Effects of metal ion concentration on electrodeposited CuZnSn film and its application in kesterite Cu₂ZnSnS₄ solar cells

Abstract

In this study, effects of concentrations of Cu(II), Zn(II) and Sn(II) ions in the electrolytic bath solution on the properties of electrochemically deposited CuZnSn (CZT) films were investigated. Study of the composition of a CZT film has shown that the metallic content (relative atomic ratio) in the film increased linearly with increase in the metal ion concentration. It is the first time that the relationship of the compositions of the alloy phases in the co-electrodeposited CZT film with the concentration of metal ions has been revealed. The results have confirmed that the formation and content of Cu₆Sn₅ and Cu₅Zn₈ alloy phases in the film were directly controlled by the concentration of Cu(II). SEM measurements have shown that Sn(II) has significant impact on film morphology, which became more porous as a result of the larger nucleation size of tin. The changes in the surface properties of the films was also confirmed by chronoamperometry characteristic (i–t) deposition curves. By optimization of metal ion concentrations in the electrolyte solution, a copper-poor and zinc-rich kesterite Cu₂ZnSnS₄ (CZTS) film was synthesized by the sulfurization of the deposited CZT film. The solar cell with the CZTS film showed an energy conversion efficiency of 2.15% under the illumination intensity of 100 mW cm⁻².

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1. Introduction

Electrodeposition has been extensively used to fabricate metal/alloy films^1–3 and materials^4,5 with specific properties for applications due to its advantages of ease, low cost, environmental friendliness and viability for large scale production. Ternary alloys based on CuZnSn (CZT) were mainly employed in industries for decorative and protective purposes in early times.⁶ During recent years, they have been widely used as an essential precursor to synthesize new sustainable photovoltaic materials such as Cu₂ZnSnS₄ (CZTS)^7–10 and Cu₂ZnSnSe₄ (CZTSe).^11–14 Solar cells using electrodeposited Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSnSe₄ (CZTSe) with efficiencies up to 8.0% have been reported.^15–18

Generally speaking, the electrodeposition of CZT metallic film can be achieved by the deposition of Cu/Zn/Sn stacked elemental layers using separate electrolyte solutions containing each metal ion via a multi-step method,^18–22 or by co-deposition using a single solution containing all the metal ions in one step.^23–27 The method based on co-electrodeposition has advantages over the stacked elemental layers method because of its simpler procedure, which is more favorable for high-throughput production. However, compared to the stacked elemental layers approach, it is more challenging to control the co-electrodeposition process of CZT film due to the complexity of the electrolyte system and the different reduction potentials of the metal ions. The simultaneous presence of copper, tin and zinc metal ions with the complexing agent in the electrolyte solution can generate various complex species that may have different solubilities and can lead to precipitation in the electrolyte solution.^28,29 Moreover, in the deposition process, the CZT film deposited from copper, zinc and tin ions may contain mixture of metals and binary alloy phases such as ƞ-Cu_6.26Sn₅, Sn and γ-CuZn₅ (ref. 30) and Zn, Sn, Cu₆Sn₅ and CuZn₂.⁸ It has been reported that the alloy phases, such as Cu₆Sn₅ and Cu₅Zn₈, in the CZT film have significant effects on the morphology of the CZTS film.¹⁰

Because a high quality CZT precursor film is one of the prerequisites to synthesize CZTS film with desired optical and electronic properties for application in solar cells, it is important to develop an effective electrodeposition route for the fabrication of a CZT film. According to the generally accepted electrodeposition principles,⁶ two types of variables can influence the properties of the deposited film: (i) electrolyte solution variables such as the concentration of metal ions, the nature of complexing agent, and the pH of the electrolyte solution and (ii) operational variables such as the electrodeposited current density or potential, deposition temperature and stirring of solution. A complexing agent is an important component in an electroplating solution because it can narrow the gap between the reduction potentials of different metals ions and enhance the solubility of metal ions.²⁸ It can also reduce the grain size of the deposits and improve the homogeneity and roughness of the deposited film.⁶ Different complexing agents, such as Copper Glo,^31,32 EDTA,³³ citric acid,²⁸ and tri-sodium citrate,^{8,12–15,17,26,27,34,35} have been used in aqueous electrolyte solutions for electrodeposition of CZT films. Among these, tri-sodium citrate is the most widely used one because of its effective complexing abilities with Cu(II), Zn(II) and Sn(II) ions and its environmentally friendly nature.^28,29 Recently, effects of the concentration of tri-sodium citrate on the electrodeposition process of CZT film and on the composition of the deposits were investigated by M. Slupska et al.,²⁹ while the effects of the applied potentials on the composition of CZT films were studied by C. Gougaud et al.²⁸ However, reports on the influence of the concentration of metal ions on the electrodeposited CZT film using tri-sodium citrate as a complexing agent are rare. The concentration of metal ions could have great impact on the CZT film formation through influencing the mass transfer process. Therefore, it is important both fundamentally and practically to distinguish the effects of metal ion concentration on the properties of electrodeposited CZT film to obtain an in-depth understanding of the formation mechanism of CZT film.

Herein, we have investigated the effect of the concentration of each metal ion on the composition, morphology, and crystal structure of the electrodeposited CZT film using static electrolyte solutions containing a constant content of tri-sodium citrate as the complexing agent. For the first time, the dependence of the formation of alloys in the co-electrodeposited CZT film on the concentrations of metal ions in the electrolyte was determined quantitatively. A kesterite CZTS film with a desired copper-poor and zinc-rich composition was synthesized by controlling the concentrations of metal ions in the electrolyte solution. The corresponding CZTS solar cell showed an energy conversion efficiency of 2.15% under 100 mA cm⁻². The research provides new insights into the preparation of high quality CZT and CZTS films for solar cell applications through the control of electrodeposition procedure.

2. Experimental

2.1 Chemicals

Molybdenum-coated soda lime glass (Mo/SLG) substrates were ultrasonically cleaned by acetone, ethanol and Milli-Q water for 5 min and dried under nitrogen gas flow. All the chemicals used were analytical reagents and supplied by Alfa Aesar unless otherwise stated.

2.2 Preparation of CZT and CZTS films

The electrodeposition of CZT films was implemented with a three-electrode configuration consisting of an Mo/SLG substrate working electrode, a Pt wire counter electrode and an Ag/AgCl/saturated KCl reference electrode using an electrochemical workstation (VSP-300 Bio-logic). To investigate the effects of the concentration of each metal ion on the property of the deposited CZT film, freshly prepared aqueous solutions containing different concentrations of metal ions were used as electrolytic baths. The detailed compositions and the pH values of each solution are shown in Table 1. The solutions containing different concentrations of Cu(II) are named as A series (A1B1C1–A5). Similarly, the solutions with different concentrations of Zn(II) are labelled as B series (A1B1C1–B5) and those with different concentrations of Sn(II) are labelled as C series (A1B1C1–C5). The starting electrolyte solution is labelled as A1B1C1, which contains 10.0 mM of each metal ion. For the electrodeposition of copper, zinc and tin individual metals, the electrolyte solution consisted of 10.0 mM of each metal sulfate and 200 mM of Na₃C₆H₅O₇·2H₂O. The pH value of each electrolyte solution was adjusted to 6.2 using sulfuric acid. Due to difficulty in the direct deposition of Zn on Mo substrate, the substrate for the electrodeposition of the metal was based on a thin layer of CZT film made by the electrodeposition of the solution A1B1C1 for 10 minutes. The reduction potential of each metal ion was determined by linear voltammetry [Fig. S1, ESI†]. A negative constant potential of −1.25 V (vs. Ag/AgCl/saturated KCl) was applied to deposit CZT films for 10 min, as well as individual metal films, at a temperature of 26 ± 1 °C. Mo/SLG substrates with an active area of 0.2 cm² (circular shape) were used in the electrodeposition for investigating the effects of metal ion concentration on CZT films, as well as for the electrodeposition of individual metals. A larger area of CZT film (2.2 cm × 2.2 cm) with a thickness of ∼800 nm was prepared for the fabrication of a thin film solar cell. The CZTS film was synthesized by sulfurizing the electrodeposited CZT film at 550 °C for 30 min in a rapid thermal processing (RTP) furnace (OTF-1200X, MTI) filled with argon (12.0 Torr). The furnace chamber was heated from ambient temperature to 550 °C in 1 minute.

Table 1 Chemical composition and pH of electrolyte solutions containing different concentrations of metal ions

Chemical composition of electrolyte solutions
Series number	CuSO4 (mM)	ZnSO4 (mM)	SnSO4 (mM)	Na3C6H5O7 (mM)	pH
A1B1C1	10.0	10.0	10.0	200	6.2
Different concentrations of copper ion
A2	12.5	10.0	10.0	200	6.2
A3	15.0	10.0	10.0	200	6.1
A4	17.5	10.0	10.0	200	6.1
A5	20.0	10.0	10.0	200	6.1
Different concentrations of zinc ion
B2	10.0	12.5	10.0	200	6.2
B3	10.0	15.0	10.0	200	6.2
B4	10.0	17.5	10.0	200	6.2
B5	10.0	20.0	10.0	200	6.2
Different concentrations of tin ion
C2	10.0	10.0	12.5	200	6.2
C3	10.0	10.0	15.0	200	6.1
C4	10.0	10.0	17.5	200	6.1
C5	10.0	10.0	20.0	200	6.1

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2.3 Fabrication of CZTS solar cell

The CZTS thin film solar cell adopted an architecture of Mo/CZTS/CdS/i-ZnO/ZnO:Al/Ni:Al with an active area of 0.34 cm². The cell was made by the following procedure. A 60 nm CdS buffer layer was deposited above the CZTS film using a chemical bath solution containing 1.0 mM Cd(CH₃COO)₂, 6.0 mM CH₃COONH₃, 5.0 mM thiourea and 25% ammonia water (5 ml in 250 ml bath aqueous solution) by placing the film in the solution for 10 min at 82 ± 2 °C. This was followed by the deposition of a window layer consisting of 50 nm i-ZnO and 500 nm ZnO:Al layers by sputtering. The cell was completed by the deposition of an Ni/Al grid contact by electron beam evaporation.³⁶

2.4 Characterization

The composition of CZT and CZTS films were determined by energy dispersive X-ray spectroscopy (EDS, JEOL 7001F) at an acceleration voltage of 20.0 kV. The relative error of the EDS detector was approximately ±2.0% for Cu, ±2.0% for Zn and ±3.0% for Sn. The surface and cross-sectional morphology of CZT and CZTS films were analyzed by field emission scanning electron microscopy (FESEM, JEOL 7001F). The X-ray diffraction patterns of CZT and CZTS films were collected using a PANalytical X'Pert Pro diffractometer operating at 40 kV using Cu-Kα radiation in Bragg–Brentano geometry. The relative molar percentages of the crystal phases in the CZT films were determined by refining the XRD patterns. Rietveld refinement against the collected XRD patterns was performed using Total Pattern Analysis Solutions (Topas V5, Bruker). Cu-Kα5_Berger emission profile was used to describe the incident X-rays. The XRD data in the range of 2θ = 36–42° and 71–76° were excluded in the refinement because these regions contained intense XRD peaks due to the Mo/SLG substrate. A Raman spectrometer (Renishaw inVia Raman microscope) was used to obtain the Raman spectrum of the CZTS thin film. Laser excitation wavelengths at 532 nm and 785 nm were used in the Raman measurements. The J–V characteristic performance of the CZTS solar cell was measured using a solar simulator with AM 1.5 spectrum and illumination intensity of 100 mW cm⁻².

3. Results and discussion

3.1 Effects of concentration of metal ions on the compositions of CZT films

The dependence of the content of electrodeposited metals (Cu, Zn and Sn) in the film on the concentration of corresponding metal ions is shown in Fig. 1a–c. The compositional result is an average of three samples prepared under the same conditions. For the electrolyte solution with a concentration of 10.0 mM for all the three metal salts (A1B1C1), a CZT film with a composition (molar ratio) of Cu : Zn : Sn = 1.48 : 1.37 : 1.00 was obtained. The deviation of the composition from the ratio of the metal ion concentration (1 : 1 : 1) in the solution suggests that the three metals have different deposition rates. In particular, the deposition rates of the metal ions should be in the order of Cu(II) > Zn(II) > Sn(II). It has been reported that the metal ions can interact with the complexing agent to form non-electroactive metal citrate complexes²⁹ such as ZnH₂Cit₂ and Zn₂Cit₂⁴⁻ or precipitation components such as Cu₃SO₄(OH)₄, Cu₃SO₄(OH)₆, Zn₂Cit, Zn₄(OH)₆SO₄, and Sn(OH)₂, which could decrease the effective deposition of metal ions on the substrate. Nevertheless, it is observed that the content of specific metal in the CZT films increases linearly with the increase of the corresponding metal ion concentration in the electrolyte solution, while the relative content of the other two metals decrease linearly. It is also observed in Fig. 1a–c that the slopes of the plots for the two metals with constant concentration (10.0 mM) in the electrolyte solution are different. This should be attributed to the different deposition rates of the metals and the fact that the electrodeposition of CZT film involves the formation of alloys including Cu₆Sn₅ and Cu₅Zn₈ phases instead of pure metals. Therefore, a change of each metal ion concentration would change the surface property of the CZT film, which in turn affects the deposition rate of each metal, as shown in the following section.

Fig. 1 Dependence of metal content in the electrodeposited CZT films as a function of different concentrations of metal ions: (a) Cu(II), (b) Zn(II), and (c) Sn(II) in electrolyte solutions.

3.2 Effects of concentration of metal ions on material phases in CZT films

X-ray diffraction was employed to characterize the material phases in the CZT film (Fig. 2a–c). It is found that alloys of Cu₆Sn₅ and Cu₅Zn₈ exist in all the films together with metallic tin. The relative contents of the material phases of Cu₆Sn₅, Cu₅Zn₈ and Sn were determined by Rietveld refinement and are correlated with the concentration of each metal ion, as shown in Fig. 2d–f. In the CZT film deposited from the electrolytic solution A1B1C1, the film is dominated by a metallic Sn phase, which accounts for 59.54%, while the alloy of Cu₆Sn₅ only accounts for 11.33% of the film. However, as the concentration of Cu(II) in the solution increases, the contents of both Cu₆Sn₅ and Cu₅Zn₈ phases increase, while the content of the Sn phase decreases rapidly. Sn phase is not detected by XRD for the film deposited with 17.5 mM of Cu(II) in the electrolyte solution. This indicates that the increase of the Cu(II) concentration favors the formation of Cu₆Sn₅ and Cu₅Zn₈ alloys and suppresses Sn in the deposited film. However, with the increase of the concentration of Zn(II) in the electrolyte bath, the content of Cu₅Zn₈ phase remain largely constant and the Sn phase slightly increases, while the Cu₆Sn₅ phase decreases until there is no Cu₆Sn₅ phase when the Zn(II) concentration reaches 15.0 mM, as illustrated in Fig. 2e. Furthermore, the increase in Sn(II) concentration is found to facilitate the formation of the Sn phase while hindering the formation of the Cu₅Zn₈ phase; however, it has almost no effect on the Cu₆Sn₅ phase, as shown in Fig. 2f. The results suggest that the formation of both Cu₅Zn₈ and Cu₆Sn₅ alloy phases is determined by the concentration of Cu(II) instead of that of Zn(II) and Sn(II).

Fig. 2 XRD patterns (a–c) and the phase compositions (d–f) of the electrodeposited CZT films from electrolyte solutions containing different concentrations of Cu(II) (a and d), Zn(II) (b and e), and Sn(II) (c and f). The contents of each material phase were calculated by Rietveld refinement of the corresponding XRD patterns.

3.3 Effects of concentration of metal ions on the morphology of CZT films

The evolution of the morphology of the CZT films deposited from different concentrations of metal ions is illustrated in Fig. 3. It is found that the morphology of the CZT films deposited from solutions (A1B1C1–A5) containing different concentrations of Cu(II) is very similar, indicating the negligible effect of copper content on the CZT film morphology. The films are mainly composed of ∼200 nm crystal grains formed from nanoscale small particles. In contrast, the morphology of the CZT film changes slightly with the increase of Zn(II) concentration. As shown in Fig. 3A1B1C1–B5, the increasing content of zinc in the film leads to more defined spaces between crystal grains and a rougher film surface. Moreover, the crystal sizes of the grains increase to more than 200 nm (B4). Bigger grains, with size around ∼300 nm, are found in B5. In addition, the effects of Sn(II) on the CZT film morphology are even more pronounced. When the concentration of Sn(II) increases, the film becomes porous and rougher, as illustrated in Fig. 3A1B1C1–C5. The agglomeration of particles is also observed at higher concentrations of Sn(II), as shown in Fig. 3C3–C5. The different morphological behavior induced by the metal ions should be related to the different nucleation mechanism associated with Cu, Zn and Sn metals in the electrodeposited film. To clarify this, the morphology of the individually electrodeposited films of copper, tin and zinc on a CZT/Mo/SLG substrate was investigated. Mo/SLG was not used as the substrate for the deposition because it was found that Zn could not be electrodeposited on the surface of Mo/SLG under the reduction potential used. The results show that the average particle size of the film with copper is around 40 nm (Fig. 4a), whereas that of zinc varies from several nanometers to a few hundred nanometers (Fig. 4b), while the film with tin is composed of micrometer-sized islands composed of crystals with a size of few hundred nanometers (Fig. 4c). This confirms the larger nucleation size of tin compared to copper and zinc. The different morphologies of the films, as presented in Fig. 4, confirm that the morphologies of Cu₆Sn₅ and Cu₅Zn₈ alloys should be indirectly affected by the nucleation size of each metal, namely, copper, zinc and tin. Obviously, tin has the most significant influence on the morphology of the CZT film. The increase of tin content (Fig. 1c) made the film rougher and more porous, as seen in Fig. 3A1B1C1–C5. The XRD pattern presented in Fig. 2c shows that the tin crystal grows along the preferred direction of (020). This is also supported by the plate-like crystals formed in the film, as marked by a blue circle in the inset plot of Fig. 3C5.

Fig. 3 SEM images of CZT films electrodeposited from electrolyte solutions containing different concentrations of metal ions: Cu(II) (A series), Zn(II) (B series), and Sn(II) (C series).

Fig. 4 SEM images of individually electrodeposited metal: (a) Cu, (b) Zn and (c) Sn, on CZT/Mo/SLG substrate.

3.4 Chronoamperometry characteristic (i–t) curve

The chronoamperometry characteristic (i–t) curve for the electrochemical deposition of CZT films can also indicate changes in the interface between the deposited metal film and the electrolyte solution during the deposition process.³⁷ The i–t curves of the CZT films deposited using different electrolyte solutions are shown in Fig. 5a–c. As can be seen, the cathodic current density decreases rapidly at the initial stage (about the first 50 s for A1B1C1) of the deposition. This is due to the decrease in the concentrations of metal ions in the vicinity of electrode surface caused by electrochemical reduction; subsequently, the current becomes stable. The i–t curve of each metal ion concentration shows different features. In the case of varying the concentration of Cu(II), all the i–t curves show a similar trend with the cathodic current becoming constant after 50 s, as shown in Fig. 5a, suggesting a constant surface area of the films during the deposition. This is in agreement with the similar morphologies of the films with different concentrations of Cu(II), as shown in Fig. 3A1B1C1–A5. However, for the deposition with different concentrations of Zn(II) in the electrolyte solution, the cathodic current at the plateau region increases with the increase of Zn(II) concentration, as shown in Fig. 5b. This indicates the enhancement of the surface area of the deposited films, which is also consistent with the more rough surface morphology of the film as shown in Fig. 3A1B1C1–B5. It should be noted that significant peaks (Fig. 5b) are observed in the i–t curves at the deposition time between 30 s and 100 s for different Zn(II) concentrations, and the peak intensity increases with the increasing concentration of Zn(II). This is attributed to the fact that the reduction of Zn(II) does not occur until sufficient CuSn are present on the surface of the Mo. This appears to occur at 30–60 s and is speculated to be due to a lack of reduction ability of Zn(II) on Mo substrate at this voltage [Fig. S2, ESI†]. Remarkably, with increase in Sn(II) concentration, the cathodic current at the plateau region increases rapidly and cannot reach a steady value at all, as illustrated in Fig. 5c. This is ascribed to the enhanced surface area of the films as a result of the formation of porous structured films, as shown in Fig. 3A1B1C1–C5. Nevertheless, a common trend of increasing cathodic currents in the plateau region with increasing concentration of each metal ion has been observed in all the electrolyte solutions (Fig. 5a–c). This suggests that the deposition current is dependent on the concentration of each metal ion.

Fig. 5 Comparison of the i–t curves for the electrodeposition of CZT films with different concentrations of (a) Cu(II), (b) Zn(II) and (c) Sn(II).

3.5 Characterization of CZTS film

A CZTS film with copper-poor (Cu/(Zn + Sn) ≈ 0.8) and zinc-rich (Zn/Sn ≈ 1.2) composition has been reported to benefit the performance of solar cells.^38,39 Therefore, it is important to control the contents of copper, zinc and tin in the CZT precursor film to fabricate CZTS films with desired properties. As shown above, this can be achieved by finely tuning the concentration of the metal ions in the electrolyte solution. By optimization of the concentrations of the metal ions, a CZT film with composition (atomic ratio) of Cu/(Zn + Sn) = 0.69 and Zn/Sn = 1.02 was deposited from an aqueous electrolyte solution containing 13.0 mM CuSO₄·5H₂O, 12.0 mM ZnSO₄·7H₂O and 10.0 mM SnSO₄ with 200 mM Na₃C₆H₅O₇·2H₂O. The thickness of the deposited CZT film is 0.83 μm according to the measurement performed by the cross-sectional SEM image (Fig. 6a). The film is uniformly composed of spherical crystallites with a size of 200 nm (Fig. 6b) and is well adhered to the Mo/SLG substrate. The CZT film was transformed into CZTS through a high temperature sulfurization process at 550 °C for 30 min. The CZTS thin film had the composition (atomic ratio) of Cu/(Zn + Sn) = 0.74, Zn/Sn = 1.33 and S/(Cu + Zn + Sn) = 1.05 based on EDS measurement, which falls in the desired composition range of CZTS films for high performance solar cells. It is found that the ratios of Cu/(Zn + Sn) and Zn/Sn in the CZTS film are higher than the ratios in the corresponding CZT film. This is attributed to the loss of tin during the high temperature sulfurization process.^40,41 The thickness of the CZTS film is 1.78 μm and large polycrystalline grains with a size of 0.3–1.0 μm can be seen in the compact CZTS film, as shown in Fig. 6c and d.

Fig. 6 SEM cross-sectional and surface images of optimal CZT film (a and b, respectively) and corresponding CZTS film (c and d, respectively).

Moreover, the crystal structure of the CZTS film adopts a kesterite phase (JCPDS 01-075-4122), as confirmed by the XRD pattern shown in Fig. 7a. Nevertheless, XRD alone cannot rule out the existence of secondary phases, such as Cu₂SnS₃ and cubic ZnS, in the film because these materials have XRD patterns similar to CZTS.⁴² Therefore, Raman spectroscopy was used to distinguish these potential impurities from CZTS. Herein, both 532 nm and 785 nm lasers were applied as excitation sources in the Raman measurement. The Raman spectra in Fig. 7b show that peaks due to Cu₂SnS₃ are not observed. However, cubic ZnS phase may exist in the film because the characteristic scattering peak of ZnS at around 350 cm⁻¹ is observed in the Raman spectrum with 785 nm laser excitation, although the intensity of the peak (located at 353 cm⁻¹) is considerably weak.⁴³ The other peaks in both spectra can be assigned to the Raman scattering of kesterite CZTS that has been reported previously.^43,44 Compared to the spectrum measured by 532 nm laser excitation, more peaks were detected by the 785 nm laser. This is attributed to the effect of vibration resonance because the 785 nm excitation is close to the bandgap (1.5 eV) of CZTS.

Fig. 7 (a) XRD pattern and (b) Raman spectra of the optimal CZTS film.

3.6 Performance of CZTS solar cell

The performance of the thin film solar cell using the CZTS film under the illumination of 100 mW cm⁻² (AM 1.5) is shown in Fig. 8. The solar cell shows an energy conversion efficiency of 2.15% with J_sc = 11.16 mA cm⁻², V_oc = 0.47 V and ff = 0.47. All these characteristic parameters are lower than those reported (η = 3.74%, J_sc = 13.4 mA cm⁻², V_oc = 0.595 and ff = 0.47) for the highly efficient solar cell made by the co-electrodeposition method.²⁴ Jiang et al.¹⁸ have shown that preheating treatment can significantly improve the performance of solar cells made from electrodeposited stacked metal layer precursor films. Similar effects would be expected for the film made by co-electrodeposition. Preheating treatment of the CZT film was employed in the study of the highly efficient co-electrodeposited CZTS solar cells²⁴ before the CZT film was sulfurized to form CZTS, while no such procedure was used in our study. Thus, the lower performance of the solar cell in our study could be related to the different thermal treatment used in the film fabrication. The series resistance (R_s = 22.69 Ω cm²) and shunt-resistance (R_sh = 290.13 Ω cm²) of the solar cell are shown in Fig. 8. It is well known that a highly efficient solar cell requires low R_s and high R_sh. The relatively high R_s value of 22.69 Ω cm² in this study compared to that reported for the highly efficient solar cell¹⁴ (R_s = 4.10 Ω cm² and η = 7.3%) should be one of the factors contributing to the lower ff in the present study. The higher R_s is probably a result of the voids and small-sized crystals formed in the bottom layer of the CZTS film (Fig. 6c). These voids may also work as recombination centers to cause lower J_sc and V_oc, and further decrease the conversion efficiency of the solar cell.

Fig. 8 J–V plot of the CZTS solar cell.

4. Conclusions

The effects of the concentrations of Cu(II), Zn(II) and Sn(II) ions in the electrolyte solution on the composition, material phases and morphologies of the electrodeposited CZT films were systematically studied in this work. It has been found that the content of metal in the CZT films was linearly dependent on the concentration of the corresponding metal ion in the electrolyte solution. Moreover, it has been found that the increase of Cu(II) concentration in the solution facilitated the formation of Cu₆Sn₅ and Cu₅Zn₈ alloy phases. However, the increase in Zn(II) and Sn(II) concentration had no influence on the formation of these alloys. The investigation of the film by SEM showed that Cu(II) had no effect on the CZT film morphology, while Zn(II) increased both crystal size and surface roughness of the film. Sn(II) showed the most dramatic effect on the CZT film morphology with the formation of a porous film. The different morphologies with different metals were attributed to the different electrochemical nucleation sizes of the metals. The changes in the surface properties of the CZT film with different content of metals were also confirmed by chronoamperometry characteristic (i–t) deposition curves. Through tailoring concentrations of the metal precursor salts in the electrolyte solution, a copper-poor and zinc-rich CZTS film with a kesterite structure was synthesized and the corresponding solar cell exhibited an energy conversion efficiency of 2.15% under the illumination intensity of 100 mW cm⁻².

Acknowledgements

T. Hreid acknowledges the PhD scholarship program of Queensland University of Technology. H. W. acknowledges the financial support from Australian Research Council (ARC) Future Fellowship (grant No. FT120100674).

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Footnote

† Electronic supplementary information (ESI) available: Supplementary data associated with this article can be found with Fig. S1 and S2, as mentioned in the main text. See DOI: 10.1039/c5ra09966h

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