Impact of heating on the properties of CZTS thin film solar cells produced through sol-gel spin-coated

Copper-zinc-tin-sulfide kesterite compounds (Cu₂ZnSnS₄ or CZTS) have attracted considerable attention in recent years due to their desirable optoelectronics properties for photovoltaic (PV) thin film solar cell applications.1 It is a promising material for solar cells because it is an abundant, low-cost, and environmentally friendly material that offers significant advantages over CIGS and CdTe.2 Both CdTe and CIGS have significant disadvantages, such as the toxicity of Cd, scarce resources of the elements In, Ga, and Te, and resulting price fluctuations that could affect their market launch in the future. Furthermore, the same underlying device architecture for CdTe and CIGS could be implemented for CZTS, which would help accelerate the development of CZTS solar cells with superior power conversion efficiencies.

CZTS has a remarkably high absorption coefficient in the visible region, which is 104 cm⁻¹.3 Its optimal forward bandgap of 1.5 eV strategically positions it to efficiently capture a significant portion of the visible solar spectrum.4CZTS is characterized by its p-type conductivity and is the ideal choice among second-generation thin film solar cells.

The theoretical limit of energy efficiency of CZTS connections is an impressive 32%.5 It is noteworthy that a breakthrough was achieved in 2012 with a record CZTS conversion efficiency of 12.6%, using a pure hydrazine-based solution route.6 Various techniques have been used to prepare CZTS thin films, including molecular beam epitaxy,7 sputtering,8 pulsed laser deposition,9 sputter pyrolysis,10 electrodeposition,11 and sol-gel spin-coating. Among these methods, the sol-gel process stands out for its simplicity, which allows it to be adapted to large surfaces and different substrates in terms of shapes and materials. In addition, the sol-gel technique offers advantages such as precise stoichiometric control and quantitative doping at the molecular level. This allows greater control over structure and composition compared to alternative methods. In this study, the sol-gel route was the main method for preparing CZTS films.

The performance of a solar cell depends mainly on the crystalline structure and the composition of the absorbing material. Therefore, it is important to understand and optimize the growth of photovoltaic materials to achieve the desired stoichiometry.12 In fact, the stoichiometry of CZTS is influenced by the annealing temperature and the atmosphere.13–15 It was found that good crystallinity requires higher growth temperatures. However, to avoid the evaporation of sulfur S during deposition, it is necessary to proceed with low temperature deposition and subsequent post-treatment at higher temperature to improve crystallinity. The sulfurization process ensures the best stoichiometry of the CZTS thin film, and the high temperature heating ensures the high crystallinity of the CZTS absorbing material, which improves the electrical and optical properties.16 Xiaomi You et al.17 studied the annealing of sol-gel based CZTS films prepared from sulfur-containing solutions in an inert atmosphere without suffixation and with different thicknesses of the deposited layers: three-layer samples annealed at 250°C for 10 min, five- and nine-layer samples were annealed at 500°C in a nitrogen atmosphere for 60 min. They showed that the CZTS phase was successfully formed in all samples, and the nine-layer sample had better crystallinity. In another study, Khottummee et al.18 studied the influence of annealing temperature on the optical properties of CZTS thin films deposited by the dip coating technique using the sol-gel method. They showed that the bandgap energy decreases from 1.72 to 1.50 eV when the annealing temperature increases from 200°C to 500°C for 1 h. In addition, many studies have been conducted on the influence of annealing temperature,19 temperature, and time20,21 on CZTS thin films prepared by sol-gel spin coating.

In this study, we investigated the effect of annealing in air at different temperatures on the structural, morphological, elemental composition, optical and electrical properties of CZTS thin films synthesized by sol-gel spin-coating technique. The aim of this study was to determine the optimal annealing conditions. In addition, using the CZTS data from this research, the theoretical performance of the solar cell with a toxic buffer layer (CdS) and a non-toxic buffer layer (ZnS) was also determined using a solar cell capacitance simulator (SCAPS-1D).

CZTS thin films were synthetized by sol-gel spin-coating technique on soda-lime glass using copper chloride (CuCl₂.2H₂O), zinc chloride (ZnCl₂), tin chloride (SnCl₂.2H₂O), and thiourea (SC(NH₂)₂) as precursors for copper (Cu), zinc (Zn), tin (Sn), and sulfur (S). The ratio of Cu:Zn:Sn:S was maintained at 2:1:1:8. The solution was stirred at 60°C for 20 min, then spin-coated onto cleaned glass substrates at 3000 rpm for 20 s and dried at 280°C for 10 min. This process was repeated to achieve the desired film thickness. The coated substrates were annealed in air at 340°C, 360°C, 380°C, and 400°C for 20 min, except for one sample left non-annealed to assess the impact. The samples were labeled as CZTS-AD, CZTS-340, CZTS-360, CZTS-380, and CZTS-400 accordingly.

The film’s crystalline structure was confirmed using X-ray diffraction (XRD) in a Bragg–Brentano geometry with CuKα radiation (λ = 1.5418 Å). Raman spectroscopy analyzed the phase composition of CZTS samples with a laser wavelength (λ = 532 nm) ranging from 200 to 500 cm⁻¹. Field-emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectrometry (EDX) were used to examine surface morphology and elemental composition of the thin films. FESEM images were taken with a fixed electron beam at 10 kV.

Optical studies were performed using a computer-controlled UV–vis–NIR spectrophotometer (UV-3101PC-SHIMADZU) covering a range of 185–1400 nm. Transmittance measurements were compared to bare glass substrates. Electrical properties were assessed by measuring the resistivity of the samples at room temperature using the van der Pauw method with an Ecopia HMS-5500 instrument.

SCAPS is a one-dimensional solar cell simulation software developed by researchers at the Department of Electronics and Information Systems (ELIS) at Ghent University in Belgium.22 We used SCAPS-1D to perform a numerical simulation of a CZTS solar cell, and determine its theoretical efficiency and key output parameters such as J–V characteristics.

XRD was used to analyze the structural properties that influence the optical and electrical properties of CZTS thin films. Figure 1 shows the X-ray patterns of CZTS-AD, CZTS-340, CZTS-360, CZTS-380, and CZTS-400°C. The sample annealed at 400°C initially shows no CZTS peaks and is therefore unsuitable for optical and electrical evaluations. However, the remaining samples show sharp peaks at 28.34°, 47.38°, and 56.43°, denoted as (112), (220), and (312), respectively, indicating the thin film structure of CZTS. The XRD patterns confirm that all thin films crystallized in the kesterite structure (ICDD #04-023-3974). This is consistent with previous research,13 which suggested that CZTS thin films should be annealed at the lowest possible temperature without sulfurization to avoid CZTS phase degradation and sulfur evaporation. Note that the peak (112) at 28.34° shows significant intensity in all samples, indicating the preferential orientation of the CZTS film along the (112) axis. The crystallinity of the films improves with increasing temperature up to 360°C, highlighting the tendency for CZTS crystalline structure to align along the (112) direction and confirming the formation of the CZTS kesterite crystallographic phase.

The crystal size D was calculated by exploiting the XRD results using the Debye–Scherrer relation:23 

where $β$ is full width at half maximum (FHWM), $λ$ is the wavelength of the Cu(kα) radiation, and $θ$ is the angle of the (112) peak. The crystalline size D versus annealing temperature is shown in Figure 2.

The CZTS-AD exhibits a minimum crystalline size of ≈8.72 nm. It is obvious that the crystalline size D with increasing annealing temperature and reaches 17.79 nm. This behavior suggests that higher annealing temperatures result in larger crystalline sizes, possibly due to the increased thermal energy of the atoms.24 It is well documented that the conversion efficiency of CZTS solar cells improves with larger grain sizes in the absorber layer. However, these average crystalline sizes are smaller than those achieved with CZTS thin films annealed through sulfurization.25 Therefore, annealing in air at temperatures exceeding 380°C could potentially affect the crystallinity and phase purity, leading to the formation of secondary impurity phases.

Raman spectroscopy was used to definitively identify the CZTS kesterite phase. The similarity between the XRD databases for cubic ZnS and tetragonal Cu₃SnS₄ and CZTS makes it difficult to distinguish CZTS based on XRD results alone. An Ar⁺ laser with a wavelength of 532 nm was used for the analysis. In Figure 3, the Raman spectra of pre-annealed and annealed CZTS thin films at different temperatures are shown. The Raman data revealed a consistent peak at 337 cm⁻¹ in all CZTS thin films, which corresponds to the main vibration mode A1characteristic of the CZTS phase.26 Consequently, both Raman spectra and XRD data confirmed the formation of a pure kesterite crystallographic phase in the CZTS thin films.

The CZTS thin films were analyzed using SEM images to investigate the influence of annealing and non-annealing temperatures on their surface morphology. SEM images are shown in Figure 4. A significant difference in morphology was observed between the pre-annealed and annealed samples. The images reveal numerous nanopores on the surface of all thin films, probably due to evaporation of precursors during the drying process, especially for thiourea precursors. Park et al.27 demonstrated through thermalgravimetric analysis that thiourea exhibited the highest weight loss compared to other precursors at 250°C. Despite drying the samples at 280°C, all samples showed particle agglomeration, non-uniform surfaces, and cracks of different sizes. Nanocavities were also present. Increasing the annealing temperature resulted in a smoother surface. The SEM image of the sample annealed at 380°C shows a dense and almost uniform surface morphology with small cracks and voids (Figure 4(d)).

The elemental composition of CZTS significantly influences its crystal structure and its efficiency in solar cells.28 Heat treatment is a crucial factor in determining the initial composition of CZTS in a solution. A higher efficiency was observed for non-stoichiometric Cu-poor, Zn-rich.29EDX coupled with SEM was utilized to analyze the composition of CZTS thin films, detecting variable atomic percentages of Cu, Zn, Sn, and S at different annealing temperatures (Figure 5 and Table 1). The CZTS-340 and CZTS-360 exhibited Cu-poor and Zn-rich configurations which are suitable for the growth of high-quality CZTS thin films. However, the element ratios Cu/Zn + Sn and Zn/Sn of the annealed CZTS thin films slightly increased, while the S/metal ratio decreased with increasing annealing temperature (Figure 6). XRD and EDX analysis demonstrated that the high crystallinity was observed for the Cu-poor Zn-rich represented by CZTS-340 (Cu/Zn + Sn = 0.67, Zn/Sn = 1.05), and also for the CZTS-360 (Cu/Zn + Sn = 0.95, Zn/Sn = 1.14). Thus, these conditions could be developed for growing high-crystallinity CZTS thin films and yielded highest conversion efficiency.

Additional elements such as O, C, and Cl were identified in the analysis. The presence of carbon could be due to the incomplete evaporation of the solvent and thiourea, while the presence of chloride could be due to the metal chloride used. The atomic content of carbon significantly influences the electrical properties of CZTS thin films. The inclusion of metal chloride is intended to reduce the carbon content, which decreases with increasing temperature rises and reaches a minimum at 360°C.

The optical properties of CZTS thin films annealed at different temperatures were analyzed by UV–visible spectroscopy in the wavelength range of 360–1100 nm. Figures 7 and 8 present the optical transmission spectra (T) and reflection spectra (R) spectra of the CZTS thin films, respectively. Consistent changes in transmittance and reflectance were observed at all wavelengths except for CZTS-AD, which remained constant. The CZTS sample annealed at 360°C exhibited the highest transmittance and reflectance, which may be due to the surface morphology and elemental composition.

The transmission and reflectance results were used to calculate the optical bandgaps for all CZTS thin films, based on the Tauc method using the following relation:30 

where E_g is the bandgap, B is a constant independent of the energy hν, and α is the absorption coefficient determined for all samples using the following formula:

where d is the thickness of the thin film estimated to be 3 µm from the SEM image from our previous work.31 All CZTS thin films show an absorption coefficient of 10⁴ cm⁻¹ in the visible region which is consistent with previous results reported in other works,15,32 and are suitable for use as absorption layers in thin-film solar cells.

The optical bandgap indicates the range of the solar spectrum that a photovoltaic cell can absorb. The bandgap energy was calculated by extrapolating the linear portion of (αhν)² versus the incident photon energy hν, as shown in Figure 9. The bandgap values range from 1.38 to 1.71 eV with increasing annealing temperatures, while CZTS-AD exhibits a minimum bandgap of 1.36 eV. The increase in bandgap with increasing the annealing temperatures is attributed to variations in crystalline quality and resulting differences in grain sizes among the samples. In our study, the decrease in S/metal ratio with increasing annealing temperature is associated with an increase in bandgap energy. Previous studies on CZTS synthesized using sputtering and thermal evaporation methods have shown that the bandgaps of CZTS thin films increase with annealing temperature, which agrees well with our results.33,34 The optical bandgaps of all obtained CZTS samples are considered suitable for the use of solar cells.35 

The electrical properties of CZTS have a significant impact on the efficiency of solar cells. To analyze the electrical properties of CZTS thin films deposited and annealed at different temperatures, the four-point method was used. Table 2 describes the resistivity, conductivity type, and hole concentration determined from Hall effect measurements. All CZTS thin films exhibited p-type conductivity. The resistivity of the annealed CZTS samples varies with temperature and is directly influenced by the film properties and crystallinity. This change is related to the increase in bandgaps and composition of CZTS with annealing temperatures. A larger crystallite size reduces grain boundaries, minimizing film defects and carrier recombination.36 The CZTS-360 sample showed minimal resistivity (1.60 × 10⁻² Ωcm) due to the low carbon content and Cu/Zn + Sn ratio observed in the EDX results. The lack of Cu led to the formation of the CuZn antisite defects, which are the main reason for the p-type conductivity and good electrical properties of the CZTS thin film.37 The resistivity values of all CZTS thin films were consistent with the literature.38 However, as the S/metal ratio in annealed CZTS decreased, the mobility and hole concentration also decreased. Petal et al. have shown that high mobility is related to Zn-rich and high S content.39 Other study revealed that the electrical properties of CZTS are influenced by the stoichiometry and atomic percentage of each element.40 All electrical parameters of the sol-gel deposited CZTS absorption layer are suitable for photovoltaic applications.

In this study section, CZTS thin films with p-type conductivity were deposited on an ITO substrate with n-type conductivity to create a PN junction, which was confirmed by their electrical properties. An Ag contact was then placed on top of the CZTS to improve current flow. Figure 10 shows the I–V properties of CZTS thin films deposited on an ITO substrate using sol-gel spin-coating in the absence of light. The semi-logarithm of I(V) is shown in the inset of Figure 10. The nonlinear I–V characteristics demonstrate rectifying and non-ohmic behavior, indicating the successful formation of a PN junction between the p-CZTS and n-ITO thin films. Furthermore, the I–V curve showed a turn-on voltage of 0.45 V. In addition, the positive part of the I–V curve, which shows an exponential variation, can be modelled using the following equation:

where I₀ is the saturation current estimated at 3.1 × 10⁻⁵ A, q is the electronic charge, V is the voltage applied to the terminals of the junction, k is the Boltzmann constant, T is the temperature in Kelvin, and n is the ideality factor which is an important parameter. However, each diode has an ideality factor. For solar cell applications, the ideality factor n is in the range of 1–2.

For the diode consisting of a heterojunction (glass/n-ITO/p-CZTS/Ag), the series resistance R_s must be taken into account due to the lattice mismatch between CZTS and ITO. This resistance is influenced by the contact resistances on the front and back. The series resistance was calculated to be 139.5 Ω based on the I–V curve analysis.

The PN junction structures (glass/n-ITO/p-CZTS/Ag) are shown in Figure 11, both in the dark and under illumination. The device showed consistent behavior under both conditions, with rectifying properties observed at the PN junction. In particular, the turn-on voltage dropped to 0.42 V and the measured ideality factor was 1.77 under lighting. Furthermore, an increase in current was observed during illumination, which is due to the photocurrent generated by the CZTS absorbing the incident photons. This process excites electrons from the valence band to the conductive band and forms electron–hole pairs,41 resulting in improved PN junction parameters. The fabricated device can be used as a UV detector36 and for photocatalytic applications.42 The solar cell can use an our PN junction to incorporate a buffer layer between p-CZTS and n-FTO, reducing potential barriers due to lattice parameter mismatch. Gunavathy et al.⁴³ studied the influence of FTO substrate temperature on CZTS thin films prepared by spray pyrolysis. The sample annealed at 350°C showed promising properties and was used as a n-type in a heterojunction with the FTO/CdS/CZTS/Ag configuration. The PN junction showed rectifying behavior. However, under illumination, the solar cell characteristics for this heterojunction structure were: open-circuit voltage of 213 mV, short-circuit current density of 490 μA cm⁻², fill factor of 0.33%, and an efficiency of 0.68%.

This section explores the impact of non-annealing and annealing at different CZTS temperatures on solar cell efficiency using the SCAPS-1D capacitance simulator. SCAPS-1D can determine key parameters of solar cells such as: efficiency (η), fill factor (FF), short-circuit current density (J_sc), and open-circuit voltage (V_oc). Figure 12(a) shows the physical device used in this study, which is composed of Mo/CZTS/(CdS or ZnS)/ZnO:I layer. The simulated band energies of the devices with CdS and ZnS buffer layers are shown in subfigures (b) and (c) of Figure 12. In this configuration, Mo serves as the back contact, CZTS as the absorber layer, ZnO:I as the window layer, and ZnS and CdS as different buffer layers. For the buffer layer, a comparative study was performed between ZnS, which is abundant and non-toxic, and CdS to reveal the performant layer.44 However, the scarcity and toxicity of cadmiums limits the cost efficiency of this buffer layer. Therefore, it is imperative to explore alternative materials to replace CdS. ZnS thin films offer several advantages over CdS as a buffer layer in solar cells.45,46 The parameters for the SCAPS-1D simulation of all layers of the solar cell are detailed in Table 3 and are taken from the literature, except for CZTS-AD, CZTS-340, CZTS-360, and CZTS-380 (Table 4). Based on the results of this study, key parameters of these absorber layers were incorporated into SCAPS-1D. During the entire simulation under AM 1.5 illumination, the physical constants remained constant at a temperature of 300 K. Figures 13 and 14 show the current–voltage results for CZTS-AD and all annealed CZTS layers at different temperatures using CdS and ZnS as buffer layers, respectively. The parameters used for the SCAPS-1D simulation for all the layers in a solar cell are listed in Table 3 which are taken from the literature, except for those of CZTS-AD, CZTS-340, CZTS-360, and CZTS-380 (Table 4). Based on the results of this study, key parameters of the absorbent layers were integrated into SCAPS-1D. Throughout the simulation under AM 1.5 illumination, these physical constants were kept constant with the temperature set at 300 K. Figures 13 and 14 show the current–voltage results for CZTS-AD and all CZTS annealed at different temperatures using CdS and ZnS as buffers layers respectively. All SCAPS-1D results can be found in Table 5. Changes in the properties of CZTS have a significant impact on the parameters of solar cells. The modification of processes or annealing temperatures influences CZTS properties, subsequently affecting solar cell performance when CdS or ZnS is used as a buffer layer. The J_sc values of CdS and ZnS decrease with increasing annealing temperature, probably due to the mobility of the sample. It is noteworthy that J_sc shows a similar trend in mobility with respect to annealing temperature. Efficient carrier generation for device output depends on timely contact arrival to avoid recombination, which requires long relaxation times and increased mobility.47 In contrast, the unannealed sample shows J_sc values of 19.28 mA cm⁻² for CdS and 15.93 mA cm⁻² for ZnS. The CZTS-AD configuration with CdS and ZnS as buffer layers exhibited the highest fill factor (64.51% for CdS and 57.56% for ZnS), which decreased with increasing CZTS annealing temperature. In contrast, the open-circuit voltage of the CZTS thin film increases with annealing temperature, with the lowest V_oc observed for CZTS-340 (0.79 V for CdS and 0.84 V for ZnS). In particular, solar cells with ZnS as a buffer layer showed higher V_oc values compared to those with CdS. Efficiency is an important parameter for determining the performance of solar cells. The simulation results of the efficiency of the solar cell with CdS and ZnS as the buffer layers are shown in Figure 15. The efficiency of solar cells with CdS or ZnS decreased with increasing annealing temperature. The solar cell with CZTS-AD as the absorbing layer achieved an average efficiency of η_CdS = 10.01% and η_ZnS = 8.91%. The efficiency values across all samples ranged from 2.99% to 11.40%. The solar cell with CZTS-340 showed the highest efficiency at 11.40% for CdS and 10.55% for ZnS. The solar cell with the CdS buffer layer showed the best performance, compared to that with the ZnS buffer layer. However, solar cells with a ZnS buffer layer (Mo/CZTS/ZnS/ZnO:I) offer numerous advantages. They are crafted from non-toxic, eco-friendly, and abundant materials, resulting in a cost-effective material with an efficiency of 10.55% for the CZTS-340, even without sulfurization. The maximum efficiency values for CdS or ZnS with CZTS-340 as the absorbent layer are within the range of numerous studies.48,49 However, the experimental results for solar cells produced by spin coating are still behind the simulated ones.27,50 This discrepancy can cause several problems in solar cell assembly due to interface problems between layers and the appearance of secondary phases in CZTS thin films. The deposition of CZTS on Mo produces MoS₂ with n-type conductivity, which affects the fill factor (FF) and increases the series resistance, which consequently reduces the open-circuit voltage (V_oc).43 The secondary phases of the CZTS layer, such as SnS and ZnS,51 influence the short-circuit current (I_sc) of the solar cell, while V_oc is limited by Cu₂SnS₃.52,53 

Figure 16 shows the efficiency of CZTS/ZnS/ZnO based on the composition map determined by the Zn/Sn and Cu/Zn + Sn ratios. The simulated results show that the most efficient performance of 10.55% is achieved with CZTS thin films annealed at 340°C in the low Cu and high Zn region. This result is in good agreement with the experimental one. In a study by Katagiri et al.,54 the influence of composition on the efficiency of CZTS solar cells was investigated by adjusting the molar ratios of Cu/Sn + Zn and Zn/Sn. They found that the highest efficiency was achieved in the Cu-poor and Zn-rich region, near the ratios Cu/Zn + Sn = 0.9 and Zn/Sn = 1.25. The CZTS samples annealed at 340°C and 360°C came to this composition with ratios of Cu/Zn + Sn = 0.67 and Zn/Sn = 1.01 as well as Cu/Zn + Sn = 0.92 and Zn/Sn = 1.14 very close, respectively, which leads to efficiencies of 10.55% and 9.27% when ZnS is used as a buffer layer. Deviations from this optimal configuration resulted in lower efficiency, as shown by the example of CZTS-380, which had the lowest efficiency with Cu/Zn + Sn = 1.65 and Zn/Sn = 3.41 in its composition.

We successfully deposited CZTS films at different temperatures on a glass substrate using a low-cost sol-gel spin-coating technique. It is clearly observed that changing the annealing temperature has a significant effect on the physical properties of CZTS thin films. XRD revealed a kesterite structure for all CZTS thin films with a preferential orientation along the (002) plane, and an increase in the crystalline size with annealing temperature was observed. The existence of the CZTS phase observed by XRD was also confirmed by the Raman spectra which showed a peak corresponding to the A1 vibration. The absorption coefficient was estimated from the U-V results and was found at the range of 10⁴ cm⁻¹ in the visible region. The optical bandgap increases from 1.38 to 1.71 eV with the increase annealing temperature due to the changing at the elemental composition. The electrical properties revealed that the CZTS thin films have P-type conductivity, and minimum resistivity was observed for the CZTS-360 of 1.60 × 10⁻² Ω cm. In addition, the I–V characterization of CZTS deposited on the ITO substrate confirms the formation of a PN junction by indicating rectifying and non-ohmic behavior. Indeed, the data of all CZTS thin films were injected into the simulation program to study the possibility of using these samples as absorbent layers in solar cells (Mo/CZTS/(CdS or ZnS)/ZnO:I). Furthermore, we investigated the performance of non-toxic and abundant solar cells with ZnS as a buffer layer by comparing it with a solar cell where CdS was used as the buffer layer. A promising efficiency was observed for the solar cell (Mo/CZTS/ZnS/ZnO:I) which varied with the annealing temperature of CZTS from 5.67% to 10.55%.

E. El Mahboub: Writing – carried out the experiment (Thesis). S. Zakria: Original draft preparation – analysis and interpretation of data (Thesis). M. Mansori: Visualization, investigation. A. El Hichou: Conception and design of study – reviewing and editing.