1 Introduction

Extensive research efforts have focused on quaternary chalcogenide materials. This surge in interest is due to the remarkable electronic and electrical properties of these compounds. These properties make them highly promising for applications in photonic devices [1]. Recently, a conversion efficiency of 14.9% has been achieved with the readily available solar absorber Cu2ZnSnS4(Se) [2], known for its high absorption coefficient (above 104 cm). Cu2CoSnS4 (CCTS) belongs to the chalcopyrite family of inorganic semiconductors and offers a comparable and viable alternative absorbing material. CCTS crystallizing in quadratic system (I4m group) [3] and has an ideal bandgap in the range of 1.2–1.5 eV making it exceptionally well suited for various photonic applications. This optimal bandgap range positions CCTS as a promising candidate for harnessing solar energy and advancing photovoltaic technology [4,5,6,7]. Its potential for integration into next-generation optoelectronic devices is further enhanced by its structural and electronic properties.

Several synthetic techniques, such as, sputtering [8], pulsed laser deposition [9], microwave method [10], thermal decomposition [11], chemical vapor deposition [12], spray pyrolysis [13, 14], solid-state reaction [15], colloidal synthesis [16], and electrochemical deposition (electrodeposition) [17,18,19,20], have been used to fabricate CCTS thin films. Among these approaches, electrochemical deposition stands out as a highly attractive method. This is due to its advantages, including economical equipment, inexpensive starting materials, and the ability to perform scalable deposition at room temperature. Some research has been conducted to investigate the factors that affect the electrodeposition process for the preparation of CCTS. Electrodeposition techniques include sequential deposition of metallic stacked Cu, Zn, and Sn thin films followed by sulfur (S) diffusion, simultaneous deposition of metallic Cu, Zn, and Sn thin films followed by sulfur (S) diffusion, and single-step deposition of CZTS thin film has been reported in several studies. Pulsed electrodeposition technique (PED) has found wide application in metal plating, effectively increasing deposition rates and refining film quality [21]. PED has been used in CZTS deposition to improve film quality [22,23,24]. However, its use in the growth of CCTS thin films has not been explored. PED differs from conventional DC electrodeposition primarily in the controlled on/off cycling of the plating currents. This results in the generation of a square wave pattern that is characterized by specific pulse parameters, including pulse duration (TON), time between pulses (TOFF), and the overall deposition duty cycle. Pulsed Electrodeposition (PED) provides the flexibility to create complex alloy compositions. By precisely adjusting metal ion concentrations, current densities and pulse durations, the formation of new crystals can be stimulated rather than the growth of existing ones. The result is the production of films with an exceptionally fine grain structure [25].

2 Experimental

The pulse electrodeposition (PED) process was performed using a standard three-electrode potentiostat (VersaSTAT 3). Fluorine tin oxide (FTO) was employed as a working electrode, a Platine electrode was used as the counter electrode, while Ag/AgCl/Cl− was used as the reference electrode (Fig. 1). The aqueous electrolytic solution consists of CuSO4·5H2O (0.02 M), CoCl2·6H2O (0.015 M), SnSO4 (0.010 M) and Na2S2O3 (0.02 M), with Na3C6H5O7·2H2O (0.2 M) acting as a complex agent. The pH of the solution was adjusted to 5.0 by the addition of an appropriate amount of tartaric acid. The CCTS thin films were prepared by potentiostatic electrodeposition using potential-pulsed electrodeposition at − 1.1 V with a pulse duration of 1 s and an off-time (0 V) of 1 s (Fig. 2c). The total electrodeposition time was 10 min (CCTS1), 20 min (CCTS2), and 30 min (CCTS3). The Cu–Co–Sn–S electrodeposited precursor films were sulfurized by exposure to an evaporated sulfur environment at T = 500 °C for 1 h in an alumina tube furnace, resulting in the formation of CCTS films. To eliminate any trace of air, the tube was purged with Ar gas for 15 min prior to sulfurization. 0.8 g sulfur (with a purity of 99.9%) was used as the source and was placed in an alumina boat that was maintained at a temperature of 25 °C.

Fig. 1
figure 1

Schematic of synthesis of CCTS thin films

Full size image
Fig. 2
figure 2

a Cyclic voltammograms in trisodium citrate solution for each individual element: Cu2+, Co2+, and Sn2+. b Cyclic voltammetry in trisodium citrate solution for combined elements: Cu2+–Co2+–Sn2+–S2−. c Current density/mA cm−2 vs. time/s in pulsed electrodeposition [E1 = 0.00 V, E2 = − 1.10 V vs. Ag/AgCl]

Full size image

The sulfurized thin films were structurally analyzed using X-ray diffraction (XRD) with CuKα radiation (λ = 1.5406 Å). A field emission scanning electron microscope (FE-SEM) (model) and an energy dispersive X-ray spectrometer (EDS) were used for surface morphological and compositional characterization. UV–visible–NIR spectroscopy (SHIMADZU, UV-2600i) was used to measure the optical properties of the thin films at room temperature.

3 Results and discussion

3.1 Electrochemical characterization

Cyclic Voltammetry (CV) is a valuable technique providing insights into the reduction potential of elements within the electrolytic bath, crucial for understanding the electrochemical behavior and reactivity of the deposition process components. To determine the deposition potential of the CCTS precursor, the individual deposition potentials of copper (Cu), cobalt (Co), and tin (Sn) were first determined using separate electrolytic baths. Subsequently, the deposition potential of CCTS films was thoroughly evaluated for the influence of complexing agents and thiosulfate elements (S2O32−). This comprehensive approach ensured a detailed understanding of the factors influencing the deposition process. The electrochemical behaviors of each element in the aqueous solution reveal significant potential differences for Cu, Co, and Sn [26].

To refine the potential window, trisodium citrate was used as a complexing agent in the electrolytic bath. This compound forms complexes with copper ions (Cu2+), effectively shifting the reduction potential of copper to more negative values. A more controlled deposition process was achieved by this adjustment [26, 27].

Figure 2a depicts the electrochemical behavior of each individual element: Cu2+, Co2+, and Sn2+. The curves display cathodic and anodic peaks observed between − 1.3 and 0.8 V vs. Ag/AgCl. The cathodic peaks of Cu2+, Sn2+, and Co2+ are located around − 0.56 V, − 0.98 V, and − 1.10 V vs. Ag/AgCl, respectively. Figure 2b presents the cyclic voltammetry (CV) curve of the co-electrodeposition bath containing Cu2+, Sn2+ and Co2+. The voltammogram showed three distinct peaks. These peaks were consistent with the curves of the individual metallic elements shown in Fig. 2a (Eqs. 1, 2, 3). This observation is an indication that the formulation was well suited for the deposition of Cu–Co–Sn films (Fig. 2b) [18, 27]. Additionally, these elements can be deposited with sulfur in the presence of S2O32− which can generate colloidal S in acidic medium (Eqs. 4, 5) (Fig. 2b) [28]. S2O32− could be introduced as a complexing agent for Cu2+, as described by (Eqs. 6, 7) [29]. The overall electrodeposition reaction of Cu–Co–Sn–S precursors is represented by (Eq. 8).

$$ {\text{Cu}}^{2 + } + 2e^{ - } \to {\text{Cu}} $$
(1)
$$ {\text{Sn}}^{2 + } + 2e^{ - } \to {\text{Sn}} $$
(2)
$$ {\text{Co}}^{2 + } + 2e^{ - } \to {\text{Co}} $$
(3)
$$ {\text{S}}_{2} {\text{O}}_{3}^{2 - } + {\text{H}}^{ + } + 2e \to {\text{S}}^{2 - } + {\text{HSO}}_{3}^{ - } $$
(4)
$$ {\text{S}} + 2e^{ - } \to {\text{S}}^{2 - } $$
(5)
$$ 2{\text{S}}_{2} {\text{O}}_{3}^{2 - } + 2{\text{Cu}}^{2 + } \to 2{\text{Cu}}^{ + } + {\text{S}}_{4} {\text{O}}_{6}^{2 - } $$
(6)
$$ {\text{Cu}}^{ + } + n{\text{S}}_{2} {\text{O}}_{3}^{2 - } \to \left[ {{\text{Cu}}\left( I \right)\left( {{\text{S}}_{2} {\text{O}}_{3} } \right)_{n} } \right]^{ - 2n + 1} $$
(7)
$$ 2{\text{Cu}}^{2 + } + {\text{Co}}^{2 + } + {\text{Sn}}^{2 + } + 2{\text{S}}_{2} {\text{O}}_{3}^{2 - } + 16e^{ - } + 12{\text{H}}^{ + } \to {\text{Cu}}_{2}-{\text{Co}}-{\text{Sn}}-{\text{S}}_{4} + 6{\text{H}}_{2} {\text{O}}{.} $$
(8)

The deposition of CCTS films was carried out using pulsed electrodeposition for durations of 10, 20, and 30 min. This involved applying a high potential (On; − 1.1 V) in a series of pulses separated by intervals of 1 s with no potential flow (Off; 0 V) (Fig. 2c) [24].

Electrochemical impedance spectroscopy (EIS) was investigated to evaluate the behaviors of the electrochemical process associated with CCTS electrodeposition. The EIS were performed at both the potential of deposition (− 1.10 V) and at the potential of (0.00 V). In Fig. 3 the dotted curves shows the experimental Nyquist diagrams in different situations, while the solid line represents the fit of the respective nyquist spectra.

Fig. 3
figure 3

In-situ electrochemical impedance spectroscopy measurements: a CCTS precursor deposition at 0.00 V, b CCTS precursor deposition at − 1.1 V, c CCT precursor deposition at − 1.1 V and b without electroactive species at − 1.1 V

Full size image

The nyquist plot in Fig. 3a shows an in-situ impedimitric spectra at applied potential of E = 0.00 V titled by an angel θ to the y-axis. This implies that the equivalent circuit comprises either a standalone capacitor C wich represents the nonfaradaic current results from charging the double-layer capacitor [30] or a capacitor C connected in series with a solution resistor Rs [31]. The circuit fitting (Fig. 3a) suggests that the impedance is not purely ohmic and includes a capacitive element (CPE) [30]. This could be due to surface effects, double-layer capacitance, or other non-idealities [32]. In another hand, the EIS spectra recored at E = − 1.10 V a semicircle at high and medium frequency is related to the double layer response and an inclined line at low frequency indicated the presence of a warburg impedance due to diffusion of ions into the electrode surface. The impedance plotted in Fig. 3b was fit to the equivalent circuit shown in the same figure. This circuit consists of a parallel combination of a series charge transfer resistance (Rct) and a Warburg element (W), along with a constant phase element (CPE) representing the double-layer capacitance [33, 34]. In addition, the solution resistance is represented by the series resistance (Rs). The fitted lines in Fig. 3b correspond to this equivalent circuit [34]. The values for the various elements are given in Table 1. The effective double-layer capacitance, Q, was determined from the constant phase element using a method explained by Mansfeld [35, 36]. Figure 3c presented the impedance spectra conducted in situ during electrodeposition of Cu–Co–Sn in a free sulfer solution. The results shows the presence of a capacitive loop without Warburg impedance. This may mean that charge transfer at the electrode/electrolyte interface is relatively fast and mass transport limitations (typically represented by Warburg impedance) are not significant during the electrodeposition process. Additionally, The absence of a Warburg impedance may indicate that the electrodeposited layer is relatively thin. The difference in the impedance spectra between Fig. 3b and c can demonstrate the interaction between S2O32− and metal ions in the electrodeposition process, as indicated by Eqs. (2) and (3). Furthermore, the electrochemical impedance behaviors of FTO electrode in the citrate electrolyte at the same potential (− 1.10 V) and in absence of the electroactive elements was studied. Figure 3d presents the impedance spectra obtained by the experiment. It reveals the presence of high-frequency capacitive loop, associated with double layer capacitance, and the appearance of inductive loop representing the relaxation phenomena. This relaxation is likely dependent of the presence of (Sn(IV))ads≡(SnH4)g as an adsorbed intermediate species in the reduction reaction tin oxyde (comprising 95% FTO [37]) [34]. This reduction reactions denoted by Eqs. (9) and (10) is induced by cathodic polarisation on the surface of the FTO electrode [38, 39].

$$ {\text{SnO}}_{2} + 4{\text{H}}^{ + } + 4e^{ - } \to {\text{Sn}} + 2{\text{H}}_{{2}}{\text{O}} + \quad E^\circ = \, - \, 0.{\text{1 V }}\left( {{\text{vs}}.{\text{ SHE}}} \right) $$
(9)
$$ {\text{Sn}} + 4{\text{H}}^{ + } + 4e^{ - } \to {\text{SnH}}_{4} \left( g \right)\quad E^\circ = \, - { 1}.{\text{1 V }}\left( {{\text{vs}}.{\text{ SHE}}} \right). $$
(10)
Table 1 Simulated impedance parameters
Full size table

3.2 Structural characterization

The precursor electrodeposited samples of CCTS were sulfurized at a temperature of 500 °C for one hour in the presence of sulfur powder.

3.2.1 XRD analysis

Figure 4a shows the XRD patterns of CCTS films obtained by pulsed electrodeposition of Cu–Co–Sn–S precursor films at 10, 20 and 30 min. All the films aligned with the tetragonal structure of CCTS, as indicated by the four distinctive peaks (112), (204), and (312) observed at 2θ values of 28.90°, 47.92°, and 56.72°, respectively, in accordance with JCPDS 96-153-3608. The secondary phases produced in all cases is CoS2 at 32.5° (card no. 00-041-1471) [40]. Additionally, the peak observed around 31.6° in samples electrodeposited for 20 and 30 min indicates the presence of CuS (card no. 00-006-0464) [41]. The other peaks located at 26.9°, 38.1°, 51.74°, 61.7°, and 65.7° are associated with the FTO substrate [27, 28, 42]. Using the Gaussian fitting (Fig. 4b), the average crystallite sizes of the CCTS films were determined by calculating the full width at half maximum (FWHM) of the peak through the application of the Debye–Scherrer equation. The CCTS film crystallite sizes resulting from electrodeposition of CCSS precursor films at duty 10 min, 20 min and 30 min were 28.36.6 nm, 27 nm, and 24 nm, respectively (Table 2). According to the observations mentioned earlier, an increase in deposition time led to a minor reduction in crystallite sizes, aligning well with the findings from scanning electron microscopy (SEM) (Fig. 6).

Fig. 4
figure 4

X-ray diffraction analysis of CCTS thin films for various pulse durations

Full size image
Table 2 Structural parameters of CCTS thin films
Full size table

3.2.2 Raman analysis

Figure 5 shows the Raman spectra (200–600 cm−1) of the CCTS samples deposited for different time periods. The spectra obtained showed a principal sharp peak at 470 cm−1 consistently in all cases, along with a less prominent peak at 265 cm−1. Both peaks are characteristic of a crystalline structure, indicating a single crystal of CuS [43]. The several peaks located at 280–290 cm−1 and 318–334 cm−1 correspond to the Cu2CoSnS4 phase. The most prominent peak occurs in the range of 318–319 cm−1 and is associated with the A1 symmetry of the CCTS stannite structure. The peak located at 490 cm−1 in 30 min sample my indicate the presence of CoS phase [44]. As a result, the samples electrodeposited for 20 and 30 min confirm the presence of a pure CCTS phase.

Fig. 5
figure 5

Raman spectra analysis of CCTS samples

Full size image

3.3 Morphological characterization

Figure 6a–c, shows SEM images of CCTS pulsed electrodeposited thin films on the FTO substrate at different deposition time. The sample deposited for 10 min shows a uniform morphology (Fig. 6a), but as the deposition time increases to 20 min (Fig. 6b), a decrease in grain size is observed. This phenomenon can be attributed to increased nucleation leading to the formation of a wider number of smaller grains. On the other hand, prolonging the deposition time to 30 min through pulsed electrodeposition (Fig. 6c) results in smaller grain sizes. Simultaneously, an additional morphology with a hexagonal sheet-like structure appears, possibly associated with the secondary phase CuS [45].

Fig. 6
figure 6

SEM-images of CCTS thin films; electrodeposited for: a 10 min, b 20 min and c 30 min

Full size image

Quantitative EDS analysis was performed on thin films electrodeposited for 10, 20, and 30 min. Figure 7 shows an EDS spectrum showing the presence of different peaks associated with Cu, Co, Sn and S, corresponding to the deposited CCTS thin film. In addition, the presence of an O peak originating from the FTO substrates was observed. The atomic composition (Table 3) indicates the sulfur-rich nature of the films. The presence of Cu is likely contributed by the CuS phases, and the 10 min sample shows Sn-rich grains, possibly due to its association with one of the FTO compositions. The ratio (Cu/(Co + Sn)) decreases, reaching approximately 1 as the deposition time increases. The proportion of Co increases while the amount of Sn decreases. The ratio (Cu/(Co + Sn)) decreases, reaching approximately 1 as the deposition time increases (Table 3).

Fig. 7
figure 7

EDS analysis of CCTS thin films; a 10 min, b 20 min and c 30 min

Full size image
Table 3 Elemental composition of CCTS thin films
Full size table

3.4 Optical characterization

The UV–Vis absorption spectra of the CCTS electrodeposited samples are shown in Fig. 8a. These spectra were measured in the absorbance mode over the wavelength range of 400–1200 nm. The absorption spectra show a decrease in intensity with increasing deposition time. This is likely due to an increase in thickness uniformity of the film as the deposition time increases. The straight-line segments of the Tauc’s plots shown in Fig. 8b are extended to the point where the absorption coefficient reaches zero, providing intercepts or values for the band gap. The band gap values for the 10 min, 20 min, and 30 min films are 1.57 eV, 1.54 eV, and 1.43 eV, respectively. The band gap (Eg) values show a slight decrease over time, increasing by 0.03 eV from CCTS 10 min to CCTS 20 min, and by 0.11 eV from CCTS 20 min to CCTS 30 min. This change is attributed to variations in structural composition and morphology [46], as illustrated in Figs. 5, 6, and Table 3. Despite the slight impact of deposition time on the Eg values of CCTS thin films due to the presence of secondary phases [18], all films have a band gap aligned with the optimum energy range for thin film solar cells, making them suitable for applications in thin film solar cells with high conversion efficiency. This value is consistent with those reported in the literature [14, 47]. Table 4 presents a comparative analysis of CCTS thin films elaborated by co-electrodeposition and our study, which investigated pulsed mode electrodeposition.

Fig. 8
figure 8

a UV–visible absorption measurements, b Tauc plots of CCTS thin films electrodeposition

Full size image
Table 4 Comparative analysis of CCTS electrodeposition results
Full size table

4 Conclusion

In this study, we present an economical approach for manufacturing CCTS thin films on an FTO-coated glass surface. The pulsed electrodeposition method (PED), followed by a sulfurization treatment at 500 °C in argon atmosphere, proved successful in the fabrication of CCTS films. The crucial parameter in pulsed electrodeposition (PED) is the pulse duration with predefined potentials. The electrochemical behaviors of CCTS precursors have been studied individually using cyclic voltammetry, as well as together in the same bath containing trisodium citrate. In-situ electrochemical impedance spectroscopy (EIS) has investigated the mechanism of deposition at the electrode–electrolyte interface. The impedance spectra reveal a loop capacitive behavior, and in the presence of S2O32−, they indicate the presence of Warburg diffusion, suggesting a diffusion phenomenon. CCTS deposited for 20 min exhibited a pure CCTS phase, compact morphology, and nearly stoichiometric composition. Conversely, pulsed electrodeposition of CCTS for 10 min revealed the presence of secondary phases. The optical gap energy is slightly affected by the deposition time and was found to be around 1.50 eV, making it well-suited for utilization as an absorber in high-performance solar cells based on CCTS.