Introducing a Dilute Single Bath for the Electrodeposition of Cu₂(ZnSn)(S)₄ for Smooth Layers

Abstract

Cu₂(ZnSn)(S)₄ (copper, zinc, tin, and sulfide (CZTS)) provides possible advantages over CuInGaSe2 for thin-film photovoltaic devices because it has a higher band gap. Preparing CZTS by electrodeposition because of its high productivity and lower processing costs, electroplating is appealing. Recently published studies reported that the electrodeposition process of CZTS still faces significant obstacles, such as the sulfur atomic ratio (about half of the whole alloy), deposits’ adhesion, film quality, and optical properties. This work introduces an improved bath that facilitates the direct electroplating of CZTS from one processing step. The precursors used were significantly more diluted than the typical baths mentioned in the last few years. An extensive analysis of the electrochemical behavior at various rotation speeds is presented at room temperature (~22 °C). The deposited alloy’s composition and adherence to the molybdenum back contact are examined with agitation. The annealing process was carried out in an environment containing sulfur, and the metal was not added at this stage. The ultimate sulfur composition was adjusted to 50.2%, about the desired atomic ratio. The compound’s final composition was investigated using the Energy-Dispersive X-ray Spectroscopy technique. Finally, X-ray diffraction analysis was applied to analyze CZTS crystallography and to measure thickness.

1. Introduction

The quaternary Cu₂(ZnSn)(S)₄ (copper, zinc, tin, and sulfide (CZTS)) compound is a possible applicant for the photovoltaic industry due to its earth-abundant materials, lower toxicity, and higher band gap compared to other thin films [1]. CZTS has similar optoelectronic properties and crystallography structure to the Cu(In, Ga)(S, Se)₂ compound, which is a highly efficient photovoltaic device, especially on a laboratory scale [2]. The CZTS compound has an almost ideal band gap of 1.5 eV and has a direct band gap. Furthermore, CZTS has a significant absorption coefficient (10⁴ cm⁻¹), which is a better value than other photovoltaic (P.V.) thin film compounds studied [1]. CZTS deposition methods include atom sputtering, electron beam evaporation, electrodeposition, hybrid sputtering, screen printing, and pulsed laser deposition. However, electron beam evaporation requires costly sophisticated equipment and procedures [3,4,5], and atom beam sputtering requires expensive equipment and a complicated procedure. Therefore, generating such thin film alloys by electrochemical methods is appealing because of its high throughput and reduced processing costs [1,2,3,4,5,6]. Electrodeposition has many advantages, such as constructing electrodeposition thin films under ambient temperature and using fewer electrolyte precursors with non-toxic complexing agents (additives). The CZTS electrodeposition has several environmental benefits, such as lower energy consumption, reduced material usage (diluted electrolyte), non-toxic materials, scalability and flexibility, and minimal waste generation (diluted electrolyte).

Furthermore, the electrochemical process can produce films with significant areas with smooth and homogenous deposits. As a result, the electrodeposition method can produce thin P.V. films on a large scale at a low cost since it does not require expensive equipment to make a vacuum atmosphere, as is the case with physical vapor deposition techniques. Two electrodeposition methods are suggested for creating CZTS thin films. The first method begins with the electrodeposition of three species precursors (Cu, Tn, and Zn). The next step is sulfurization in a sulfur atmosphere or H₂S during annealing [7,8]. The second method is one-step electrodeposition using aqueous electrolytes or ionic liquids [9,10]. The technique’s main challenge is the wide range of the standard potential of all reducing elements. In addition, some metallic precursors are reduced under their limiting currents while others are under kinetic control. The suitable complexing agents and the right electrolyte composition must be optimized experimentally. The sulfur content is the most challenging to incorporate into a single bath process because it is around 50% of the final deposit.

CZTS is the most promising candidate for substituting silicon solar cells due to its optical properties, low cost, and abundant precursors. The effects of additives on CZTS electrodeposition were investigated in the literature [7]. CZTSSe research is moving forward to produce low-cost, high-efficiency devices [8,9,10]. About 12.6% is the record device efficiency for a P.V. cell made by thermal evaporation with the subsequent structure: soda lime glass/Mo/CZTSSe/CdS/i-ZnO/ZnO:Al [11]. The CZTS electronic properties are limited due to crystallography defects and imperfections at interfaces between the compounds with different layers. These defects must be controlled, and the imperfections must be limited to improve the feasibility of CZTS’s large-scale production.

Furthermore, recent studies have produced CZTS with a single-step electroplating technique, mainly using ten mM to 80 mM of Cu, Zn, Sn, and S precursors in an acid medium [12,13,14,15,16,17,18]. However, even though a considerable contribution of hydrogen gas production determines the final composition’s influence and the film’s morphology, currently, no study has reported on understanding the phenomena associated with the reaction’s transference of mass and kinetics that separately govern each of these precursor’s behavior.

With its many benefits, particularly cost-effectiveness, electrodeposition is usually used for creating thin-film photovoltaic devices. Essentially, electrodeposition is a low-cost way of making thin-film solar cells for the following principal reasons:

• Waste is reduced by the exact controllable material deposition made possible by electrodeposition with low electrolyte concentration. Materials are only placed where they are required, which minimizes overuse.
• Compared to rare and costly materials like indium and gallium utilized in other thin-film technologies, many materials used in electrodeposition, such as zinc, copper, and tin, are cheap and readily available.
• Electrodeposition can be carried out at ambient temperature and atmospheric pressure, saving energy and equipment expenses in contrast to other processes requiring high temperatures or vacuum conditions.
• The technology is appropriate for mass production because it can be readily scaled from small laboratory settings to big industrial production without requiring significant setup changes compared to other production methods.
• Electrodeposition requires comparatively low-cost and essential equipment. Usually, it consists of an electrolyte solution bath, electrodes, and a power source with minimal maintenance requirements.
• The procedure tolerates fast production rates of large quantities with continuous production lines.

The current work aimed to achieve electrodeposit in a single step to produce CZTS film with various sulfur atomic fractions. Specifically, this study introduced a dilute electrolyte composition, which leads to better control of operational parameters and generates smooth deposits. Therefore, this solution is approximately eight times more diluted than the conventional bath, providing better deposit properties. The deposit’s weight balance is discussed to distinguish the hydrogen reduction’s contribution. This weight analysis aids in understanding the efficiency of the electrochemical process and revealing an association of hydrogen production with the film’s morphology in the Rotating Disk Electrode (RDE) study. The deposition parameters’ adhesion and optimization are also discussed. Besides the CZTS main layer electrodeposition, an entire P.V. device consisting of SS/Mo/CZTS/CdS/ZnO/ZnO-Al layers is built and tested. Finally, this study allows us to associate the rates of hydrogen formation concerning the precursor species in more detail to improve the performance required for the P.V. compound’s formation. As a result, this association is crucial to forming high-quality films with significant areas.

2. Materials and Methods

2.1. Fabrication Process

A three-electrode system with an RDE setup was used in laboratory experiments (PINE Research, Durham, NC, USA). The experimental setup consisted of a 0.3167 cm² stainless-steel (406 SS) disk (covered by a back-contact (thin molybdenum) layer embedded in a shielding Teflon cylinder. The platinum mesh was used as the counter electrode, and a Saturated Calomel Electrode (SCE) served as a reference electrode. However, in this study, all the voltages applied were reported vs. the normal hydrogen electrode (NHE). The power source was a Bio-Logic potentiostat/galvanostat Model VSP (Seyssinet-Pariset, France). The pH solution was adjusted to 1.8 by adding sulfuric acid. The practical work was conducted at an ambient temperature of about 20 °C and was applied at 0 to 500 rpm rotational rates.

After rinsing it with acetone and deionized water, the substrate was allowed to dry naturally. Before electroplating, the stainless-steel disk was electro-activated in 0.1 M of H₂SO₄ (Sigma-Aldrich, St. Louis, MO, USA) for a few seconds at 1.8 V vs. NHE to improve the adhesion. The bath composition was 3.2 mM CuSO₄ (Sigma-Aldrich, 99.9%), 2.1 mM ZnSO₄ (Sigma-Aldrich 99.9% trace metals basis), 3.3 mM SnCl₂ (Strem-Chemicals 99.9%, New Buryport, MA, USA), and 0.3 M Na₂S₂O₃ (Sigma-Aldrich, 99% Reagent Plus, Burlington, USA). The electrolyte was buffered to pH ≅ 2 using the Hydrion pH buffer solution and 0.72 M LiCl (each bag contained a precise buffer solution when the buffer was dissolved in 100 mL of purified water) and was incorporated into the bath as a supporting electrolyte. All chemicals were acquired from Sigma-Aldrich, St. Louis, MO, USA. The conventional bath was 0.4 M CuSO₄, 0.031 M ZnSO₄, 0.09 mM SnCl₂, and 0.54 mM Na₂S₂O₃. These experiments were conducted potentiostatically, with the electroplating potential falling between 0 and −1.55 V vs. NHE.

2.2. Instrumental Equipment and Techniques

The whole CZTS apparatus was generated experimentally. The thin film solar cell was completed using electrochemical techniques with the proper atomic composition and thickness. The layer after the CZTS absorber layer was CdS, with about 60 nm of thickness. The cadmium sulfide (buffer layer) electroplating was performed at 400 rpm (−0.8 V vs. SCE) for 11 min at an average temperature of about 75 °C. The precursors’ concentrations were 0.31 M CdCl₂, 0.008 M Na₂S₂O₃, and 0.41 M KCl. Hydrochloric acid drops fixed the electrolyte’s pH. The next layer was 200 nm of the intrinsic zinc oxide layer. The electrodeposition process of the final layer was carried out at 250 rpm at 75 °C by applying −0.84 V/SCE for 32 min from a solution consisting of 0.23 M Zn(NO₃)₂ and 0.51 M KCl. NaOH drops were added to adjust the pH bath to 6.5. The top layer of the P.V. cell was the window layer (doped zinc oxide). This top layer was electroplated to form 500 nm at 75 °C from a conventional nitrate bath. For the electrolytes 0.22 M Zn(NO₃)₂, 0.86 mM InCl₃, and 0.5 M KCl, −1.2 V/SCE was applied for 50 min at 200 rpm. The pH bath was adjusted to 3.5 using NaOH. The Energy-Dispersive X-ray Spectroscopy (EDS) technique (FEI Tecnai F30, Hillsboro, OR, USA) determined the final composition. The X-ray diffraction (XRD) technique was used to analyze CZTS crystallography (Bruker Discover D8 X-ray diffractometer, Billerica, MA, USA). The Scanning Electron Microscopy (SEM) images were obtained with FEI Tecnai F30 equipment (Hillsboro, USA) (2 μm CZTS deposit on stainless-steel disk). The CZTS main layer (absorber layer) was deposited on the stainless-steel substrate and treated with molybdenum. The deposit was cleaned with distilled water, and the surface was washed with acetone. The film was mounted onto an SEM sample holder.

2.3. CZTS Electrodeposition—Overriding Considerations

One of the main obstacles to CZTS electrodeposition is the significant variation in the typical values for the four co-deposited metals’ standard deposition potentials. The following equations could be used to represent the Cu, Zn, S, and Sn cations’ reduction potential [7,8,9,10]:

Cu²⁺ + 2e⁻ → Cu; E = 0.093 V + (RTln[Cu²⁺]/2F) (vs.SCE)

(1)

Zn²⁺ + 2e⁻ → Zn; E = −1.007 V + (RTln[Zn²⁺]/2F) (vs.SCE)

(2)

Sn⁴⁺ + 2e → Sn; E = −0.096 V − (RTln[Sn⁴⁺]/2F) (vs.SCE)

(3)

Sn²⁺ + 2e⁻ → Sn; E = −0.384 V + (RTln[Sn²⁺]/2F) (vs.SCE)

(4)

SO₄²⁺ + 2e⁻ → S; E = −0.93 V + (RTln[Sn²⁺]/2F) (vs.SCE)

(5)

All other species (Zn, Cu, and Sn) must electroplate at a more negative potential than the sulfur standard potential (−0.93 V). In this case, they reduce to their standard potential to a very cathodic voltage, which puts them near or higher than the metals’ limiting current, which is the electroplated area that is extremely sensitive to convection and transport. One way to express the ionic species’ diffusion flux toward the electrode is as follows [15]:

$$N_j={\displaystyle \frac{D_j\left(C_{b,j}-C_{e,j}\right)}{\left(1-t_j\right){\delta }_j}}$$

(6)

Here, the ionic flow, transport number, and diffusivity of ionic species j are denoted by N_j, t_j, and D_j, respectively. The total concentration (bulk) and the species j concentration at the electrode are denoted by C_b,j, and C_e,j, respectively, and the corresponding Nernst diffusion layer thickness is given by δ_j [15]. The latter is somewhat dependent on the ionic species. While the diffusion coefficients of the relevant species do not fluctuate significantly, we can assume that δ for each species behaves independently and mainly depends on the dominant transport mode. The transport number (t_j~0) in a well-supported solution, such as the CZTS setup, can be close to zero. Additionally, the concentration at the electrode is zero at the limiting current conditions (C_e = 0); so, Equation (5) produces the restricted transport flux condition (N_j, L):

$$N_j={\displaystyle \frac{D_j{C}_{b,j}}{{\delta }_j}}$$

(7)

As a result, the corresponding limiting current density has the following expression:

$$i_{j,L}={\displaystyle \frac{n_{j}F D_j{C}_{b,j}}{{\delta }_j}}$$

(8)

• F = Faraday’s constant;
• n_j = the number of electrons transported in the electrode reaction.

As a result, we anticipate that agitation and convective flow will be crucial during the electrodeposition process since the mass movement of copper, zinc, and tin will predominate. It should be noted that almost all commercial electroplating procedures operate under the mode of convective flow, usually involving air agitation and pumping to electrode surfaces. Nevertheless, most practical procedures are designed under conditions based on kinetic parameters because those processes are hard to scale and quantify. Furthermore, the rough and powdery deposit surface is typically generated under mass transport process control.

According to experiments, any rotation speed higher than 500 rpm is associated with bad adhesion, powdery deposit, and low current efficiency. Furthermore, calculating the hydrogen evolution by assembling the deposit weight according to the partial current was impossible at high speeds. Therefore, stopping at 500 rpm is likely a compromise to optimize the mass transfer and the overall process efficiency.

3. Results

3.1. CZTS Electrodeposition

The different electrochemical behavior and the range of standard deposition potentials of the four metals add complexity to CZTS electroplating. It is necessary to perform the electrodeposition process at a more negative voltage than the sulfur reduction (E_Applied < −0.93 V vs. NHE). Due to this fact, the electrodeposition of most other metals will be controlled by mass transfer, which occurs when limiting the current density is presented, resulting in a rough deposit and a challenge to managing the process.

According to the literature and this work, CZTS electrodeposition made it challenging to obtain the desired final atomic composition from a single bath process. In addition, the CZTS film was powdery and rough with poor adhesion. This work diluted the electrolyte about eight times to improve the deposit properties. This diluted solution yielded better deposit properties using a single bath process.

High-quality CZTS absorber layer production depends on the electrolyte solution’s optimum dilution. The concentration of precursor salts in the solution influences the deposited films’ atomic stoichiometry, shape, and quality. The authors examine the effects of various dilutions to ascertain the perfect circumstances and the desirable atomic composition. About eight times to ten times the dilution of the four species concentration produces a promising result, while further dilution more than ten times does not produce a continuous deposit.

The improved electrolyte yielded a smooth film with no proof of residue generation on the SS-disk surface or the solution at 500 rpm. Electroplating using this diluted system achieved the desired atomic composition at the deposition potential of −1.52 V vs. NHE, as shown in Figure 1. The deposit was adherent, uniform, and an acceptable grain size (Figure 1). Thus, during that interval, zinc operated at the kinetic control in the electrodeposition of the metal compound.

3.2. Quantifying Convective Flow Effects

The RDE system characterized flow effects on CZTS electroplating. The rotation speed varied while the electrolyte concentration and the potential were fixed. This process was carried out at −1.52 V/NHE at room temperature for 45 min. This procedure was repeated at several rotation speeds. The deposit composition was checked using X-ray diffraction (Hitachi SEM). See

Table 1. Atomic % composition (EDS analysis) and the weight composition of the electrodeposition of CZTS obtained from lower concentration electrolytes at different rotation rates after annealing. Electrodeposition time is 45 min at −1.52 V vs. NHE.

Rotation Rate		Cu	Sn	Zn	S
500 rpm	Atomic % Composition	24.93	14.2	9.9	50.8
	Weight [mg] Total Weight 1.67 mg	0.416	0.237	0.165	0.85
300 rpm	Atomic % Composition	23.9	11.1	9.1	55.8
	Weight [mg] Total Weight 1.63 mg	0.39	0.18	0.148	0.91
100 rpm	Atomic % Composition	23.2	12.8	9.7	54.1
	Weight [mg] Total Weight 1.2 mg	0.278	0.15	0.116	0.65

.

The data in Figure 2 illustrate how the agitation rate affects the deposition current, as evidenced in the plateau region, due to low hydrogen evolution at the limiting current. Thus, at the highest measured negative potential, the current density rose as the rotation rate increased (Figure 2). The electroplated metals, at deficient concentrations, perform as they are all mass transport-controlled, as shown in Figure 2, because all species are affected almost similarly to the agitation rate. Accordingly, the film’s atomic composition will be less affected by the agitation rate, as Table 1 shows.

To use these phenomena, we have introduced a solution in which the significant components’ concentrations have been diluted about eight times compared to typical baths. This upgraded electrolyte chemistry yielded good quality deposits regarding adhesion composition and electric properties at the high rotation speed of 500 rpm.

Electroplating experiments were conducted at the desired potential of −1.52 V vs. NHE for 45 min and at ambient temperature (~22 °C). The desirable atomic ratio of the compound was obtained in every experimental setup by adjusting the species concentration. The S-S substrate was weighed before electrodeposition and after annealing to determine the film’s total weight and calculate the current efficiency by the EDS technique, the deposited metals’ atomic ratio percentage. As noted in Table 1, less weight is observed at lower rotation speeds than the higher speed introduced due to good adhesion at lower and higher rotation speeds.

The electrode potentials (E) for the CZTS electrodeposition vary with the concentration of the individual metal ions. At around −1.1 V/NHE, copper, zinc, and tin are deposited at their limiting current. For sulfur, the standard potential is −0.93 V/NHE. The amount of sulfur deposit is too low at any potential of less than −1.3 V/NHE. The desirable sulfur content is about 50% of the total deposit. At potential −1.2 V/NHE (0.6 M Na₂S₂O₃), the amount of sulfur was only 18% of the total CZTS deposit, while at potential −1.4 V/NHE (0.6 M Na₂S₂O₃), the amount of sulfur was only 43% of the total CZTS deposit. At potential 1.52 V/NHE, the amount of sulfur was 61% at dilute concentration, while 56% was used for the conventional system. Then, several experiments were conducted to optimize copper, zin, and tin concentration according to the deposit’s final atomic ratio [Table 1].

3.2.1. Polarization Curves

After reaching the steady state, polarization curves were obtained on the RDE using different potentials for 0 to 1.3 V vs. NHE using potentiostat output. This process was run to find information about the electrochemical behavior of the four metals. Indeed, the RDE is convenient for analyzing the electrochemical redox reactions under controlled parameters and distinguishing regions in which the process is governed by the kinetics of the reaction, mass transfer, or both effects under the potential window studied for each salt precursor. Measurements on the polarization curve achieved from the improved electrolyte in Figure 2 exhibited different plateau behavior at 300 and 500 rpm than at conventional concentrations, as shown in Figure 3a. However, the cathodic branches of Figure 2 and Figure 3 are roughly similar for the lower rotation rate of 100 rpm. Additionally, the hydrogen evolution can be present at further negative deposition potentials than 0 V vs. NHE according to the equation described below:

$$2{\mathit{H}}^{+}+2{\mathit{e}}^{-}\leftrightarrow {\displaystyle \frac{1}{2}}{\mathit{H}}_{2\left(gas\right)} \mathbf{0} V vs. NHE$$

(9)

Then, it is conceivable to generate hydrogen gas toward the cathodic direction or enhance the amount of hydrogen evolution by gradually decreasing the potential at more negative values than 0 V vs. NHE. The film obtained has smooth and continuous coverage and an adherent deposit.

As estimated, the electrodeposition current and hydrogen evolution were significantly low due to the species’ low concentration and the smooth deposit. The open circuit potential was almost the same at different rotation speeds due to no change in the electrochemical reaction.

3.2.2. Hydrogen Evolution

Polarization curves were conducted to describe and calculate the magnitude of hydrogen evolution associated with this process. The polarization curves were created by investigating the film’s atomic ratio (EDS analysis) and calculating the individual species’ deposited film weight and partial current densities. The calculated curves were drawn from the partial current density, as shown in Figure 3b. The total weight from electrodeposition can be calculated as follows:

W_T = W_Cu + W_Sn + W_Zn + W_S

(10)

The equation used to obtain the weight of each element of the compound can be obtained from the current density measured from the polarization curve as described below:

$$W_j={\displaystyle \frac{A_{disk}{M}_j{i}_{j}t}{n_{j}F}}$$

(11)

From Equation (3), $W_j$ is the weight of species j, $A_{disk}$ is the geometric area of the electrodeposit, $M_{j }and i_j$ are the molecular weight and the current density of the species j, respectively, and $\mathit{t}$. is the time of the deposit. Finally, $n_j$ is the number of electrons participating as a part of the reduction reaction of the species j.

The assembled polarization curves of hydrogen evolution exhibited considerably low current density compared to the conventional concentration, corresponding to about 18% of the total current, as shown in Figure 3b. In other words, conventional baths with higher concentration systems contain a significant hydrogen evolution current, which leads to low deposition current efficiency and a smooth and good adhesion deposit (Figure 3b).

Figure 4 describes the difference in the current density of the CZTS when the measurements are collected via measurements from a potentiostat with the chronoamperometry method (polarization curve) or via weight measurements. The differences in both current densities of the compound are associated with the influence of the reduction of hydrogen protons to produce the evolution of hydrogen gas. In other words, lower concentration systems contain a small hydrogen evolution current, which leads to high deposition current efficiency and a rough and bad adhesion deposit (Figure 4).

3.3. Thermal Annealing

Post-thermal annealing is essential after ambient temperature deposition to improve the crystallography structure and reduce the recombination defects [17,18,19,20,21,22]. Parameters of the annealing process depend on the deposited layer thickness, film composition, binary stacked layers, the four species’ partial vapor properties, argon or nitrogen flow rate on the tube furnace, and temperature ramp rate [19].

Electroplating experiments were conducted at the desired potential of −1.52 V vs. NHE for 45 min and at the ambient temperature (~22 °C). The desirable atomic ratios of the compound were obtained in every experimental setup by adjusting the species concentration, and the atomic composition was investigated using the EDS method. In the following step, the electroplated device was placed in a quartz tube furnace filled with hydrogen sulfide (around 10 kPa) for 55 min, and the annealing was carried out at 535 °C. The annealing process heating rate was 15 °C/min, and then it was allowed to cool naturally. The atomic composition was investigated using the EDS method. The desired temperature was selected by checking the CZTS deposited film composition before and after annealing (see

Table 2. CZTS deposited film chemical composition before and after annealing. The applied potential is −1.52 V vs. NHE for 45 min at a rotation rate of 500 rpm before the annealing process: 3.2 mM CuSO₄, 2.1 mM ZnSO₄, 3.3 mM SnCl₂, and 0.3 M Na₂S₂O₃. A Hydrion buffer pH solution is used to fix the solution to pH ≅ 2 and 0.71 M LiCl. The post-treatment stage was conducted on a quartz tube furnace filled with hydrogen sulfide (around 10 kPa) for 55 min, and the annealing was carried out at 535 °C.

Metal	Before Annealing	After Annealing	Experimental Errors (%)
Copper	23.2%	24.93%	4.1
Tin	15.0%	14.2%	3.9
Zinc	8.0%	9.9%	5.2
Sulfur	54.1%	50.3%	6.05

).

The electrodeposition of CZTS was repeated several times under identical conditions to minimize procedural errors (Table 2). The elemental composition of copper, zinc, tin, and sulfur showed good repeatability with minimal errors in the repeated electrodeposition of CZTS thin films, as stated in the last column of Table 2. The films’ consistent composition and suitability for use in solar devices are guaranteed by the finding that the error ranges fall below reasonable bounds for real-world applications. The slight deviations seen are expected for the electrodeposition process. They can be reduced even more by adjusting the electroplating parameters and strictly controlling the experimental setup.

3.4. Crystallography of CZTS Solar Device

Following the post-treatment step, the deposit was investigated by XRD analysis to determine the crystallography and morphology (Bruker Discover D8 X-ray diffractometer). The film’s most important diffraction peaks were found at the (112), (200), and (220) planes, which match the crystal structure of CZTS (Figure 5). The investigation accomplished by XRD analysis shows that the electroplated film has a single Cu(ZnSn)S₄ crystallography structure; no other peaks were detected. According to the literature, the most significant peak is located at 27.92°, the desirable CZTS crystallography. The CZTS film thickness is about 1.72 µm, according to profilometer measurements (P-6 stylus profiler). The thickness measurements show that the CZTS absorber layer was deposited on the molybdenum-coated stainless-steel disk. After distilled water to clear the deposit, acetone was used to wash the surface. The deposit (0.31 cm²) is mounted on the profilometer stylus apparatus. The topography of a surface is measured with a profilometer in terms of vital dimensions such as step, curvature, and flatness. After touching the surface vertically, a diamond stylus is pushed laterally across the sample for a predetermined distance. Operating at the precise contact force is made possible by force feedback. The probe scans the thickness and roughness of the CZTS sample. The total thickness is compared with the calculated weight from the current efficiency section (Equations (2) and (3)).

4. Optical Characterization Test

The CZTS solar device was studied using an electroplating process in a single step. Characterization examinations were conducted on the CZTS completed device like in previous work [15,16].

The CZTS devices developed from thin films generated from the lower concentration electrolyte were about 3% efficient (current–voltage, I–V) (Figure 6). Device characterization was conducted at AM1.5 illumination (1000 W/m²). The investigation showed approximately 14.7 mA/cm² of open short-circuit current and 0.285 V of open-circuit voltage. Figure 6 illustrates the fill factor of the CZTS device, which was approximately 44%.

The ratio of excited electrons to incident photons is known as the external quantum efficiency. The appropriate wavelength is used to calculate the current generated per incoming photon. The solar cell device’s incident photon conversion efficiency, or external quantum efficiency (EQE), is measured using the QEX10. The corresponding wavelength is used to calculate the current generated per incoming photon:

EQE = Elecron/(Sec)/Photon/Sec

The quantum efficiency of CZTS was measured using the QEX10 quantum efficiency measurement instrument. This apparatus determined the CZTS absorber layer’s band gap. The quantum efficiency for the CZTS solar device under illumination (1000 W/m²) is displayed in Figure 7. This characterization examination demonstrates that the CZTS absorbs photons mainly in the visible light spectrum range. The CZTS film produced from the low-concentration system has acceptable quality, which explains why the quantum efficiency was satisfactory (Figure 7). The band gap evaluation matches the practical one, nearly 1.41 eV. Figure 7 illustrates the quantum efficiency at the CZTS band gap, which was approximately 0.78.

The performance was re-investigated to check the long-term stability of the CZTS device. The aging process for CZTS cells was carried out at room temperature, in the dark, and with light. After 120 days, aging in the dark did not decline performance. Generally, the zinc oxide (ZnO) layer adds another degree of protection to the zinc surface and is comparatively stable. Also, there is no change in the use of the peeling test, which is similar to previous work [15].

5. Conclusions

This work achieved an excellent quality deposit of CZTS thin film by applying a single bath electrodeposition process with a dilute electrolyte. The mass transport of the final composition and its quality were characterized by the dilute and conventional electrolytes. Agitation effects on the process were optimized, and this work yielded a thin film with the desired atomic composition of the four metals. A weight analysis was also performed to characterize the primary function of hydrogen gas co-evolution in the CZTS electrodeposition process and its efficiency. The deposited compound was homogeneous, adherent, and within well-controlled experimental parameters. The sulfur loss was controlled during the annealing and optimized to gain the desirable final atomic composition. Several analyzing methods were applied to investigate the final CZTS composition, thickness, and film properties. The CZTS device layers were completed as mentioned in previous work [15]. The research conducted for this project advances the development of a more practical method for controlling the electrodeposition process to produce large-scale CZTS devices. The optical properties and quantum efficiency of the CZTS film were characterized. The optical characterization revealed an open short-circuit current of 14.7 mA/cm² and an open-circuit voltage of 0.285 V. In addition, the CZTS device had a fill factor of around 44%. The CZTS device’s quantum efficiency at the optical band gap was 0.78. Finally, this work on the CZTS system yielded optimistic results regarding deposit quality and optical characteristics.