Optimization of electrodeposition time on the properties of Cu2ZnSnS4 thin films for thin film solar cell applications

The Electrochemical deposition was used to create a quaternary CZTS (Cu2ZnSnS4) kesterite thin layer. An aqueous solution of CZTS was used to deposit a thin layer over Indium Tin Oxide. The effects of deposition time (variation) on CZTS thin films under ambient conditions were investigated in this study. Several available characterization systems were used to study the samples as they were produced. The polycrystalline description of the layer is investigated by X-ray diffraction. The SEM as well as AFM study show that the deposition time improved surface morphology and topography of CZTS thin films which increase several nm in grain size. Furthermore, depending upon the deposition duration that affect the thickness and crystallinity of the films prepared, the optical study reveals an acceptable band gap in a range of 1.71–1.42 eV. Characteristics of high-quality CZTS absorber layers for solar cell applications are determined by deposition time variation. To check the effect of this band gap variation (1.71–1.42 eV, depending upon the deposition time) on the performance of a CZTS based thin film solar cell, a simulation software SCAPS-1D is being used.


Introduction
Researchers are looking at the Copper-Zinc-Tin-Sulfur (CZTS) absorber material as a promising alternate for CdTe and CIGS thin-film technology (Syafiq et al. 2020). To date, CdTe and CIGS-based devices have achieved photo-conversion efficiency of 22.5 percent and 23.4 percent, respectively (PCE) (Green et al. 2019). The lack of In and Ga, as well as a toxicity of Cd, may limit the usage of such materials in solar systems. CZTS is regarded for its low cost, ease of manufacturing, environmental friendliness, earthrichness, non-toxicity, and good absorption coefficient (10 4 cm −1 ) when used with direct band gap material (Ratz et al. 2019). The quaternary CZTS (Cu 2 ZnSnS 4 ) kesterite make it an ideal absorber thin film for solar applications. The best CZTS efficiency to date is 12.6 percent, which is still below the theoretical Shockley-Queisser limit efficiency of about 32 percent (Wang et al. 2014;Ki and Hillhouse 2011). Secondary phases, high intrinsic defect density, compositional variation, Cu-rich and Cu-deficient structure, and rapid carrier recombination might all explain the difference between theoretical and reported efficiency (Kumar et al. 2015;Valle Rios et al. 2016). Several techniques have been utilized for fabrication of quaternary CZTS (Cu 2 ZnSnS 4 ) kesterite thin films such as dip-coating (Prabeesh et al. 2018), SILAR (Suryawanshi et al. 2016), spray pyrolysis (Kim et al. 2012), thermal evaporation (Redinger et al. 2014), and pulsed laser deposition (Moholkar et al. 2011).
In this work, CZTS has been deposited by electro-deposition due to its cost effectiveness, easy-to-control deposition parameters, homogeneity, and large-scale deposition. To enhance the performance of CZTS thin films, the deposition duration impact was examined in this study, and the effect of deposition time was resulted in giving considerably better film crystallography and morphology. The films were further examined by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy and optical transmission and absorption. At the end simulation is performed to examine the effect of deposition time variation (band gap variation, other parameters are kept constant as mentioned in the Table 3) on the characteristics parameters of CZTS based thin film solar cells.

Experimental details
CuSO 4 , ZnSO 4 , SnSO 4 , Na 2 S 2 SO 3 , Na 3 C 6 H 5 O 7 , and HCL for pH correction were bought from Sigma Aldrich to fabricate CZTS thin film. Without any additional purification, all of the ingredients were combined in a chemical bath aqueous solution as supplied. As previously reported, commercial ITO-coated soda lime glass (10-17 sq-ft) was ultrasonically cleaned (Shafi et al. 2016(Shafi et al. , 2018Amal et al. 2019a). The electro-deposition was done in three-electrode configuration (Amal et al. 2020) 0.1 M Trisodium citrate and diluted hydrochloric acid were used to adjust the pH of the solution upto 5.16 pH. Cyclic voltammetry in the range of 0.5 to − 1.3 mV, containing all the precursors in the chemical bath, was used to ensure proper and precise deposition. The reduction potential was determined to be − 1.05 mV, and the deposition procedure was carried out with the same reduction potential. After 300, 480, 720 and 900 s, the as-deposited samples were removed from the mixture with sample, elaborated as, CZTS-1, CZTS-2, CZTS-3, CZTS-4 respectively. The as deposited samples were washed with distilled water to remove loosely bounded particles before being dried with nitrogen gas. Subsequently, all prepared four as-deposited samples were annealed in a vacuum with the presence of sulfur powder in a small box at 450-500 °C for 45 min.
A Rigaku Ultima IV diffractometer with CuKα radiation (λ = 1.54060 Å) was used to analyze the phase and structural information of the resulted films. Surface morphology was analyzed by SEM. The geography of the films was investigated by AFM Bruker Multimode 8 AFM Nanoscope V controller. UV-Visible spectroscopy was used to examine optical transmission and absorption. Figure 1 shows the schematic flow of the procedures/techniques that have been carried out in this experimental study.  Figure 2 shows the cyclic voltammograms (CV) of CZTS solutionat 300 K temperature obtained at the range of 0.5 mV to − 1.3 mV versus Ag (reference electrode). CZTS bulk reduction is ascribed to the cathodic processes at − 1.05 mV, indicating that CZTS may be effectively deposited on ITO substrate even without electrolyte interference. The CZTS combination decomposed at − 1.05 mV, to form CZTS thin films on the ITO substrate. Figure 3 shows XRD diffraction patterns of the CZTS thin films deposited at various times. The XRD analysis confirms the formation of polycrystalline nature of CZTS thin film.

Results and discussions
The obtained CZTS thin film exhibited various dominant peaks at 2 theta angle,18.20°, 28.53°, 32.99°, 37.97°, 47.33°, 56.18°, 58.97°, and 69.23° corresponding to approximately the planes of CZTS peaks, (101), (112), (200), (211), (220), (312), (224), and (008) respectively to the standard JCPDS card 00-026-0575 (Khalil et al. 2016). It was observed that the intensities of the main peaks (112) increased with the increasing time of deposition as compare to the rest of the samples.  Besides, there are some secondary phases in the structure of the films, the reason may the formation of these secondary phases within the structure is interface layer between the ITO and CZTS layer, ion diffusion that can pass through the substrate during annealing time (Unveroglu and Zangari 2016). Furthermore, the structural parameters of the obtained thin films are summarized in the Table 1. The grain size (D) has been calculated for CZTS thin films via Scherer's equation (Shafi et al. 2020a).
where D is the crystallite size, λ = 0.1540 nm) is the wavelength of the X-ray radiation, k is a Scherer's constant (0.9), β is the full-width at half maximum (FWHM) of the peak (in radians), and θ is the Bragg's diffraction angle at the peak position. The dislocation density (δ) and lattice strain (ε) have also been computed using the formulae below (Amal et al. 2019b;Shafi et al. 2020b).
The obtained results of crystallite size (D), dislocation density (δ) and lattice strain (ε) are summarized in Table 1. As noticed the deposition times strongly affect the film structure parameters. The typical crystallite dimension improves from 10 to 24 nm with deposition time of 15 min. The dislocation density (δ) and lattice strain (ε) have been founded to decrease with increase in deposition time. In conclusion that crystallite size (D), dislocation density (δ) and lattice strain (ε) strongly depend on deposition time. The dislocation density (δ) and lattice strain (ε) are reduced because the stress is released in the formation of high-quality film during deposition.
Furthermore, surface morphology of CZTS thin film deposited by different deposition times is shown in Fig. 4a-d. The SEM analysis display that the porous morphology and the agglomerated nanoparticles are clearly visible. Moreover, the nonuniform distribution of the grains were observed when the deposition time increased from 300 to 900 s. the morphology of the CZTS films evolves from porous to compact and improvement was observed in the grain size as increase the deposition time see Table 1. It's clear that if the grains size becomes bigger that means the crystallinity increased, the results is in good agreement with the XRD analysis.
The topography of the CZTS thin films were performed by AFM measurement sees Fig. 5 with different time frame. The AFM analysis confirmed that the grain size and surface roughness of CZTS thin film increased with increasing deposition time. The variation of the roughness was observed as the deposition time increases from 300 to 900 s. a significant improvement was founded in the CZTS thin film where, for 300, 480, 720 and 900 s the roughness was 77 nm, 109 nm, 130 nm and 146 nm respectively. Nevertheless, the deposition time appear to be useful for enhancing the crystallinity and the phase formation of the thin film, because an increased in roughness and grain size lead to lower the recombination rate, which enhances the photovoltaic performance of de device (Park et al. 2008).
(1) D = k cos Figure 6 shows the energy band gap values of CZTS thin films. The absorption, transmission spectrum was recorded in a range of 300-1000 nm with UV-Vis spectroscopy. It is observed from the results that the absorption decreased as we increase deposition time this change may be due to the excess CZTS film layer deposition. The optical band gap is calculated using the Tauc formula (4).

Optical analysis
where the formula parameters are, α is absorption coefficient, h is energy of a photon with frequency υ, n is an optical conversion for direct band gap its value is 2 and for indirect band gap is 1/2. We measured a direct band gap by extrapolating a straight line to α = 0 axes. The estimated band gap for CZTS thin films is around 1.71-1.42 eV depending upon the deposition time (Fig. 7). Deposition time for CZTS-1, CZTS-2, CZTS-3, and CZTS-4 corresponding to 300 s 480 s, 720 s, and 900 s respectively. As the time of deposition was increased, the band gap decreased because the deposition become more strong and absorb more visible light in the lower wavelength range which summarized in the Table 2. (4) αhυ = A(hυ − Eg) n

Numerical simulation
To observe an effect of deposition time variation on the performance of solar cell through the change in band gap, SCAPS-1D software is being used (Shafi et al. 2021(Shafi et al. , 2022a. The simulation parameters of CZTS as absorber layer, CdS as buffer layer and ZnO as buffer layer are taken from literature as shown in Table 3. But the band gap of CZTS absorber layer is taken from our experimental calculated data where band gap is varying from 1.42 to 1.71 eV depending upon the deposition time.
To check the performance of CZTS based solar cell with respect to the band gap variation of CZTS thin film as an absorber layer, a simulation tool SCAPS-1D is used, where CdS is used as a buffer layer and ZnO as a window layer, having solar cell model "Back Contact/CZTS/CdS/ZnO/Front Contact/Glass". Table 4 shows the front and back contact parameters that are taken from the SCAPS simulation tool as well.
As the CZTS-1 thin film is initially deposited for 300 s, so its band gap is 1.42 eV. By applying this band gapV oc achieved 0.764 V, J sc of 26.665 mA/cm 2 . Similarly fill factor was 66.17% and efficiency of 13.89% was achieved. Table 5 shows the characteristics parameters of CZTS based solar cell depending upon deposition time.
Similarly for 480, 720 and 900 s the V oc calculated as 0.783, 0.876 and 1.03 V, J sc of 26.02, 23.09 and 18.52 mA/cm 2 , fill factor in percent was 68.34, 69.05 and 69.84, where the efficiency of 13.92, 13.97 and 13.39% achieved respectively. Here it can be observed that at 720 s time deposition, the proposed solar cell is giving maximum efficiency, because at 300, 480 and 900 s the efficiency is less as compared to at 720 s. CZTS-3 with 720 s deposition time is the sample having optimized thickness which results the matching band gap to generate the maximum efficiency. It is also observed that the CZTS-1 was giving better efficiency as compared to CZTS-4, because in CZTS-4 sample with maximum deposition caused to increase the electron hole recombination rate, hence as a result the efficiency decreased (Shafi et al. 2022e).
The effect of deposition time is observed in the Fig. 8 that with increase in deposition time the performance of the cell is being varied. The Fig. 8a is showing the J-V characteristic curve and Fig. 8b showing P-V characteristics curve (Shafi et al. 2022f) of CZTS based solar cell with respect to deposition time. The P-V curve shows that the maximum power is at 720 s deposited CZTS thin film. Figure 9 is showing the External Quantum Efficiency (EQE) curve. Here in this graph we can also observe the effect of deposition time in corresponding of band gap variation on the external quantum efficiency of CZTS based solar cell. From the above results it has been concluded that on the basis of characterization, there are four regions as: Fig. 8 J-V and P-V characteristics curves for 2 and 10 cycles as deposited and annealed respectively • Region.1where λ varies from 300 to 390 nm. In this region EQE reduced because of the recombination of electrons and holes on the front surface of solar cell. • Region.2 where λ varies from 390 to 650 nm for CZTS-4, 390 to 700 nm for CZTS-3, 390 to 750 nm for CZTS-2, 390 to 800 nm for CZTS-1. In this region EQE reduced because of reflection and low diffusion length of solar irradiance. • Region.3 where λ varies from 650 to 720 nm for CZTS-4, 700-810 nm for CZTS-3, 750-870 nm for CZTS-2, and 800-890 nm for CZTS-1. In this region EQE reduced because of low absorption at long wavelengths, surface recombination and low diffusion length of solar irradiance. • Region.4 where λ > 720 nm for CZTS-4, λ > 810 nm for CZTS-3, λ > 870 nm for CZTS-2, and λ > 890 nm for CZTS-1. In this region EQE reduced to zero because of zero absorption of light below E g at the longer wavelengths.
From the above calculated results we concluded that the CZTS-3 sample with 720 s deposition time with 1.54 eV band gap is the most promising sample as compared to other ones because the thickness of this sample is the optimized for the capturing of photons and as a result generation of holes and electrons.

Conclusion
It was concluded that CZTS kesterite thin films were successfully prepared using the electrochemical deposition technique, so this technique is a cost-effective way to control the crystallography and structure-property of CZTS using ITO back contact. It is observed that the variation of deposition time has a great impact on the CZTS thin film preparation. The XRD analysis of the CZTS-3 thin film indicated pure kesterite phases with high crystalinity among the other samples deposited for 720 s. The SEM and AFM analysis also confirmed the formation of homogenous and increase in grain size up to 324 nm. The optical band gap values were determined between 1.42 and 1.71 eV. Using SCAPS-1D simulation software, it is observed the effect of deposition time on the performance of a solar cell upon its characteristics parameters. From the simulation and experimental data, it can be concluded that deposition for 720 s is a suitable time for high-quality of the CZTS absorber layer.