3.1. EDX and Structural Studies
A summary of the EDAX results of (Cu
1−xAg
x)
2ZnSnS
4 thin layers is recorded in
Table 1. The X-ray diffraction (XRD) patterns of (Cu
1-xAg
x)
2ZnSnS
4 thin film with varying Ag content are presented in
Figure 1(a). The XRD results confirm the formation of crystalline CAZTS phases. The CAZTS thin layers exhibit diffraction peaks at 28.53°, 47.32°, and 56.19°, corresponding to the diffraction planes of (112), (220), and (312), respectively. These planes are extracted from JCPDS card with no.26-0575. This provides strong evidence for the formation of CAZTS in a tetragonal phase. Furthermore, the CAZTS layers exhibit a sharper peak at (112) plane, indicating an improvement in crystal quality with an increase in Ag content.
Figure 1 (b) illustrates the peak shift of (112) plane. A shift is observed in the diffraction angle towards smaller angles with increasing Ag content. The shift towards smaller diffraction angles in XRD (X-ray diffraction) curves with increasing Ag (silver) content can be 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 are substituted for other atoms in the crystal lattice, the interatomic distance between the atoms changes, 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 results in a decrease in the interatomic distance, and thus a shift towards 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 [
24,
25,
26].
The Debye-Scherrer equation is commonly used to determine the average crystallite size in polycrystalline materials from X-ray diffraction data. The equations relates 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 [
26,
27]:
β is the widening equivalent of the difference in profile width between the films (βobs) and the standard silicon (βstd).
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 increasing Ag concentration. The size of crystallites rises when Ag content increases 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 decrease as the Ag content rises. This is since Ag atoms have a higher atomic radius than the host metal atoms, which can cause the crystal lattice to be distorted. More Ag atoms are integrated into the lattice as the Ag concentration rises, which can aid in restoring the optimum crystal structure and lowering lattice strain. Overall, as a result of Ag's influences on crystal development and lattice distortion, an increase in Ag concentration causes crystallites to grow larger and lattice strain values to drop.
3.4. Spectroscopic Ellipsometry
The optical constants (n, 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 polarisation 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 acquaintance of the film 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.
The formula connecting the spectroscopic ellipsometric parameters Ψ and ∆ Fresnel's factor of the polarized light can be given as follows [
29,
30,
31]:
Here, r
p and r
s portray Fresenal's factor of the parallel polarized wave (p) and perpendicularly polarized wave (s). The ellipsometric (ψ
exp and Δ
exp) of CAZTS /glass layers among a outcome of studied CAZTS is illustrated in
Figure 6. The measurements are frequently made in spectroscopic ellipsometry at various wavelengths (in this case, every 5 nm from 300 to 1100 nm), and the values of and are noted at each wavelength. Using 1'modeling software, 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 surface normal of the CAZTS film are 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's Cauchy 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) is used to model rough layers, which is a useful tool for determining the morphology of multilayers [
32,
33,
34,
35].
Figure 7(a, b) illustrates the spectral various of ψ
exp., Δ
exp for the last sample, which agree with the computed ψ
ical, Δ
ical 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' Ag content grew from 0 to 0.5%Ag and the CAZTS layer's surface roughness increased 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 [
36]. The main fitted (n and k) constants of studied layers are illustrated in
Figure 8 and Figure 9, respectively. Due to the increase in crystallinity caused by the larger crystallize size, the n-spectra grows as the Ag content increases in CAZTS layers [
37,
38].
Figure 8 shows the k-spectra of the CAZTS/glass layer obtained using the model averred above. Importantly lower k values are obviously detected at the absorption edge, verifying that the light is completely absorbed by the fabricated layers [
39].
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 a different of 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 [
40].
CAZTS materials are quaternary semiconductors that have undergone substantial research in preparation for their prospective application in photovoltaic solar cells. Efficiency as a solar absorber is greatly influenced by the material's optical properties, particularly its energy band gap. For extracting the energy band gap 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 [
41,
42,
43]:
In this formula, p and
α0 incarnate exponent and constant. The index value, p is a parameter that depends on the nature of the band gap (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 is dominant with (p = 1/2) [
44,
45,
46].
Figure 10 shows the plotting of (αhν)
2 versus hν for different Ag contents of CAZTS layer. The energy band gap
was determined by subtracting the measured data's 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 increases, the band gap value for the CAZTS/glass film drops (see
Figure 11) as sigmoidal behavior. This is 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 band gap of the material decreases as a result, the impact of doping Ag (silver) atoms into a CAZTS (Copper Zinc Tin Sulfide) lattice, specifically how it affects the band gap 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 towards itself in a chemical bond. Because Ag has a lower electronegativity, it's 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 band gap decreases. Lowering the band gap means it requires less energy to excite an electron from the valence band (where electrons are normally located) to the conduction band [
47,
48,
49],. This can have implications for the material's optical and electronic properties. Several other factors besides doping can affect the decrease of band gap of a material. These include structural factors[
50], defects in the lattice and stress in the material's surface [
51,
52,
53].
The transmission and reflection spectra of (Cu1−xAgx)2ZnSnS4 thin films with different Ag concentrations (0, 0.1, 0.2, 0.3, 0.4, 0.5) are displayed in
Figure 12. The transmittance in transparent regions increases as the Ag content rises, according to these figures, whereas the reflectance decreases as the Ag level rises. The improvement in transmittance with increasing Ag content for Cu
1−xAg
x)
2ZnSnS
4 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, Higher Ag content might result in smoother film surfaces, reducing scattering losses and allowing for better transmission of light through the material. But, the decrease in reflectance with increasing Ag content suggests 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. Increased Ag content might be improving 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 like photovoltaics, where controlling absorption profiles is essential for maximizing efficiency. The described changes in the optical properties of (Cu1−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 13. One should be aware 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 the Ref. [
53] provides the remaining properties of the manufactured diode.
Figure 14(a) shows the illumination (J-V) characterization of junctions.
Figure 14(b) 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 14 (b). The
PCE = Pmax × (Pin)−1% formula [
54,
55], (which
Pmax incarnates the maximum power in
Figure 13 (b), and
Pin 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 side, the fill factor (FF) is computed utilizing
[
54,
55]. Here, V
oc is the open-circuit voltage, the voltage measured across the terminals of the PV 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
Vmax and
Imax are the voltage and the current corresponding to the maximum power points
Pmax. 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 15 (a) shows the behavior of J
sc and V
oc and
Figure 15 (b) also shows the behavior FF-η curves as a function of Ag content of (Cu
(1-x)Ag
x)
2ZnSnS
4 thin films. As shown in
Figure 15 (b) the (PCE) in the illumination state with increasing Ag content which corresponds to x = 0.4 and 0.5 % Ag results confirms that the layer with the largest silver content is the best for use in the solar cells in this work. By substituting Ag for Cu in the kesterite structure, the essentially tuning the electrical properties of the material. Silver has a higher electrical conductivity and incorporating it into the lattice can enhance the charge carrier mobility within the semiconductor. Higher mobility reduces 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)
2ZnSnS
4 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.