Facile Growth of Cu2ZnSnS4 Thin-Film by One-Step Pulsed Hybrid Electrophoretic and Electroplating Deposition

The use of costly and rare metals such as indium and gallium in Cu(In,Ga)Se2 (CIGS) based solar cells has motivated research into the use of Cu2ZnSnS4 (CZTS) as a suitable replacement due to its non-toxicity, abundance of compositional elements and excellent optical properties (1.5 eV direct band gap and absorption coefficient of ~104 cm−1). In this study, we demonstrate a one-step pulsed hybrid electrodeposition method (PHED), which combines electrophoretic and electroplating deposition to deposit uniform CZTS thin-films. Through careful analysis and optimization, we are able to demonstrate CZTS solar cells with the VOC, JSC, FF and η of 350 mV, 3.90 mA/cm2, 0.43 and 0.59%, respectively.

the time consuming pre-alloying/soft annealing inter-layer diffusion and interlayer rinsing steps associated with SEL approaches. Likewise, in comparison to MAE and QED, the formation of the Cu 2−x S nanoparticles reduces the complexity of the electrolyte chemistry, offering better bath stability and a potentially faster overall deposition process. Finally, this method avoids the time consuming and complex process of CZTS nanoparticle preparation required when using an exclusively EPD deposition process 7,8 . In this work, we use a pulsed hybrid electrodeposition (PHED) 9,10 method as opposed to a more conventional potentiostatic methods, which enables the formation of more uniform CZTS thin-films. A final sulfurization step is performed on the films by placing then in a sulfur vapor atmosphere for 1 hr. The materials characteristics and device performance were investigated in detail.

Results and Discussion
Our PHED deposition of CZTS thin-films was carried out using a three-electrode cell, consisting of an Mo coated soda lime glass substrate as the working electrode (WE), a graphite bar as the counter electrode (CE) and a silver/silver chloride (Ag/AgCl) electrode as the reference electrode (RE). Figure 1(a) illustrates a schematic representation of the system with an electrolyte solution of deionized (DI) water containing 0.0022 M copper sulfate (CuSO 4 ), 0.011 M zinc sulfate (ZnSO 4 ), 0.005 M tin chloride (SnCl 2 ), 0.08 M sodium thiosulfate (Na 2 S 2 O 3 ) and 0.05 M tri-sodium citrate (Na 3 C 6 H 5 O 7 ) used as a complexing agent for the Sn 2+ ions 11 , respectively. The pH value of electrolyte was 5.81 with no further pH adjustment applied. To deposit the CZTS films, the pulsed electrodeposition method was applied using a 10% duty cycle (on-time duration of 0.25 sec at − 3 V, off-time duration of 2.25 sec at 0 V) as shown in Fig. 1(b). The set potential of 0 V during the off-time is the potential difference between applied voltage and open circuit voltage, which is the steady state without any disturbance of applied potential. During the off-time, the relatively unstable as-deposited CZTS film will re-dissolve into electrolyte again, which will happen on the top part of film. This is beneficial to the film thickness uniformity. So we can observe that the current is still flowing to the positive direction during the off-time. The off-time allows the distribution of the precursor species within the electrolyte to return to a homogeneous state, thus allowing a higher deposition rate during the on-time when compared to potentiostatic deposition methods 12 . Our PHED method integrates a two-stage chemical process, enabling the use of both electroplating and electrophoretic deposition in a single 'one pot' deposition method. In this process, we first dissolve our metal ions and Sn complexing agent (Na 3 C 6 H 5 O 7 ) in DI water as electrolyte. Shortly afterwards, we add our Na 2 S 2 O 3 , initiating the formation of a Cu 2−x S nanoparticle precipitate, in the electrolyte.
The key to preparing a suitable electrolyte lies in the sequence and timing, in which the various precursors are added. The order of addition from first to last is Na 3 C 6 H 5 O 7 → SnCl 2 → ZnSO 4 → CuSO 4 → Na 2 S 2 O 3 , respectively. There is a specific duration needed before the addition of Na 2 S 2 O 3 so the complexation reaction between the Na 3 C 6 H 5 O 7 and Sn 2+ can be controlled since the reaction duration will affect the elemental composition of the electrodeposited CZTS film. Figure 1(c) shows the results of the compositional analysis obtained by energy-dispersive X-ray spectroscopy (EDS), and we observe that when the reaction duration is longer, the more complete complexation of the Sn in the electrolyte will hinder the reduction of Sn during the final electrodeposition. Therefore, the relative Sn and Zn percentages decrease while the Cu and S percentages increase. Na 2 S 2 O 3 is a common sulfur source used in CZTS electrodeposition, and here it is also used as a reducing agent for the Cu. After the oxidation-reduction reaction between copper sulfate and sodium thiosulfate, the Cu 2+ ions are reduced to + Cu 2 2 ions and the Cu 2−x S then precipitates 13 .
First, Cu 2+ will be reduced by thiosulfate by the reaction Then, S 2− will form by the reactions Finally, S 2− will react with Cu and precipitate insoluble copper sulfide by Thus, the color of the electrolyte solution changes from blue to a rusty orange color [ Fig  composed of Cu (~48.7 at %) and S (~42.5 at %) as observed by EDS (details in Table 1). Once the electrolyte is prepared, the Sn, Zn and Cu 2−x S were deposited onto the Mo coated SLG substrate using our PHED process. Following 400 cycles of PHED (16:40 min), we obtain a 1 μ m thick CZTS thin-film on our Mo coated (400 nm) soda lime glass (SLG) substrate. Top and cross sectional scanning electron microscopy (SEM) images of the CZTS film morphology before sulfurization are shown in Fig. 2 Schematic illustrations of the complexation process for the Sn ions and formation of the Cu 2−x S nanoparticle precipitate are shown in Fig. 3(a). In order to form a kesterite absorber suitable for solar cell applications, the as-deposited CZTS thin-films must undergo a final sulfurization process whereby the films, along with S and SnS pellets, are placed together in a graphite box, and then placed into a furnace under a H 2 (20 sccm) and N 2 (180 sccm) atmosphere at a pressure of 60 torr for 1 hr. The films are heated according to the temperature profile illustrated in Fig. 3(b) across a range of temperatures (450 °C to 600 °C). The high temperature sulfurization process is applied via the following reactions 14 : A schematic of the entire film deposition and sulfurization process is shown in Fig. 3(c). During the sulfurization process, the chalcogenides of Cu 2 S, ZnS and SnS will form first, followed by a further reaction with the excess S vapor to form CZTS. Note that the excess S vapor present during the elevated temperature process facilitates the formation of CZTS, while the presence of the SnS pellets helps prevent the loss of Sn 15,16 . However, if care is not taken, the equilibrium S vapor pressure will rise as the sulfurization temperature increases 14 , and insufficient S vapor can result in the thermal decomposition of CZTS into secondary phases 17 .
To shed light on the crystallinity of the CZTS, the XRD spectra for each of the films were acquired [ Fig. 4  Analysis of the FWHM of the CZTS (112) plane shows that the crystallinity is initially enhanced with increasing sulfurization temperature as summarized in Table 2. Notably, little enhancement is observed when the sulfurization temperature is increased beyond 500 °C. With only a 1.5% decrease of the (112) XRD FWHM observed as we increase the sulfurization temperature from 500 to 600 °C. The same films with different sulfurization temperatures from 450-600 °C were also examined by Raman spectroscopy as shown in Fig. 4(b) with distinct Raman-active vibration modes at 287, 338 cm −1 and B Raman-active vibration modes at 374 cm −1 for CZTS found for all sulfurization temperatures 18 . At each sulfurization temperature point, we can clearly observe the expected kesterite related CZTS Raman modes 19 . No significant change was observed in any of the peak intensities or positions with no Cu 2 S, ZnS or SnS related peaks being observed at any temperature, suggesting that sulfurization is being performed within an appropriate process window. Note that various temperatures were investigated with the best performing devices obtained following a 550 °C sulfurization process. The elemental compositions of these 'best performing' films pre and post-sulfurization are listed in Table 3 (obtained by EDS analysis), with a post-sulfurization elemental compositions of ~27.18 at %, ~14.44 at %, ~12.13 at %, and ~46.25 at % for Cu, Zn, Sn, and S elements, respectively. In general, it has been observed that moderately Cu-poor and Zn-rich films (such as observed in our pre-sulfurized CZTS) lead to enhanced solar cell performance 20 . In our case, a slight elemental loss was observed during the high temperature sulfurization process, leading to a slight adjustment to our     Fig. 5(a-d) reveal similar surface morphologies for all of the CZTS thin-films sulfurized at temperatures from 450 to 600 °C. However, the optical properties of the CZTS thin films measured using UV-Vis spectroscopy vary significantly. The calculated band gap 21 from the plots of (α hυ ) 2 versus incident photon energies in Fig. 5(e) reveal CZTS films sulfurized at 550 °C show the best optical properties in terms of suitability for kesterite solar cell fabrication, with a significantly wider band gap of 1.48 eV 4 . The films sulfurized at relatively lower temperatures of 450 °C and 500 °C reveal smaller band gaps 22 , this may be due to the non-complete formation of kesterite CZTS and the secondary phase formation of Cu 2 SnS 3 16,23 . For the film sulfurized at 600 °C, its reduced band gap is likely due to thermal decomposition of the CZTS as a result of the S vapor being below the equilibrium pressure required to form CZTS 24 .

SEM images illustrated in
To complete the fabrication of the CZTS solar cell devices, we deposited a cadmium sulfide (CdS) buffer layer followed by intrinsic zinc oxide (i-ZnO), an indium doped tin oxide (ITO) layer and a final aluminum electrode (details in the experimental section). The detailed device configuration is shown in inset of Fig. 6 and the corresponding J-V characteristics of CZTS solar cells measured under air mass 1.5 illumination are shown in Fig. 6. The extracted performance data for the complete set of CZTS films is shown in Table 4. Our results show relatively high R S and low R SH , resulting in poor current density and open circuit voltage and thereby degradation in overall cell performance. Importantly, the poorer performance of the cells fabricated utilizing the 450 °C, 500 °C and 600 °C  Table 3. Elemental composition of PHED CZTS thin-films before and after sulfurization at an optimized temperature of 550 °C. sulfurization processes can be attributed to the poor CZTS absorber quality as determined from a combination of the optical band gap and XRD measurements. Clearly, the cells prepared using a 550 °C sulfurization process exhibit markedly higher performance with a V OC , J SC , FF and η of 350 mV, 3.90 mA/cm 2 , 0.43 and 0.59%, respectively. The significantly higher R SH observed in these devices is one contributing factor to their enhanced performance, and is likely a consequence of the optimum sulfurization process. The wider band gap of the films prepared using the 550 °C sulfurization process is expected to be the biggest factor in enhancing their outright performance. However, we expect these both of these factors to be inter-related. We believe the remaining cells performances are inhibited by insufficient sulfurization of the CZTS at 450 °C, 500 °C and the onset of thermal decomposition of CZTS at 600 °C as discussed previously. Table 5 lists the progress of quaternary electrodeposition of CZTS. In our work, we demonstrate a facile electrolyte solution preparation method, where in comparison to conventional electroplating methods performed, a reduction in the number of complexing agents is required. As such, we can potentially reduce the complexity and cost of the CZTS absorber fabrication while concurrently increasing the speed at with the films can be deposited. One part    of our method that is still open to further enhancement is the optimization of the sulfurization process in order to enhance the cell performance. Specifically, it would be beneficial if we could afford better control of the required S vapor during the elevated sulfurization temperature. We consider this a potential avenue toward achieving further enhanced kesterite CZTS solar cells performance, and will be the subject of future work. Finally, we would like to add that our PHED process is an extremely low-cost process when compared to costly vacuum based thin-film solar cell fabrication methods, and as such offers a viable route toward low-cost sustainable energy.

Conclusions
In summary, a one-step pulsed hybrid electrophoretic & electroplating deposition method was demonstrated for the deposition of kesterite CZTS thin-films, enabling the formation of high quality films with appropriate elemental compositions suitable for solar cell applications. A facile electrolyte solution preparation method was proposed where in comparison to conventional electroplating methods, a reduction in the number of complexing agents required in the solution at the time of deposition provides a more stable electrolyte environment for film formation, in turn leading to highly uniform films. Likewise, we remove the complexity traditionally associated with nanoparticle preparation required for CZTS electrophoretic deposition. We have investigated the effect of the sulfurization temperature on the films structural and optical qualities, and are able to observe that films exposed to a 550 °C sulfurization process possess the most suitable characteristics for solar cell fabrication. A significant widening of the optical bandgap is observed in films prepared at this temperature, leading to enhanced performance, with our best CZTS solar cell devices having a V OC , J SC , FF and η of 350 mV, 3.90 mA/cm 2 , 0.43 and 0.59%, respectively. Furthermore, our PHED process is an extremely low-cost process compared to costly vacuum based thin-film solar cell fabrication methods, offering a viable route toward low-cost sustainable energy.

Methods
The PHED deposition of CZTS thin-films was carried out using a three-electrode cell, consisting of an Mo coated soda lime glass substrate as the working electrode (WE), a graphite bar as the counter electrode (CE) and a silver/ silver chloride (Ag/AgCl) electrode as the reference electrode (RE ions, respectively. The pH value of electrolyte was 5.81 with no further pH adjustment applied. To deposit the CZTS films, the pulsed electrodeposition method was applied using a 10% duty cycle (on-time duration of 0.25 sec at − 3 V, off-time duration of 2.25 sec at 0 V). The set potential of 0030 V during the off-time is the potential difference between applied voltage and open circuit voltage, which is the steady state without any disturbance of applied potential. Surface morphologies were examined using a Hitachi S-8010 field-emission scanning electron microscope (FE-SEM). Raman spectra for the CIGS films were obtained using a using a HORIBA Jobin-Yvon, LabRAM HR800 system, equipped with a 632.8 nm laser. The quality and crystal structure of the films were confirmed by X-ray diffraction (XRD) using a Shimadzu XRD-6000 with Cu Kα radiation (0.154 nm). Optical reflectance was measured using a Hitachi U-4100 UV-Visible-NIR Spectrometer. The current-voltage characteristics were measured using a Keithley 4200-SCS parameter analyzer under AM 1.5 G solar simulator at a constant temperature of 25 °C.