1 Introduction

The third generation of solar cells mainly contains copper zinc tin sulphide solar cells (CZTS), dye-sensitized solar cells (DSSC), Polymer solar cells, organic solar cells, quantum dot-sensitized solar cells (QDSSC), etc. [1,2,3,4,5]. The main objective of third-generation approaches is to achieve higher efficiency. The DSSC was highly efficient. However, DSSC is more actively working for lower intensity than for high intensity; hence, DSSC has mainly been proposed for indoor application [6, 7]. DSSC has several disadvantages because of their use of a degradable liquid electrolyte for higher intensity and temperature [8,9,10], working of QDSSC is mainly based on interfacial charge separation between the short bandgap (donor) and large bandgap (accepter) semiconducting materials. [11, 12]. In many respects, QDSSC is much better than DSSC the projected efficiency of QDSSC is high, reaching 42%, which is exceeding the Shockley–Queasier efficiency limit. QDs are inorganic, because this QDs are more stable. QDs possess a tunable bandgap, sensitization to diffuse light, generation of multiple excitons, and high extinction coefficient. QDS can be easily synthesized because of the increasing demand for scientific and industrial research [13]. The performance of a solar cell is influenced by various parameters such as the photoanode, sensitizers, electrolytes, and counter electrodes. This study focuses on the synthesis of counter electrodes and their impact on solar-cell performance. The main function of the counter electrodes is to catalyze the reduction reaction of the electrolytes via external circuits. Good counter electrodes exhibit high catalytic activity, conductivity, and good mechanical and chemical stability. In QDSSc counters, electrodes such as Pt, Au, Cu2S/ Brass, CuxS/FTO, CuxSe/FTO, ITO/Cu2S, GH-CuS/Ti, CoS, PbS, NiS, FeS2, MoS2, Cu2SnSe3, CZTS, CZTSe, CNT, and C60 were used [14, 15]. Here, we synthesize ternary material like copper tin sulfide (CTS) as a counter electrode because it has a band gap of 0.9–1.35 eV and an absorption coefficient of 104 cm−1 [16]. Copper tin sulfide is a promising material for electrodes compared to carbon electrodes, owing to its unique properties. Its high electrical conductivity allows for efficient charge transport in electronic devices such as solar cells. Their optical properties make them ideal for photovoltaic devices, where they convert solar energy into electrical energy. CTS is chemically stable, which makes it suitable for electrochemical applications. Its high specific capacity makes it ideal for energy storage applications. Its low-cost and abundant raw materials make it an attractive alternative to expensive and rare materials. CTS’s tunable properties of CTS allow the optimization of specific applications.

Several methods are available for the synthesis of CTS Solvothermal [17], sol–gel [18], chemical bath deposition [19], and SILAR [20]. The CBD and SILAR methods offer advantages such as simplicity, cost effectiveness, and control over deposition. CBD requires basic laboratory equipment and is cost effective, providing uniform coatings that are suitable for large-scale production. It also allows for control over the deposition rate and thickness by adjusting the deposition parameters. However, this method has limitations such as limited materials and complexity in multilayer deposition. On the other hand, SILAR allows precise control over the film thickness and composition, facilitating multilayer deposition. They are versatile and can be used in a wide range of materials and substrates. However, this is time consuming and requires careful control of the reaction conditions and parameters to achieve the desired film properties. Choosing the appropriate method depends on specific research requirements and balancing factors, such as cost, material compatibility, and control over film properties.

In this study, we used the SILAR method to fabricate CTS counter electrodes for solar-cell applications. In this study, a chemical bath deposition method was employed to deposit a compact ZnO layer on nanocrystalline ZnO films, followed by a porous ZnO layer using the doctor-blade method, and sensitization with CdS using the SILAR method. The crystallite size is approximately 5 nm, indicating a strong confinement effect. As CdS has shown excellent optoelectronic properties, it has been utilized in multiple optoelectronic devices, such as photocatalysts, solar energy conversion, and nonlinear optical devices [21].

In this study, a CTS counter electrode was fabricated using a low-cost (modified chemical bath deposition) method. Utilization of CTS as a counter electrode in a CdS/ZnO sandwiched solar cell. In the present study, we used CTS as a counter electrode in a ZnO/CdS-based collar-cell configuration for the first time.

2 Experimental details

2.1 ZnO/CdS electrode preparation

This study used chemicals from SRL, Sigma Aldrich, Thomas Baker, and HPLC without further purification to prepare ZnO/CdS electrodes. Here, we used the deposition of a ZnO compact film to avoid back contact of the cells. The chemical bath deposition method is cost effective and does not require sophisticated instrumentation. A ZnO compact was deposited on a transparent conducting oxide such as (FTO). The substrates were cleaned via ultrasonication in absolute ethanol, and thoroughly cleaned with acetone. The seed layer was prepared using a chemical bath deposition method, using a 0.05 M zinc nitrate solution in double-distilled water and 20% diluted ammonia as a precursor. The layers were deposited at 60–70 °C for 15 min, cleaned, dried in an incubator, and annealed at 450 °C for 1 h. The preparation of paste is a very important part of the fabrication of solar cells here we use commercial ZnO nanopowder, ethyl cellulose, and terpineol for paste preparation [22]. The process involved preparing 1 g of ZnO nanopowder, adding 15 ml of ethanol, sonicating for 1 h, adding ethyl cellulose as a pore-filling agent, adding terpinol as a solvent or dispersant, and adding half a milliliter of acetylacetone as a stabilizing agent. The paste was collected in a suitable container or bottle and incubated in an incubator to allow the excess ethanol to evaporate. This paste preparation method is suitable for applications that require well dispersed and stable ZnO paste. The prepared films were annealed at 450 °C. The mesoporous and transparent ZnO photoanode was sensitized with CdS nanoparticles using the successive ionic layer adsorption reaction (SILAR), with cadmium nitrate [Cd(No3)2] and sodium sulfide (Na2S) as precursors for cadmium and sulfide ions, respectively, and ethanol and methanol as precursors for cadmium nitrate and sodium sulfide, respectively. The detailed methodology and analysis of the ZnO/CdS photoanode have been described previously by Pathan et al. [23].

2.2 Preparation of CTS and carbon counter electrode

SILAR is a novel method for the deposition of metal chalcogen films. The thickness of the film could be controlled by changing the number of deposition cycles. Room-temperature deposition can be achieved by the SILAR method, which is why we used the SILAR method to fabricate a CTS counter electrode. Fluorine-doped tin oxide films were used as the substrates. The FTO substrate was cleaned with a soap solution and hot water, after which an ultrasonic bath was used for film cleaning, sonicated substrates are cleaned with ethanol after drying for a few minutes in an incubator, and the FTO substrates were used for the deposition of the CTS film on it. The deposition steps for the CTS counter electrode are shown in Fig. 1. Distilled water was used as solvent. The cations source contains 0.17 M CuCl2 + 0.11 M SnCl2 + DW, and the anions were 0.11 M Na2S + DW. An EDTA solution was used as the complexing agent [18].

Fig. 1
figure 1

Steps involved in the deposition of CTS films by SILAR method

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FTO was used as the substrate for the synthesis of the carbon counter electrode, and FTO was used as a substrate. A clean FTO substrate was placed on a burning candle, and carbon soot was uniformly deposited on the substrate. The deposited film was used as the carbon counter electrode [22].

Repetitive 25 SILAR cycles resulted in uniform deposition of the films. Deposited films were annealed at 150 °C, and the prepared CTS films were used as counter electrodes for CdS/ZnO sandwiched solar cells. In this study, we prepared polysulfide electrolytes using sodium sulfide (Na2S), sulfur powder, ethanol, methanol, and double-distilled water. First, Na2S flakes were ground using a mortar and pestle prepared a 0.5 M solution in ethanol, and then the fine sulfur powder was ground and 0.1 molar solutions in 5 ml methanol and 5 ml double-distilled water were added.

3 Results and discussion

3.1 Structural properties

An X-ray diffractometer (Bruker D8) with CuKα (1.54 Ǻ) was used to analyze the XRD pattern of a CTS film deposited using the SILAR method. The XRD spectra of the CTS film, as shown in Fig. 2, are in good agreement with JCPDS 01-089-4714 [24–26]. The X-ray diffraction (XRD) analysis of the CTS film revealed diffraction peaks at specific 2θ values, indicating a similar crystal structure. These peaks correspond to the (112), (220), and (312) crystallographic planes of CTS, reflecting the atomic arrangement within the crystal lattice. The Debye–Scherrer formula calculated the crystallite size of the CTS material to be 31 nm, indicating its structural coherence and order. XRD analysis aligns with the tetragonal crystal structure, highlighting the material’s inherent properties and behavior, and contributing to a deeper understanding of its structural characteristics and potential applications.

Fig. 2
figure 2

XRD pattern of CTS thin film deposited by SILAR method

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3.2 Morphological properties

The surface morphologies of the compact and doctor-bladed ZnO films were examined using a JEOL JSM-6360A scanning electron microscopy [SEM]. Figure 3(A–C) shows an SEM image of a ZnO compact layer film with low and high magnifications, which was deposited using the chemical bath deposition method. The sample exhibited agglomerated submicron-sized small flowers, similar to the morphology of the film. The particle size of the compact ZnO film in the SEM image is in the range of 200–600 nm. The SEM images show the sizes of the agglomerated particles. SEM revealed a uniformly deposited crack-free film with full substrate coverage, which facilitated rapid electron collection and reduced charge recombination. [27].

Fig. 3
figure 3

SEM image of ZnO film deposited by CBD (A–C)

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Figure 4(A–C) shows the SEM images of the ZnO doctor-blade films showing a granular morphology. Figure 4(D–F) show that as the number of cycles is increased, the deposition of CdS improves and shows good coverage of the ZnO surface. The atomic percentages of Zn, O, Cd, and S were confirmed using EDAX analysis (see Supporting Information Table S1).

Fig. 4
figure 4

SEM image of ZnO doctor-bladed (A–C) film and CdS/ ZnO film by SILAR method (D–F)

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SEM images of the CTS film at various magnifications are shown in Fig. 5G–I. From these images, it can be seen that the spongy and agglomerated nanoparticles are CTS films. Here, the agglomeration of the CTS nanoparticles resulted in the foremost homogeneous grain formation. The CTS film surface exhibited a uniform distribution of nanoparticles with a spherical morphology. Figure 6 shows the FESEM image of the CTS film, which has an average thickness of CTS film is 9.1 µm.

Fig. 5
figure 5

SEM image of CTS film deposited by SILAR (G–I)

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Fig. 6
figure 6

FE-SEM of CTS film

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Figure 7 shows the EDAX spectra of the CTS films after (25 SILAR cycles). EDAX confirmed the presence of Cu, Sn, and sulfur in CTS films. The figure shows the quantitative elemental composition of the CTS films, including copper (Cu), tin (Sn), and sulfur (S). Copper (Cu) has a weight percentage of 30.4%, an atomic percentage of 2.26%, and an atomic percentage of 1.4%. Tin (Sn) has a weight percentage of 13.4% and an atomic percentage of 1.4%. Sulfur (S) is the most abundant element, accounting for 58% of the total weight and 4.32% of the total atoms. Copper follows with a weight percentage of 30.4% and an atomic percentage of 2.26%, whereas tin accounts for 13.4% by weight and 1.4% by atoms. This analysis is crucial for understanding the stoichiometry and elemental distribution of CTS films and provides insights into their structural properties and potential applications in thin-film technologies, solar cells, sensors, and catalysis.

Fig. 7
figure 7

EDAX of SILAR deposited CTS film

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3.3 Optical properties

The optical properties of the CdS/ZnO films were measured using a JASCO UV–Vis spectrometer. Figure 8 shows the UV–Vis spectra of CdS/ZnO films. The obtained spectra clearly show absorption bands in the ultraviolet to visible regions. The unsensitized ZnO doctor-bladed film exhibited absorption in the ultraviolet region, and the absorption width was extended when CdS was loaded on the ZnO photoanode. As the SILAR cycles vary from 4 to 8, the UV–Vis spectra are broadened towards the maximum wavelength region, resulting in a maximum increase in absorbance magnitude. A strong absorption shoulder appeared in the 525–550 nm range for the CdS/ZnO photoanode, revealing the optical properties of the CdS quantum dot [28].

Fig. 8
figure 8

UV–Visible spectra of CdS/ZnO films

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Optical absorption analysis of the CTS thin films was conducted from 200 to 1100 nm, as shown in Fig. 9a. Figure 9b shows that optical absorption measurements can be used to determine the optical transition scenery and bandgap of the material. The bandgap energy of a material is a crucial parameter for determining its photovoltaic performance in solar-cell applications. The energy difference between the valence band and the conduction band of a material directly influences the absorption of light, the generation of electron–hole pairs, and the efficiency of charge separation and collection in a solar cell. In the context of photovoltaics, bandgap energy affects several aspects of solar-cell performance. It determines the wavelength of light that can be absorbed by the material, allowing photons with energy higher than the bandgap energy to excite electrons from the valence band to the conduction band, creating electron–hole pairs that contribute to the generation of electricity in a solar cell. A material with bandgap energy well matched to the solar spectrum can efficiently absorb sunlight and convert it into electrical energy. Effective charge separation is essential for preventing recombination and maximizing the photocurrent in a solar cell. A suitable bandgap energy ensures effective charge separation and collection, leading to higher photovoltaic efficiency. The bandgap energy also affects the open-circuit voltage (Voc) and short-circuit current (Jsc) of a solar cell, thereby affecting the quality of electrical contacts and charge extraction. The optical absorption of the CTS film shows a broad range, covering almost the entire visible region, making it suitable for industrial applications such as solar-cell window layers. The experimental results reveal a sharp absorption variation around 500 nm, close to the CTS thin-film effective bandgap, with high optical absorption coefficients and a narrower bandgap energy. The band gap of the CTS film was calculated using the Tauc plot method, and Tauc’s Eq. (1)

$$h\nu = A\left( {h\nu - E_{\text{g}} } \right)^n$$
(1)
Fig. 9
figure 9

A UV–Vis spectra of CTS film deposited by SILAR. B Tauc Plot of CTS film

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The constant A is related to the effective masses in the valence and conduction bands, and n is determined by the nature of the transition, with 1/2 for direct transitions and 2 for indirect transitions. The optical absorption of the CTS film shows a broad range, covering almost the entire visible region, making it suitable for industrial applications, such as solar-cell window layers. The experimental results reveal a sharp absorption variation around 500 nm, close to the CTS thin-film effective band gap, with high optical absorption coefficients and a narrower band gap energy. The high optical absorption coefficients of CTS thin films, with a band gap of 1.5 eV, are attributed to the optical absorption edge variable.

3.4 XPS analysis

Figure 10a shows the XPS spectra of the CTS film deposited by the SILAR method, and the core-level spectra of Cu 2p, Sn 3d, and S 2p are shown in Fig. 10b–d respectively. Binding energy peaks of Cu 2p 3/2, and Cu 2p ½ are appeared at 952.4 and 932.4 eV, respectively, which are in good agreement with the literature of Cu [29]. Figure 10c shows high-resolution spectra of Sn, Binding energy peaks of Sn appeared at 486 and 494.4 eV attributed to 3d 5/2 and 3d 3/2, respectively, which corresponds to Sn2+ [30]. Figure 10d shows core-level spectra of S 2p shows two peaks at 161.9 and 162.9 eV, which imply that S is in sulfide. The peak at 169.4 eV also indicates the existence of sulfur [31, 32].

Fig. 10
figure 10

a survey scan of CTS film deposited by SILAR method. b the core-level photoelectron spectra of Cu 2p. c the core-level photoelectron spectra of Sn 3d. d the core-level photoelectron spectra of S 2p

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3.5 JV characteristics

A schematic of the CdS/ZnO sandwiched cell with a CTS counter is shown in Fig. 11.

Fig. 11
figure 11

Schematic diagram of CdS/ZnO sandwiched cell with CTS counter

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The performance of the cells was measured using a 100 mw/cm2 xenon lamp with polysulfide as the electrolyte, the most preferable electrolyte in CdS-based solar cells [29], and CTS and C as counter electrodes. The JV curves of CdS/ZnO-based solar cells are shown in Fig. 12.

Fig. 12
figure 12

JV curve of CdS/ZnO with CTS as counter electrode and JV curve of CdS/ZnO with Carbon counter electrode

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Table 1 compares the performance of CdS/ZnO solar cells using different counter electrodes, CTS and carbon, in converting light energy into electrical power. The CTS counter electrode has a higher fill factor of 29.3%, indicating better charge extraction and reduced losses. The CTS cells had a higher open-circuit voltage (0.46 V, indicating efficient charge separation and voltage generation. The CTS cells also had a higher short-circuit current of 3.6 mA/cm2, indicating enhanced light absorption and charge carrier generation. This study examines the performance of CdS/ZnO solar cells with different counter electrodes, focusing on the bandgap dependency. The choice of counter-electrode material, such as CTS or carbon, affects the band alignment, charge carrier transport, and collection mechanisms. The CTS counter electrode achieved a higher fill factor of 29.3%, attributed to optimized band alignment, reduced recombination losses, and enhanced charge carrier extraction efficiency. It also showed superior efficiency of 0.49%, indicating effective utilization of bandgap energy for photon absorption, charge separation, and collection processes.

Table 1 Cell parameters of CdS/ZnO films with CTS and Carbon counter electrodes
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4 Conclusion

The CTS counter electrode was fabricated using a simple, low-cost SILAR method. Using the SILAR method, tetragonal CTS was obtained on an FTO substrate. The prepared CTS/FTO substrate was used as the counter electrode in ZnO/CdS-sandwiched solar cells. The performance of ZnO/CdS-based solar cells with a CTS counter was 0.49% this boost is mainly because of the optimization of the counter electrode. Compared to carbon counter electrodes, CTS counter electrodes are preferable for CdS/ZnO-based solar cells. This study highlights the importance of using CTS as the counter electrode in semiconductor-sensitized solar cells. This may provide an important breakthrough in counter electrodes for third-generation solar cells. This paper suggests future study directions for CTS counter-electrode fabrication, including optimizing the SILAR method parameters, exploring low-cost fabrication methods, evaluating long-term stability and durability in CdS/ZnO-based solar cells, and exploring the potential of incorporating CTS counter electrodes in third-generation solar cells for various photovoltaic applications. These recommendations aim to improve the efficiency of semiconductor-sensitized solar cells and explore the potential of CTS counter electrodes in photovoltaic applications.