CuZnS thin films were prepared using the chemical bath deposition (CBD) method, which allows low-cost, uniform, and large-area thin film growth. The precursors chosen were 5 mL 0.1 M copper chloride (CuCl₂) and 5 mL 0.1 M zinc chloride (ZnCl₂) as copper and zinc sources due to their high solubility, purity, and wide availability. 10 mL 0.1 M Thiourea was selected as the sulphur source because it provides controlled release of sulphide ions at moderate temperatures. 4 mL Triethanolamine (TEA) and 3 mL ammonia were used as complexing agents to control the release rate of metal ions, stabilise the precursor solution, and promote uniform film growth.

The concentrations and volume of the precursors were optimised based on our previous studies [15]. We could prepare stoichiometric CuZnS thin films with good surface coverage on microscopic glass substrates of dimension 75 mm × 25 mm × 1.35 mm. Initially, a copper complex was formed by mixing 5 mL CuCl₂ with 2 mL TEA, and a zinc complex was prepared by mixing 5 mL ZnCl₂ with 2 mL TEA and 3 mL ammonia. The two solutions were combined to ensure homogeneous mixing, followed by the addition of thiourea as the sulphur source. The pH of the precursor bath was maintained at 11.5, which is favourable for controlled sulphide ion release and stable film growth in the CBD process. The reaction bath temperature was kept at 80°C, and the deposition was carried out for 1 h, which provides sufficient thermal energy for uniform nucleation and growth of CuZnS thin films on glass substrates. The glass substrates were ultrasonically cleaned using a standard sequential cleaning procedure involving detergent solution, distilled water, acetone, and ethanol to remove surface contaminants and enhance film adhesion. After deposition, the substrates were removed from the bath, thoroughly rinsed with distilled water to eliminate loosely bound particles, and dried under ambient conditions. The as-deposited CuZnS thin film was designated as CZS1. Post-deposition, the films were annealed at 100°C, 200°C, and 300°C for 1 h, and the corresponding samples were labelled CZS2, CZS3, and CZS4, respectively. Annealing was performed to improve crystallinity, remove residual defects, enhance grain growth, and tune the optoelectronic properties of the films. The annealing temperatures were selected based on the thermal stability of CuZnS and prior literature, ensuring structural improvements without decomposition of the material [16,17].

Structural and compositional analyses were performed using XRD (Bruker Kappa Apex II), XPS (K-Alpha Thermo Scientific), and SEM-EDAX (JEOL 6390LA/Oxford XMX N). Optical and electrical properties were evaluated using UV–Vis–NIR spectrophotometry (JASCO V-670) and a Hall measurement system (Ecopia HMS-3000). All measurements were carried out on representative samples under identical preparation conditions.

For the structural and compositional evaluation of the synthesized and annealed CuZnS samples, X-ray Photoelectron Spectroscopy (XPS), a powerful and surface-sensitive analytical technique, was employed [18,19,20,21]. The survey spectra provided information on the elemental composition and the relative intensities of the constituent elements. Fig. 1 and Fig. 2 represent the full survey spectra along with high-resolution scans of Cu 2p, Zn 2p, and S 2p core levels. Characteristic peaks corresponding to Cu, Zn, S, C, and O, along with Auger peaks such as Cu LMM and Zn LMM, are observed in all samples (CZS1–CZS4).

The Cu 2p_3/2 and Cu 2p_1/2 peaks appeared at approximately 931 eV and 951 eV, respectively, with a spin–orbit splitting of 20 eV, confirming the presence of Cu²⁺. The Zn 2p_3/2 and Zn 2p_1/2 peaks were located at 1021 eV and 1042 eV, indicating Zn²⁺ states. The S 2p spectra of the as-deposited sample (CZS1) showed two peaks at 162.5 eV and 167 eV, associated with metal–sulphide and oxidised sulphur species, respectively. With increasing annealing temperature, the intensity of the S 2p peak at 162.5 eV decreased significantly, suggesting partial sulphur loss due to thermal evaporation during annealing.

Quantitative elemental composition was determined using CasaXPS software based on peak-area integration and sensitivity-factor correction. The uncertainty in the atomic percentages was estimated to be within ±2–5 at.%, arising mainly from background subtraction, peak fitting, and instrumental sensitivity factors. Despite this uncertainty, the observed compositional trends with annealing temperature are systematic and exceed the estimated error margins. The relative intensities and atomic percentages of Cu and Zn increased with annealing, while those of sulphur decreased, consistent with the quantitative elemental analysis presented in Table 1 and Table 2. These changes indicate an enhancement in metal content and a corresponding reduction in sulphur concentration upon thermal treatment. Among all samples, CZS3 exhibited a comparatively balanced stoichiometry (Cu: 27.13 ± 2.0 at.%, Zn: 23.98 ± 2.0 at.%, S: 48.90 ± 3.0 at.%). It also showed stronger metal–sulphide peak intensities, suggesting improved compositional uniformity and enhanced suitability for photovoltaic absorber applications.

Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS) were employed to determine the bulk and surface compositions, respectively, of the as-deposited and annealed CuZnS thin films. The quantitative results are summarised in Table 1 and Table 2, comparing both techniques. Both EDX and XPS results reveal a progressive decrease in sulphur content with increasing annealing temperature, accompanied by a relative increase in Zn and Cu concentrations. This compositional shift can be attributed to the thermal desorption of volatile sulphur species at elevated temperatures, leading to a more metal-rich surface. Simultaneously, cation diffusion and rearrangement occur, enabling Zn atoms to occupy thermodynamically stable lattice sites, which enhances overall crystallinity. Notably, the CZS3 sample (annealed at 200°C) exhibits a more balanced cation–anion stoichiometry with a Cu/Zn ratio close to unity (≈1.1 in XPS and 0.89 in EDX). Such a near-stoichiometric composition is advantageous for photovoltaic applications, as it minimises deep defect levels and improves carrier transport within the absorber layer. In contrast, highly Cu-rich or Zn-rich films (e.g., CZS1 and CZS4) may contain antisite or vacancy defects that degrade device efficiency. The discrepancies between XPS and EDX values arise from the difference in analysis depth: XPS probes only the top few nanometers of the film, whereas EDX provides an average bulk composition. Despite these differences, both techniques show consistent compositional trends, validating the observed cation–anion adjustments during annealing. Fig. 3 displays the representative EDX spectrum of the CZS3 sample, confirming the presence of Cu, Zn, and S without detectable impurities. The compositional optimisation achieved in the CZS3 sample supports its improved optoelectronic behaviour, as discussed in the subsequent sections.

X-ray Diffraction (XRD) was employed to study the structural and crystallographic characteristics of the as-deposited and annealed CuZnS thin films [22]. The diffraction patterns were recorded in the 2θ range of 10°–90°, as shown in Fig. 4. The XRD patterns reveal broad and low-intensity peaks, suggesting the formation of nanocrystalline or partially amorphous CuZnS structures. The broadening of diffraction peaks is characteristic of nanocrystallite sizes, which were calculated using the Debye-Scherrer equation. D = K λ β cos θ where,

D = average crystallite size

K = Scherrer constant ~ 0.94 (depends on shape factor)

λ = X-ray wavelength in nm or Å, 1.5406 Å for Cu Kα lines

β = Full width at half maximum of the peak in radians

θ = Bragg angle

The calculated crystallite sizes of all the films were found to be below 5 nm. Such nanoscale crystallites are typical in chemically deposited thin films due to limited adatom mobility during growth. With increasing annealing temperature, a gradual enhancement in peak intensity and a slight reduction in peak broadening were observed, indicating improved crystallinity and reduced lattice disorder. This enhancement can be attributed to thermally induced atomic rearrangements and grain coalescence, which promote more ordered crystalline domains within the film matrix. The characteristic diffraction peaks correspond to the (002) and (022) planes, which are consistent with the reported patterns for cubic CuZnS phases [23,24,25]. The absence of additional impurity peaks confirms the phase purity of the films. Overall, annealing promotes the structural ordering of CuZnS thin films, transforming them from an initially disordered (amorphous-like) state toward improved crystallinity. This enhancement is beneficial for photovoltaic applications, as higher crystallinity reduces defect density and facilitates efficient charge transport.

Scanning Electron Microscopy (SEM) was employed to study the surface morphology and grain evolution of the as-deposited and annealed CuZnS thin films [24,25]. Fig. 5 presents the SEM images of the CZS1, CZS2, CZS3, and CZS4 samples, all recorded with appropriate scale bars for accurate dimensional analysis. The as-deposited film (CZS1) shows a compact surface with densely packed fine grains, while annealed samples display a noticeable increase in grain size and improvement in surface uniformity.

The average grain size was estimated from the SEM micrographs using ImageJ software by measuring the diameters of a statistically significant number of grains selected from different regions of each image, and the mean value was calculated. The grain sizes, summarised in Table 3, range from 149 nm to 178 nm. The CZS3 sample (annealed at 200°C) exhibits the most well-defined spherical grains, indicating enhanced grain coalescence and crystallinity. The surface appears smooth and homogeneous, with fewer voids and grain boundary defects compared to other samples. At higher annealing temperatures (300°C, CZS4), the surface morphology becomes less distinct, and the SEM image appears slightly blurred, suggesting the onset of surface roughening or possible sulphur evaporation, which may lead to non-uniform composition. No cracks or pinholes were observed in any of the samples, confirming the good adhesion and compactness of the films. The observed grain growth upon annealing is attributed to thermally activated diffusion and recrystallisation, which facilitate atomic rearrangement and grain boundary migration. Similar grain enlargement behaviour has been reported for Cu₂ZnSnS₄ and CuZnS thin films [25,26].

Ultraviolet–visible (UV–Vis) spectroscopy was employed to investigate the optical properties of the CuZnS thin films. The optical absorption spectra were recorded in the wavelength range of 300–900 nm, as shown in Fig. 6. The absorption coefficient (α) was calculated from the measured absorbance (A) using the relation α = 2.303 A t where t is the film thickness, and expressed in cm⁻¹, the optical bandgap (Eg) was calculated using Tauc’s relation, (αhν)ⁿ = B(hν − Eg) where n = 2 and 1/2 correspond to direct and indirect allowed transitions, respectively. Both direct and indirect transition models were examined, and the direct transition (αhν)² versus hν was found to provide a better linear fit near the absorption edge. Therefore, the bandgap values were extracted assuming a direct allowed transition.

Fig. 7 presents the corresponding Tauc’s plots and direct bandgap estimations for the CZS1–CZS4 thin films. The obtained bandgap values for samples CZS1–CZS4 varied from 2.50 eV to 2.73 eV (Table 4). These values lie within the previously reported range of 1.8–3.6 eV for CuZnS thin films prepared by spray pyrolysis and are consistent with the 2.4–2.6 eV range reported for films grown by the SILAR technique [26,27]. A slight increase in the bandgap with rising annealing temperature was observed. This behaviour can be attributed to several factors, including improved stoichiometry, reduction in defect density, and enhanced crystallinity due to annealing. The loss of sulphur during annealing may also induce slight compositional modifications that influence the electronic band structure. Additionally, the increase in average grain size reduces localised states near the band edges, leading to a modest widening of the bandgap. The optical results confirm that annealing effectively tunes the bandgap energy within the desirable range for photovoltaic absorber layers. The CZS3 film, with an intermediate annealing temperature of 200°C, exhibits a balanced optical response and a suitable bandgap, making it a promising candidate for solar cell applications.

The Hall effect measurement is a reliable and widely adopted technique for evaluating the electrical properties of semiconductor thin films. Table 5 presents the Hall parameters of the CuZnS thin films (CZS1–CZS4). Among the samples, CZS1, CZS2, and CZS3 exhibit p-type conductivity, while CZS4 shows a transition to n-type behaviour. The results indicate that with increasing annealing temperature, the bulk carrier concentration and sheet resistance decrease, whereas carrier mobility and electrical conductivity increase. These enhancements can be attributed to improved crystallinity, reduced defect density, and better atomic ordering induced by annealing.

According to the literature, CuZnS thin films generally exhibit carrier concentrations in the range of 10¹⁷–10²⁰ cm⁻³, carrier mobility between 0.1–10 cm² V⁻¹ s⁻¹, and conductivity values between 0.01 and 20 S cm⁻¹ [28,29,30,31,32]. In the present study, the measured carrier concentration is of the order of ~10¹³ cm⁻³, which is lower than the typical range, while the mobility values are comparable to the reported values. Although the conductivity (~10⁻⁶ S cm⁻¹) is lower than that reported for heavily doped or non-stoichiometric films, such behaviour is characteristic of wide-bandgap semiconductors with low intrinsic carrier concentration. From a device perspective, lower carrier concentration can be advantageous in absorber layers by reducing recombination losses and enhancing charge separation, provided sufficient carrier mobility is maintained. In this context, the relatively stable mobility values observed in the present study suggest that charge transport is not severely hindered. Among all samples, CZS3 (annealed at 200°C) exhibits the most balanced combination of carrier concentration, mobility, and conductivity, indicating its potential suitability for photovoltaic and optoelectronic applications.