by dissolving mixtures of the four precursors in equal proportions equivalent to one mole/liter. In order to produce a translucent solution, after 10 min of mixing and good stirring of the aqueous solution in the preparation flask, metal ions from the aforementioned precursors were added to citric acid (C₆H₈O₇) and deionized water. As a complexing agent, citric acid (C₆H₈O₇) was used. At 150 °C, 6 mL of sulfide amine solution was added to the flask and allowed to react for roughly 30 min. Following that, the CAZTS nanoparticles were separated via a centrifuge. Centrifugation was used to separate the nanoparticles from the solvent. This process exploits differences in particle size and density to isolate the nanoparticles at the bottom of the centrifuge tube. The centrifugation step helped to concentrate the nanoparticles into a more manageable form for subsequent processing. Once separated, the nanoparticles were wet and needed to be dried to remove any residual solvent and achieve a dry powder form. Drying was performed in a vacuum oven to minimize oxidation and to ensure thorough removal of the solvent without damaging the nanoparticles. Vacuum drying is preferred because it allows for lower temperatures to be used compared to conventional drying methods, reducing the risk of nanoparticle aggregation or phase transformation. After drying, the resulting product was referred to as (Cu_1−xAg_x)₂ZnSnS₄ (CAZTS) powder. This powder consists of kesterite nanoparticles with the desired composition and characteristics. The (Cu_1−xAg_x)₂ZnSnS₄ films were prepared by thermal evaporation in a vacuum chamber. The vacuum unit or the DV 502 A-type coating device was set and calibrated at a deposition rate close to 20 angstrom/second and at high pressure in a vacuum chamber of 10⁻⁷ mbar, to obtain virgin thin films via the thermal evaporation method. The influence of film thickness was minimized by putting the same weight on the boat with the same thickness of thin films using the FTM6 thickness monitor. With astonishing accuracy, the film thickness was determined using a spectroscopic ellipsometry method. The required temperature depends on the vapor pressure of the substance at a given vacuum level. The powder will not melt or sublimate in the traditional sense but will evaporate directly into the gas phase. The substrates were heated to elevated temperatures (often at 150 °C) to promote adhesion and crystallinity. In order to enhance the crystallinity, reduce defects, and adjust the film composition, the films were annealed to 500 °C according to TGA analysis.

2.3. Characterization and Measurements

The XRD patterns with Cu-K_α1 radiation (λ = 1.54056), measured by a Philips diffractometry (1710) device, were created to find the structure phase and the structural parameters, such as the crystallite size and strain. The Raman spectroscopy technique was employed to further confirm the crystal structure and identify any structural defects or impurities. For studying the surface morphology, a scanning electron microscope (SEM) with a 30 kV operating voltage was used. The spectroscopic ellipsometry (SE) method was used to investigate the optical characteristics. Using the SE technique (with VASE, J. A. Woollam Co., Inc., Lincoln, NE, USA), data were collected. Two parameters (ψ_exp and Δ_exp) were measured as a function of the wavelength. The obtained data were gathered in the range from 300 to 1100 nm with an incidence angle of ~70°. The J. A. Woollam Complete Ease software program (version 4.05) was used to analyze the SE data. The optical transmission and reflection spectra for the thin films were investigated using a double-beam computer-controlled spectrophotometer (UV-2101, Shimadzu, Kyoto, Japan) at a spectral range of wavelength from 300 to 2500 nm. The heterojunction, “ITO (100 nm)/n-CdS/p-CAZTS/Mo(100 nm)”, fabrication involved layering different materials using thermal evaporation (for CdS, CAZTS, and Mo) and electron beam gun deposition (for ITO). These layers collectively form heterojunctions crucial for the functioning of optoelectronic devices, such as solar cells. Each layer’s deposition method and material composition were carefully chosen to achieve the desired device performance characteristics.

On the other hand, the Keithley 2400 device was used to measure the illumination (current–voltage (I–V)) features, with AM1.5G and P_in = 1800 mW/cm², produced by a 150 W lamp under standard conditions for experimentation.

3. Results and Discussion

3.1. EDAX and Structural Studies

A summary of the EDAX results of (Cu_1−xAg_x)₂ZnSnS₄ thin layers is recorded in

Table 1. Summary of the EDAX results of (Cu_1−xAg_x)₂ZnSnS₄ thin films with various Ag contents (0, 0.1, 0.2, 0.3, 0.4, and 0.5).

x	Cu (mV)	Ag (at/%)	Zn (at/%)	Sn (at/%)	S (at/%)
0	21.06	0	11.71	11.83	55.4
0.1	19.11	2.03	11.81	11.94	55.11
0.2	17.21	4.05	11.78	11.93	55.03
0.3	15.04	6.12	11.68	11.91	55.25
0.4	12.88	8.52	11.82	11.87	54.91
0.5	10.88	10.21	11.85	11.93	55.13

. The X-ray diffraction (XRD) patterns of the (Cu_1−xAg_x)₂ZnSnS₄ thin film with varying Ag contents are presented in Figure 1a. The XRD results confirmed the formation of crystalline CAZTS phases. The CAZTS thin layers exhibited diffraction peaks at 28.53°, 47.32°, and 56.19°, corresponding to the diffraction planes of (112), (220), and (312), respectively. These planes were extracted from JCPDS Card No. 26-0575. This provides strong evidence for the formation of CAZTS in a tetragonal phase. Furthermore, the CAZTS layers exhibited a sharper peak at the (112) plane, indicating an improvement in crystal quality with an increase in Ag content. Figure 1b illustrates the peak shift of the (112) plane. A shift was observed in the diffraction angle toward smaller angles with increasing Ag content. The shift toward smaller diffraction angles in XRD (X-ray diffraction) curves with increasing Ag (silver) content was attributed to a decrease in the interatomic distance between the atoms in the crystal lattice of the sample. The position of the diffraction peaks depends on the spacing between the atoms in the crystal lattice, which is determined by the atomic radii and the crystal structure. As the Ag atoms were substituted for other atoms in the crystal lattice, the interatomic distance between the atoms changed, leading to a shift in the position of the diffraction peaks. As Ag has a smaller atomic radius compared to the Cu element, the substitution of Ag atoms for Cu atoms resulted in a decrease in the interatomic distance, and thus a shift toward smaller diffraction angles in the XRD curve. This behavior can be used to identify the presence of Ag in a sample and to study the impacts of Ag substitution on the crystal structure and properties of the material [22].

The Debye–Scherrer equation is commonly used to determine the average crystallite size in polycrystalline materials from X-ray diffraction data. The equations relate the crystallite size (D) and lattice strain (e) to the main X-ray wavelength (λ), the diffraction angle (Bragg’s angle θ), and the full width at half-maximum (β) of the diffraction peak [23,24]:

$$D=\left({\displaystyle \frac{0.94}{\beta }}\right).{\displaystyle \frac{\lambda }{\cos \theta }} \& e={\displaystyle \frac{\beta }{4\tan \theta }} \mathrm{and} \beta =\sqrt{{\beta }_{obs}^2-{\beta }_{std}^2}$$

(1)

where β is the widening equivalent of the difference in profile width between the films (β_obs) and the standard silicon (β_std).

The size of the crystallites increased as a result of increasing the Ag content, as seen in Figure 2, whereas the lattice strain values dropped with the increasing Ag concentration. The size of crystallites increased when Ag content increased because it serves as a site of nucleation for the development of new crystals or as a catalyst for the production of existing crystals. This is due to the possibility of extra nucleation sites for crystal growth being provided by the existence of Ag atoms, which encourages the building of bigger crystals. Lattice strain, on the other hand, measures the departure from the ideal crystal structure and can be caused by a variety of things, such as impurities or flaws in the crystal lattice. Because more Ag atoms can aid in reducing strain in the crystal lattice, the lattice strain values decreased as the Ag content rose. This is because Ag atoms have a higher atomic radius than the host metal atoms, which can cause the crystal lattice to become distorted. More Ag atoms were integrated into the lattice as the Ag concentration increased, which could aid in restoring the optimum crystal structure and lowering the lattice strain. Overall, as a result of Ag’s influences on crystal development and lattice distortion, an increase in the Ag concentration caused crystallites to grow larger and lattice strain values to drop.

3.2. Morphological Properties

With a focus on the impact of increasing the Ag concentration, SEM was utilized to find the surface morphological state of the CAZTS thin layers. The surface characteristics of the samples under study are depicted in the SEM images in Figure 3, along with the particle size diagrams. According to the SEM images, the first CAZTS film had a rough, uneven surface and obvious voids in its structural frame. The CAZTS film’s surface structure, however, was significantly impacted by the addition of Ag content. The films became more homogeneous and compact, with tightly packed grains that had specific structural characteristics. As a consequence, the rest of the films had fewer voids and appeared smoother than the first. The grain and its limits could be seen more clearly in the SEM images as the Ag content rose. Overall, the SEM images showed that increasing the Ag concentration significantly enhanced the surface morphology of the CAZTS thin films, leading to more uniform, tightly packed, and identifiable grain patterns. This may result in a smoother, more predictable surface morphology and a more equal distribution of atoms on the surface. As a result, the grain patterns were more clearly defined and closely packed. Additionally, the crystal structure of the CAZTS thin films can be changed by the addition of Ag. As a result, more homogeneous and smaller grains may form, further enhancing the thin films’ surface morphology. This phenomenon can be linked to the fact that Ag’s crystal structure differs from CAZTS’s, which can alter how the thin films’ crystals arise. The SEM images generally demonstrated that raising the Ag content in CAZTS thin films can greatly improve their surface morphology, resulting in more regular, densely packed, and recognizable grain patterns. This enhancement can be attributed to Ag’s capacity to change the crystal structure of the thin films, in addition to its surfactant effect on the formation of the thin films.

3.3. Raman Spectra

Raman spectroscopy is a powerful technique used to analyze the vibrational modes of materials, providing insights into their molecular and crystal structures. In the case of (Cu_(1−x)Agx)₂ZnSnS₄ thin films, Raman spectroscopy can reveal valuable information about the composition, crystallinity, and structural changes induced by varying the Ag content (x). Raman spectra typically exhibit peaks corresponding to specific vibrational modes of the material. For (Cu_(1−x)Agx)₂ZnSnS₄, these modes could include vibrations of Cu, Ag, Zn, Sn, and S atoms within the crystal lattice. Each element contributes to distinct Raman-active modes that can be identified in the spectrum. To determine the phase structure and crystallinity of the CAZTS thin films, Raman spectroscopy was used (Figure 4a). The CAZTS samples’ acquired Raman spectra, which in turn were captured in the spectral range between 100 and 600 cm⁻¹, were discovered to be remarkably similar. The three unique Raman modes in the spectra, with strong peaks at 286 cm⁻¹ and 340 cm⁻¹, corresponded to the kesterite structure of CAZTS. These results imply that the CAZTS thin films were naturally crystalline and extremely pure. Consequently, the Raman analysis offered additional evidence in favor of the development of CAZTS thin films with desired properties [25,26]. Figure 4b displays a graphic depiction of the Raman spectra. The primary peak seen at 340 cm⁻¹ in the Raman spectra transitioned to lower energies as the Ag content increased. The presence of internal stress in the CAZTS layers was responsible for this shift. This phenomenon has been previously reported in CAZTS layers [27,28]. According to the Raman spectroscopy data, weak peaks were observed at 476 cm⁻¹, which can be due to the presence of the Cu₂S phase. As the Ag content (x) varied in (Cu_(1−x)Agx)₂ZnSnS₄, a lower shift in the peak positions might be observed due to changes in the atomic masses and bond strengths. This can provide information about the alloying effects and the substitution of Ag for Cu. Changes in peak intensities can indicate variations in the local symmetry or disorder within the crystal structure. For (Cu_(1−x)Agx)₂ZnSnS₄, the reduction in intensity (see Figure 4a) could reflect differences in the Ag concentration. This characterization is essential for optimizing the material’s properties for various applications, such as photovoltaics or optoelectronics.

3.4. Spectroscopic Ellipsometry

The optical constants (n and k) and thickness (d) of CAZTS thin films can have a considerable impact on their performance attributes, and spectroscopic ellipsometry (SE) is a widely used method to precisely characterize these features. SE is an important optical method that monitors polarization shifts in light reflected from surfaces. By analyzing the changes in the polarization of light, SE can provide information on the optical constants of thin layers, including their refractive index, extinction coefficient, and thickness. SE can also be used to study the anisotropic properties of materials, such as birefringence. For CAZTS thin films, accurate knowledge of the optical constants is crucial for optimizing the performance of the films in photovoltaic and optoelectronic applications. The n and k constants of the films determine the absorption and reflection of light, which can have a significant impact on the efficiency of solar cells or other devices that rely on the absorption of light. By using SE to measure the reflectance and phase shift of light at multiple wavelengths and incident angles, it is possible to accurately determine the optical constants of CAZTS layers. These measurements can then be used to model the optical properties of the films and optimize their performance in specific applications. Additionally, SE can also be used to determine the thickness of the CAZTS layers, which is another critical parameter for device performance. Accurate measurement of the films’ thickness can be utilized to optimize the fabrication process and ensure consistent performance across multiple devices. In summary, SE is a powerful technique for accurately characterizing the optical properties of CAZTS films, which is crucial for optimizing their performance in photovoltaic and optoelectronic applications.

In the formula connecting the spectroscopic and ellipsometric parameters, Ψ and ∆, Fresnel’s factor of the polarized light can be given as follows [29,30]:

$$\rho ={\displaystyle \frac{r_p}{r_s}}=\mathit{tan}\psi \mathit{exp}(i\Delta )$$

(2)

Here, r_p and r_s portray Fresnel’s factor of the parallel polarized wave (p) and perpendicularly polarized wave (s). The ellipsometric parameters (ψ_exp and Δ_exp) of CAZTS/glass layers are illustrated in Figure 5. The measurements are frequently conducted in spectroscopic ellipsometry at various wavelengths (in this case, every 5 nm from 300 to 1100 nm), and the values are noted at each wavelength. Using 1’modeling software version 4.05, these data can then be utilized to determine the complicated refractive index and thickness of the CAZTS film. The plane of incidence, which is the plane where light beams are incident and reflected, and the normal surface of the CAZTS film were at an angle of 70 degrees. In order to examine the anisotropic characteristics of thin films, this angle is frequently employed in spectroscopic ellipsometry.

A three-layer optical approach, with the substrate composed of glass as the first layer, the CAZTS absorber layer as the following one, and the surface roughness layer as the last layer, was used to calculate n, k, and d of the studied CAZTS layers. In addition, the Complete EASE software, Cauchy’s version of the belt, was used to simulate the glass layers, while the B-spline computational method was used to model the CAZTS layers. Effective medium approximation (EMA) was used to model rough layers, which is a useful tool for determining the morphology of multiple layers [31,32]. In the spectroscopic ellipsometry software EASE (Essential Macleod for Analysis, Synthesis, and Education), the Cauchy model (n(λ) = A + B\λ²) refers to a specific model used to describe the optical properties (typically the refractive index) of a glass substrate. Here, (A) and (B) are coefficients that were determined through fitting experimental data or through theoretical calculations. By fitting the Cauchy model to experimental ellipsometry data (which provide information about how polarized light interacts with the material), the software can extract parameters, such as the refractive index and extinction coefficient, across a range of wavelengths. The software then adjusts the model parameters (A) and (B) to minimize the difference between the measured data and the model prediction, thereby determining the best-fit refractive index as a function of the wavelength. The B-spline computational method is particularly suitable for modeling CAZTS layers in spectroscopic ellipsometry due to its ability to accurately represent the optical properties of thin films and layered structures. There are several reasons why the B-spline method was chosen for this purpose. The first is that the B-splines can model complex curves and surfaces with high fidelity. This flexibility is essential when dealing with the varying thicknesses and compositions that may characterize CAZTS layers, as spectroscopic ellipsometry measurements can reveal subtle variations in optical properties across these layers. The second is that B-splines provide a parameterization of the layer structure that can be directly related to physical properties, such as the refractive index, extinction coefficient, and layer thickness. This parameterization facilitates the extraction of important optical constants from the ellipsometric data, which are crucial for understanding the material properties of CAZTS layers. This adaptability is beneficial when modeling CAZTS layers that may exhibit variations in composition or thickness.

Figure 6a,b illustrates the spectral variations of ψ_exp and Δ_exp for the last sample, which agreed with the computed ψ_i^cal and Δ_i^cal data observed via the mentioned model. When these figures were fitted, the low mean square error (MSE) values ranged from 2.46 to 2.35 as the studied thin films and the surface roughness decreased from 3.70 nm to 2.75 nm. The coherence of the reflection pairs in the thin film is another factor that contributes to the interference arrangement in the spectrum of light [33]. The main fitted n and k constants of the studied layers are illustrated in Figure 7 and Figure 8, respectively. Figure 8 shows that the n-spectra grew as the Ag content increased in CAZTS layers due to the increase in crystallinity caused by the larger crystallite size [34,35]. Figure 9 shows the k-spectra of the CAZTS/glass layer obtained using the model outlined above. Importantly, lower k values were obviously detected at the absorption edge, verifying that the light was completely absorbed by the fabricated layers [36].

The formula: α = 4πk/λ, relates the absorption coefficient (α) to the absorption index (k) and wavelength (λ). The proportion of incident light absorbed by a substance per unit route length is indicated by the absorption index (k), a dimensionless quantity. The absorption coefficient (α), which is impacted by different variables, including the composition, thickness, and microstructure of the material, offers information on how strongly the material absorbs light at a specific wavelength in the instance of CAZTS/glass layers with varied Ag contents. One may ascertain the optical characteristics of these materials and their prospective uses in optoelectronics, photovoltaics, and other sectors by determining the absorption coefficient for varied Ag concentrations of CAZTS/glass layers [37].

CAZTS materials are quaternary semiconductors that have undergone substantial research in preparation for their prospective application in photovoltaic solar cells. Their efficiency as a solar absorber is greatly influenced by the materials’ optical properties, particularly the energy bandgap. For extracting the energy bandgap of semiconductors, researchers frequently utilize the Tauc expression. It is based on a material’s ability to absorb photons of a particular energy, as determined by the material’s absorption coefficient. The following can be used to express the Tauc expression [38,39,40]:

$$\left(\mathsf{\alpha }\mathrm{h}\mathsf{\upsilon }\right)={\mathsf{\alpha }}_{\mathrm{o}}(\mathrm{h}\mathsf{\upsilon }-{\mathrm{E}}_{\mathrm{g}}^{\mathrm{o}\mathrm{p}\mathrm{t}}{)}^{\mathrm{p}}$$

(3)

In this formula, p and ${\mathsf{\alpha }}_{\mathrm{o}}$ represent the exponent and constant. The index value, p, is a parameter that depends on the nature of the bandgap (direct or indirect) and determines the transition type from VB to CB. For the polycrystalline character of the CAZTS layers under investigation, the permitted direct transition was dominant, with p = 1/2 [41,42]. Figure 9 shows the plotting of (αhν)² versus hν for different Ag contents of CAZTS layers. The energy bandgap, $E_g^{opt}$, was determined by subtracting the measured data intercept from the absorption coefficient’s linear extrapolation to zero after the measured data had been fitted to the Tauc expression. As the Ag concentration in the CAZTS layers increased, the bandgap value for the CAZTS/glass film dropped (see Figure 10), demonstrating sigmoidal behavior. This was due to the possibility that the addition of Ag will cause some Zn and/or Cu sites in the CAZTS lattice to partially substitute. The bandgap of the material decreased as a result, showing the impact of doping Ag (silver) atoms into a CAZTS (copper zinc tin sulfide) lattice, specifically how it affects the bandgap of the material. Silver (Ag) has a lower electronegativity compared to zinc (Zn) or copper (Cu). Electronegativity is a measure of an element’s tendency to attract electrons toward itself in a chemical bond. Because Ag has a lower electronegativity, it is more willing to give up its electrons. When Ag atoms are doped into the CAZTS lattice, they contribute electrons to the lattice. This is due to the lower electronegativity of Ag. These extra electrons can affect the electronic structure of the material. The extra electrons from the Ag doping contribute to the conduction band of the material. The conduction band is the energy band in a material, where electrons are free to move and conduct electricity. When there are more electrons in the conduction band, the bandgap decreases. Lowering the bandgap means it requires less energy to excite an electron from the valence band (where electrons are normally located) to the conduction band [43]. This can have implications for the material’s optical and electronic properties. Several other factors besides doping can affect the decrease of the bandgap of a material. These include structural factors [44], defects in the lattice, and stress in the material’s surface [45].

The transmission and reflection spectra of (Cu_1−xAgx)₂ZnSnS₄ thin films with different Ag concentrations (0, 0.1, 0.2, 0.3, 0.4, and 0.5) are displayed in Figure 11. The transmittance in transparent regions increased as the Ag content rose, according to these figures, whereas the reflectance decreased as the Ag level rose. The improvement in transmittance with increasing Ag content for (Cu_1−xAg_x)₂ZnSnS₄ thin films could also be related to changes in the crystal structure or morphology of the thin films. Ag substitution could lead to a more orderly crystal structure or reduce defects that otherwise absorb or scatter light, thus enhancing transparency. Also, a higher Ag content might result in smoother film surfaces, reducing scattering losses and allowing for better transmission of light through the material. However, the decrease in reflectance with the increasing Ag content suggested alterations in the surface and electronic structure that make the surface less reflective. This could be due to a smoother surface, as mentioned, or changes in the electronic states at the surface, affecting how light interacts with the material. An increased Ag content might improve the optical impedance matching between the film and its substrate, thus reducing the amount of light reflected from the surface. This effect would enhance the efficiency of optical devices by minimizing energy losses due to reflection. A decrease in reflectance, coupled with an increase in transmission in specific regions, might also suggest that absorption in other wavelength regions could be changing. This is critical for applications such as photovoltaics, where controlling absorption profiles is essential for maximizing efficiency. The described changes in the optical properties of (Cu_1−xAgx)₂ZnSnS₄ thin films with different Ag concentrations have significant implications for photovoltaic applications, particularly in designing more efficient solar cells. By tailoring the Ag content, it might be possible to optimize these materials for maximum light absorption and conversion efficiency, leveraging the enhanced transmission and reduced reflectance.

3.5. Features of p-n Junction

The main diagram of the examined junction is shown in Figure 12. It should be noted that CAZTS extracts the key solar cell fabrication parameters to predict how the illumination (J–V) characteristics will behave. The applied voltage and the current are connected, and the formula described in [46] provides the remaining properties of the manufactured diode. Figure 13a shows the illumination J–V characterization of junctions. Figure 13b shows the illumination P–V characterization of junctions under illumination in forward bias conditions. When a P–V cell is illuminated and operated in forward bias, it generates power due to the photovoltaic effect. The power conversion efficiency (PCE) of these devices is a critical parameter, indicating how efficiently they can convert sunlight into electrical energy. In the forward bias, the P–V characteristics in the illumination case are illustrated in Figure 13b. The $\mathrm{P}\mathrm{C}\mathrm{E}={\mathrm{P}}_{\mathrm{m}\mathrm{a}\mathrm{x}}\times {{\mathrm{P}}_{\mathrm{i}\mathrm{n}}}^{-1}\%$ formula [47,48] (in which ${\mathrm{P}}_{\mathrm{m}\mathrm{a}\mathrm{x}}$ is the maximum power (in Figure 13b) and ${\mathrm{P}}_{\mathrm{i}\mathrm{n}}$ is the experimental value of input power) helped us in the calculation of the power conversion efficiency (PCE) for the generated devices. On the other hand, the fill factor (FF) was computed utilizing $FF=\left((V_{\max }.I_{\max })\times {(V_{OC}.I_{SC})}^{-1}\right)=P_{\max }\times {(V_{OC}.I_{SC})}^{-1}$ [49,50]. Here, V_oc is the open-circuit voltage—the voltage measured across the terminals of the P–V cell when no current is flowing (i.e., when the circuit is open). I_sc is the short-circuit current, the current that flows when the cell’s terminals are shorted together (i.e., when the voltage across the cell is zero). The ${\mathrm{V}}_{\mathrm{m}\mathrm{a}\mathrm{x}}$ and $I_{\max }$ are the voltage and the current corresponding to the maximum power points, ${\mathrm{P}}_{\mathrm{m}\mathrm{a}\mathrm{x}}$. The fill factor is a dimensionless number that ranges between 0 and 1, often expressed as a percentage. Higher fill factor values indicate a more square-like I–V characteristic curve, suggesting that the solar cell is capable of operating closer to its V_oc and I_sc under load, which in turn implies higher efficiency and quality of the solar cell. Figure 14a shows the behavior of J_sc and V_oc and Figure 14b shows the behavior of FF–η curves as a function of the Ag content of (Cu_(1−x)Ag_x)₂ZnSnS₄ thin films. As shown in Figure 14b, the PCE in the illumination state with increasing Ag content, which corresponds to x = 0.4 and 0.5% Ag, confirmed that the layer with the largest silver content was the best for use in the solar cells in this work. Incorporating silver (Ag) into the (Cu_(1−x)Agx)₂ZnSnS₄ thin films (Ag from 0 to 0.5) improved the performance of the solar cells because, first, silver has higher electrical conductivity compared to copper (Cu). By substituting Ag for Cu in the kesterite structure (Cu_(1−x)Agx)₂ZnSnS₄, the overall electrical conductivity of the material can be enhanced. This improvement in conductivity can facilitate better charge carrier transport within the semiconductor. Second, increasing the Ag content from 0 to 0.5 in the (Cu_(1−x)Agx)₂ZnSnS₄ thin films led to enhanced I_sc and V_oc. Typically, these changes are monitored because they directly influence the overall efficiency of the solar cell. Third, the FF parameter represents how efficiently the solar cell converts incident light into electricity by considering factors such as charge carrier mobility and recombination losses. Incorporating silver can potentially improve FF by enhancing the charge carrier mobility. Fourth, the power conversion efficiency is the ultimate measure of how effective a solar cell is at converting sunlight into electricity. By improving Isc, Voc, and FF through the incorporation of Ag, the overall PCE of the solar cell can be increased. According to the results, the thin film with the highest Ag content, corresponding to x = 0.4 or 0.5%, showed the best performance. This suggests that there is an optimal amount of silver incorporation that maximizes the electrical conductivity and consequently enhances the solar cell efficiency. Therefore, the layer with the highest silver content was identified as the most suitable for use in solar cell applications in this study.

Also, higher mobility reduced the resistive losses within the cell, allowing for more efficient charge transport from the point of generation to the collection electrodes. The introduction of Ag, known for its strong plasmonic resonances, can significantly impact the optical properties of the solar cell. These resonances can lead to improved light trapping within the semiconductor layer, as metallic nanoparticles or inclusions scatter and absorb light, increasing the path length of photons in the material. This results in more light absorption and, consequently, a higher generation rate of electron–hole pairs, which is critical for enhancing the cell’s photocurrent and overall efficiency. The doping or substitution process involving Ag can also influence the crystalline structure of the (Cu_(1−x)Ag_x)₂ZnSnS₄ thin films. By potentially forming a more ordered crystalline lattice or healing defects that would otherwise act as recombination centers, Ag incorporation can enhance the quality of the semiconductor. Fewer recombination sites mean that once generated, charge carriers (electrons and holes) have a higher probability of reaching the electrodes before recombining, effectively increasing the carrier lifetime and the device’s efficiency. The combined effects of increased charge carrier mobility, enhanced light trapping due to plasmonic effects, and improved material quality leading to reduced recombination all contribute to the overall increase in the solar cell’s efficiency.

4. Conclusions

This work investigated the effects of varying the Ag content on the structural, morphological, and optical properties of thin layers of CAZTS, which were prepared via the thermal evaporation method. The XRD patterns revealed that the thin layers had a kesterite band structure, with a crystallite size ranging from 11 to 33 nm. The optical properties of the CAZTS/glass film were found to improve with the increasing Ag content, as demonstrated by an increase in the optical constants n and k. Additionally, the direct transition energy decreased from 1.74 to 1.55 eV. Since Ag atoms increase electrons to the CAZTS lattice when they are doped, the results also established that the produced heterojunctions’ optical properties improved with the increasing Ag content. Ag has a lower electronegativity, which clarifies this result. The material’s electrical structure may be impacted by these additional electrons. The material’s conduction band is improved by the additional electrons from the Ag doping. When a photovoltaic cell works in forward bias with illumination, it produces power. One important factor that shows how well these devices can change sunlight into electrical energy is their power conversion efficiency (PCE). A more square-like I–V characteristic curve, indicated by greater fill factor values, proposes that the solar cell can operate closer to its V_oc and I_sc under load, indicating higher efficiency and quality of the solar cell. The layer with the highest silver concentration was the best for use in the solar cells in this work, as confirmed by the PCE in the illumination state with increasing Ag content, which corresponded to x = 0.4 and 0.5 Ag. All things considered, this work offered insightful information on the characteristics and behavior of CAZTS thin layers with different Ag contents, information that may help advance the formation of more effective solar cell technologies.