Sulfurization of Electrodeposited Sb/Cu Precursors for CuSbS2: Potential Absorber Materials for Thin-Film Solar Cells

CuSbS2, as a direct bandgap semiconductor, is a promising candidate for fabricating flexible thin-film solar cells due to its low grain growth temperature (300°C–450°C). Uniform and highly crystalline CuSbS2 thin films are crucial to improving device performance. However, uniform CuSbS2 is difficult to obtain during electrodeposition and post-sulfurization due to the “dendritic” deposition of Cu on Mo substrates. In this study, Sb/Cu layers were sequentially pulse electrodeposited on Mo substrates. By adjusting the pulse parameters, smooth and uniform Sb layers were prepared on Mo, and a flat Cu layer was obtained on Sb without any dendritic clusters. A two-step annealing process was employed to fabricate CuSbS2 thin films. The effects of temperature on phases and morphologies were investigated. CuSbS2 thin films with good crystallinity were obtained at 360°C. As the annealing temperature increased, the crystallinity of the films decreased. The CuSbS2 phase transformed into a Cu3SbS4 phase with the temperature increase to 400°C. Finally, a 0.90% efficient solar cell was obtained using the CuSbS2 thin films annealed at 360°C.

Compared with other deposition methods, electrodeposition has the advantages of low equipment cost, simple operation, and high material usage rate (Lincot et al., 2004;Bhattacharya et al., 2012;Oliva et al., 2013;Vauche et al., 2016). Therefore, fabricating CuSbS 2 thin films via electrodeposition has been favored by researchers. The highest efficiency of CuSbS 2 thin films fabricated via electrodeposition is 3.13% (Septina et al., 2014), which is far lower than the maximum theoretical conversion efficiency of 22.9% (SLME) (Yu et al., 2013). The main factor limiting the further improvement of CuSbS 2 thinfilm solar cells are 1) rough electrodeposited metal precursors lead the thickness and composition distribution to be nonuniform (Yuan et al., 2009;Gao et al., 2020)and 2) secondary phases, such as Sb 2 S 3 , Cu 12 Sb 4 S 13 , and Cu 3 SbS 4 , easily form during annealing (Kang et al., 2018;Pal et al., 2020). Zhang et al. (2016) have demonstrated the crystallinity of CuSbS 2 thin films fabricated by electrodepositing Mo/Cu/Sb metal layers followed by sulfurizing in 20% H 2 S + Ar atmosphere for 1 h at 450°C. However, the surface roughness of the Mo/Cu/Sb metal layers was large, which further deteriorated the uniformity of CuSbS 2 thin films. Although Bi et al. (2016) eliminated the dendritic morphology of Cu on Mo via a pulse current electrodeposition method, the problem of dendritic clusters appeared again after the Cu thickness was reduced. To fabricate dendritic-cluster-free and smooth Cu films via electrodeposition, numerous studies have been conducted (Norkus et al., 2005;Moffat et al., 2007;Lee et al., 2012). Among them, adding organic additives in the electrodepositing solution could help reduce the surface roughness of Cu films and improve their flatness (Long et al., 2006;Favry et al., 2008). However, organic additives could easily be incorporated into the films as impurities, which would deteriorate the properties of the devices.
In this study, to solve the problem of rough metal precursors on Mo substrates, first, a smooth Sb layer was electrodeposited on a Mo substrate, and then, a Cu layer was electrodeposited on the Sb layer. A compact, flat, and uniform Sb/Cu layer was fabricated on a Mo substrate using the pulse current electrodeposition method. The metal stacking layer with the Sb/Cu atomic ratio of 1.7 was employed to fabricate CuSbS 2 . In addition, the influences of temperature on structures, compositions, phases, and morphologies of CuSbS 2 thin films were systematically studied. Ultimately, 0.90% efficient CuSbS 2 solar cells were fabricated with a 13.58-mA/cm 2 current density.

Preparation of the Sb/Cu Precursor
Mo back contact layer (1 μm) was fabricated on a clean soda-lime glass (SLG) via DC magnetron sputtering. Before the electrodeposition process, the fabricated SLG/Mo substrate was ultrasonically cleaned in alcohol for 30 min, followed by cleaning with deionized water to obtain a clean, oil-free Mo substrate. Afterward, Sb and Cu metal layers were fabricated on the Mo substrate via pulse electrodeposition at room temperature. An Sb solution was prepared using 0.3-M SbCl 3 and 2.2-M HCl, whereas a Cu solution was prepared using 0.8-M CuSO 4 and 0.76-M H 2 SO 4 . The diagram of the square wave pulse is shown in Supplementary Figure S1. There are three independent parameters: pulse current density (i m ), pulse on time (T on ), and pulse off time (T off ) in pulse electrodeposition. T off is beneficial to recover ion concentration near a cathode and improve the coating quality. The T on /T off ratios of the electrodeposited Sb film were set to 1:1, 1:3, and 1:5; the pulse current densities were 31.25, 62.5, and 125 mA/cm 2 ; and the pulse frequency was 10,000 Hz. The T on /T off ratio of the electrodeposited Cu film was 1:3, the pulse current density was 62.5 mA/cm 2 , and the pulse frequency was 10,000 Hz. The deposition charge densities of the Sb layer were varied from 0.5 to 1.275 C/cm 2 . The pulse working conditions were provided by GKPT-FB4-24 V/10 A pulse/DC power supply manufactured by Shenzhen Shicheng Company, China.

Sulfurization of the Metal Precursor
CuSbS 2 thin films were fabricated by annealing Mo/Sb/Cu precursors in a double temperature quartz tube furnace. The alloyed precursor and 1-g sulfur powder (excess) were, respectively, placed in different temperature zones of the double temperature quartz tube furnace. The sulfur source temperature was maintained at 260°C with a 10°C/min heating rate. First, the metal stacks were annealed at 320°C in Ar atmosphere for 10 min and then sulfurized at 300°C-400°C for another 30 min. After free cooling to room temperature, the CuSbS 2 thin films were removed from the furnace.

Preparation of Devices
CuSbS 2 thin-film solar cells were prepared with the traditional structure of Mo/CuSbS 2 /CdS/i-ZnO/ZnO: Al/Ni/Al. A CdS buffer layer of 50 nm was prepared via the chemical water bath method on CuSbS 2 thin films. The i-ZnO thin films with approximately 50-μm thickness were deposited on CdS thin films by AC magnetron sputtering; then, the Al-ZnO thin films with 500-800 nm thickness were deposited by DC sputtering. Finally, a Ni layer of 50 nm and an Al layer of 1 μm were deposited by electron beam evaporation as the collector. The effective area of the cell was 0.12 cm 2 .

Characterization
The structure of the CuSbS 2 thin films was analyzed using X-ray diffractometry (XRD, Rigaku Smart Lab) and Raman spectrometry (Raman, Renishaw). The excitation wavelength of the Raman spectrometer was 532 nm. The surface roughness of the films was observed via atomic force microscopy (AFM, Dimension ICON). The morphology of the films and the elemental composition and distribution (EDS) of the samples were observed via field emission scanning electron microscopy (FE-SEM, Quanta FEG 250). The element composition and chemical states of the films were measured via energy-dispersive X-ray spectrometry (XPS, ESCALAB250Xi). The J−V characteristics of CuSbS 2 solar cells were measured under a standard AM_1.5 spectrum using a solar simulator; the illumination intensity was 1000 WM −2 . Figure 1 shows the SEM images of Cu and Cu/Sb deposited on the Mo substrate. From Figure 1A, a "dendritic" morphology was formed by directly depositing the Cu layer on the Mo substrate. It was because the growth of Cu on Mo showed a three-dimensional island mode (Mercier et al., 2013). The growth rate of the Cu nucleus tended to be higher than the nucleation rate, inducing Cu to form a dendritic morphology (Budevski et al., 2000). The Sb layer electrodeposited on such a rough Cu underlayer usually resulted in a coarse Cu/Sb layer with bulging clusters on the Sb surface ( Figure 1B). The non-uniform composition distribution of the rough Cu/Sb precursor tended to form copper-rich and -poor phases during annealing, which would increase the leakage current of the device and deteriorate its performance (Oliva et al., 2013;Kwon et al., 2014).   To eliminate the "dendritic" morphology, the Mo/Sb/Cu stack was employed to obtain a flat and uniform metal precursor. Figure 2 shows the SEM images of the Sb layer on Mo prepared via the pulse current electrodeposition method. In Figures 2A-C, the T on /T off ratio was set to 1:1, 1:3, and 1:5, with the pulse current density of 62.5 mA/cm 2 ; small spherical grains can be observed. The size of the spherical grains was relatively non-uniform with  the T on /T off ratio of 1:1. The spherical grain size tended to be uniform with the decrease in the T on /T off ratio, and the surface of Sb films became smooth and compact. When the ratio decreased to 1:5, the Sb grains became nonuniform again. It might be because the decreased T on /T off ratio increased the peak current density, which led the growth rate of the Sb nucleus to be higher than the nucleation rate (Grujicic and Pesic, 2002). Figures 2D-F show the FE-SEM images of Sb deposited with different pulse current densities: 31.25, 62.5, and 125 mA/cm 2 . The T on /T off ratio was set to 1:3. The Sb surface was covered with a layer of insect-like particles with a pulse current density of 31.25 mA/cm 2 . It might be because the Sb film was not fully nucleated and grew up when the current density was too low. When the pulse current density increased to 62.5 mA/cm 2 , a uniform and compact Sb film was obtained. However, an uneven surface appeared when the pulse current density increased to 125 mA/cm 2 , which might be due to the concentration polarization near the surface of Sb film at high pulse current density. Finally, a uniform, flat, and compact Sb film could be fabricated on the Mo substrate with a T on /T off ratio of 1:3 and a pulse current density of 62.5 mA/cm 2 .

Characteristics of the Sb/Cu Metal Precursors
The Sb film deposited with a pulse current density of 62.5 mA/ cm 2 and T on /T off of 1:3 ( Figure 2B) was chosen as the substrate to deposit the Cu layer. The Cu was deposited with a pulse current density of 62.5 mA/cm 2 and a pulse frequency of 10,000 Hz. Figure 3 shows a smooth, compact Cu layer without any dendritic clusters being obtained. Figure 4 presents the AFM images of Mo/Cu, Mo/Cu/Sb, Mo/Sb, and Mo/Sb/Cu layers. The average roughness (R a ) of each layer was calculated using an atomic force microscope assistant software ( Table 1). The average roughness (R a ) of Mo/ Cu (96 nm) was larger than that of Mo/Sb (21.8 nm). As the underlying layer, the surface roughness of the Mo/Cu/Sb layer (112 nm) was larger than that of the Mo/Sb/Cu layer (30.4 nm), indicating that the uniformity of Mo/Sb/Cu was higher than that of Mo/Cu/Sb. Therefore, the new structure of Mo/Sb/Cu was chosen as the metal precursor to fabricate CuSbS 2 absorbers.

Analysis of the Composition, Phase, and Morphology of the CuSbS 2 Thin Film
Sb 2 S 3 is easy to evaporate in the sulfurization process due to its high saturated vapor pressure, resulting in the loss of Sb elements. Therefore, the Sb/Cu metal-stack layers with an atomic ratio of 1.7 were sulfurized to prepare CuSbS 2 absorbers. Figure 5A shows the XRD patterns of the CuSbS 2 thin films annealed at 300, 320, 340, 360, 380, and 400°C for 30 min. The sample sulfurized at 300°C detected the CuSbS 2 chalcostibite compound (JCPDS No. 44-1417) and weak diffraction peaks of Sb 2 S 3 (JCPDS No. 42-1393) and Cu 2−x S. The CuSbS 2 phase was generated by the reaction of Cu and Sb chalcogenides. Cu was completely transformed into Cu-S at 200°C, whereas Sb reacted with S to form Sb 2 S 3 at approximately 300°C. When the temperature was sufficiently high (300°C), the binary sulfides of Cu and Sb reacted to form the CuSbS 2 ternary compound. The specific reaction equations are as follows (Colombara et al., 2012): With the increase in temperature, the intensity of the Sb 2 S 3 secondary phase decreased and the crystallinity of CuSbS 2 thin films improved. In the temperature range of 320°C-340°C, the films mainly comprised CuSbS 2 and Sb 2 S 3 phases. However, the diffraction peak intensity of CuSbS 2 gradually increased with the temperature increase, whereas the intensity of the Sb 2 S 3 peak gradually decreased. At 360°C, the typical bimodal structure of the CuSbS 2 phase was observed in the range of 28°-31°. In addition, the CuSbS 2 thin film showed the (301) preferred orientation. No other obvious secondary phases were observed, indicating that the film was relatively pure. Sb 2 S 3 might have evaporated during high-temperature sulfurization due to its relatively high saturated vapor pressure (Colombara et al., 2011;Colombara et al., 2012). When the temperature increased to 380°C, the diffraction peak of CuSbS 2 began to decrease; the preferential orientation of the CuSbS 2 thin films changed from (301) to (410). When the annealing temperature increased to 400°C, the films became Cu-rich due to the loss of a large amount of Sb, and the CuSbS 2 phase transformed into the Cu 3 SbS 4 phase . CuSbS 2 tended to decompose into Cu 3 SbS 4 and Sb 2 S 3 . The diffraction peak of Sb 2 S 3 was not detected, because Sb 2 S 3 evaporated due to its low melting point. The decomposition reaction was as follows: 9CuSbS 2 (s) → 3Cu 3 SbS 4 (s) + 2Sb(s) + 2Sb 2 S 3 g (4) Figure 5B presents the Raman spectra of films annealed at different sulfurization temperatures. Raman vibration peaks at 249, 317, and 339-332 cm −1 all corresponded to the CuSbS 2 phase, as reported previously (Vinayakumar et al., 2017;Chalapathi et al., 2018). Moreover, the Raman peak at 471 cm −1 of the film sulfurized at 300°C corresponded to the Cu 2−x S phase (Hurma and Kose, 2016), which agreed with the XRD results. Besides, the Raman peak at 271 cm −1 of the films sulfurized in the temperature range of 300°C-340°C corresponded to the Sb 2 S 3 secondary phase (Efthimiopoulos et al., 2016). No Cu 2−x S phase was detected in the films annealed in the temperature range of 320°C-340°C, indicating that Cu 2−x S and Sb 2 S 3 had reacted completely to form CuSbS 2 . At 360°C, there was a prominent peak at 332 cm −1 , which corresponded to CuSbS 2 . With the increase in the crystallization degree, the secondary peaks at 317 cm −1 disappeared and only the main peak located at 332 cm −1 remained. At 400°C, a shoulder at 322 cm −1 appeared due to the existence of the Cu 3 SbS 4 phase (Chalapathi et al., 2018).  The elemental compositions of the films at different annealing temperatures were determined via EDS ( Table 2). The compositions of Cu, Sb, and S were very sensitive to the annealing temperature. The films were rich in copper and poor in antimony due to the high volatility of Sb 2 S 3 . The Cu/ Sb ratio was approximately 2.5 in the temperature range of 300°C-340°C, and the ratio decreased with the increase in temperature. This is because Sb was not fully sulfurized into Sb 2 S 3 and did not fully participate in the reaction to generate CuSbS 2 . When the temperature increased to 360°C, CuS and Sb 2 S 3 fully reacted to produce CuSbS 2 , the film showed only a slight Cu-rich composition and the Cu/Sb ratio of the film decreased to 1.06. Sb loss was significant when the temperature further increased to 400°C. The Cu/Sb ratio increased to 2.81 at 400°C, which should be due to the decomposition of the membrane into Cu 3 SbS 4 . Figure 6 shows the FE-SEM images of the films sulfurized at different temperatures. The morphologies of the CuSbS 2 thin films changed with the increase in temperature. As shown in Figure 6A, fine grains and white rod-shaped grains were  observed on the film surface. EDS analysis showed that the white rod-shaped grains were Cu-and S-rich phases. In the temperature range of 320°C-340°C, grains began to aggregate and grow up. With the increase in temperature, the grain sizes increased. In the temperature range of 360°C-380°C, micronsized rod-like grains were formed and the crystallization degree and compactness of CuSbS 2 thin films improved. Notably, the surface morphology of the film changed significantly with the increase in temperature due to the low reaction and formation temperature of CuSbS 2 . However, further increasing the growth temperature to 400°C, the pores formed on the film surfaces. Combined with the XRD and Raman analyses, this phenomenon might be due to the decomposition of the CuSbS 2 phase-where a large amount of Sb loss destroyed the completeness of the film. A relatively pure CuSbS 2 thin film could be obtained in the temperature range of 360°C-380°C. Figure 7 shows the cross-sectional FE-SEM images of the films sulfurized at 360 and 380°C. The thickness of the films was approximately 1.2 μm without fine-grain layers at the bottom of the film. At 360°C, the adhesion between the annealed Mo and CuSbS 2 thin films was good and columnar large grains were formed. The CuSbS 2 grain sizes increased with the increase in the sulfurization temperature to 380°C. Compared with the films prepared at 380°C, the films prepared at 360°C were more uniform and denser. The crystallinity of the CuSbS 2 thin films annealed at 360°C was high and more suitable to prepare CuSbS 2 devices.

XPS Analysis of the CuSbS 2 Thin Films
XPS was employed to measure the chemical states of elements in the CuSbS 2 thin films sulfurized at 360°C (Figure 8). From Figure 8A, the main elements of CuSbS 2 thin films were Cu, Sb, and S, indicating no additional doping element in the CuSbS 2 thin film. Figures 8B-D, the binding energies of Cu 2p 1/2 and 2P 3/2 were 931.9 and 951.7 eV, respectively, with an interval of 19.8 eV, which agreed with the reported binding energies of Cu + in CuSbS 2 (Wan et al., 2016). The binding energies of Sb 3d 5/2 and 3d 3/2 were 529.6 and 539.0 eV, respectively, with an interval of 9.4 eV, which agreed with Sb 3+ (Vinayakumar et al., 2017). The peak at 532.1 eV corresponded to the oxygen adsorbed on the thin film. The binding energies of S 2p 3/2 and 2p 1/2 were 161.4 and 162.6 eV, respectively, with an interval of 1.2 eV, which corresponded to the binding energies of S 2− in CuSbS 2 (van Embden et al., 2020). The results agreed with the chemical states of standard CuSbS 2 , which proved the pure phase CuSbS 2 thin films to be prepared at the annealing temperature of 360°C.

As shown in
The CuSbS 2 thin films annealed at 360°C and 380°C were selected to prepare solar cells. The J-V curves of the CuSbS 2 thin-film solar cells are shown in Figure 9. The device parameters are listed in Table 3. The photovoltaic conversion efficiency (η) of the device, based on the CuSbS 2 thin films annealed at 360°C, was 0.90% with an open-circuit voltage (V oc ) of 0.21 V, a current density (J sc ) of 13.58 mA/cm 2 , and a filling factor (FF) of 31.77%. However, the photovoltaic conversion efficiency of the device, based on the CuSbS 2 film annealed at 380°C, was only 0.36% with an open-circuit voltage(V oc ) of 0.10 V, a current density of 11.75 mA/cm 2 . The CuSbS 2 thin films annealed at 360°C were more compact than those annealed at 380°C ( Figures  6D,E). The voids in the CuSbS 2 thin films could influence carrier transportation, which would reduce the current density. Combined with the XRD analysis, the crystalpreferred orientation of the CuSbS 2 thin films changed from (301) to (410) with an increase in annealing temperature. Based on the analysis of the Cu (In, Ga) Se 2preferred orientation (Kim et al., 2018), the changedpreferred orientation of CuSbS 2 might induce different defect densities, which would influence the open-circuit voltage. In addition, the band structure diagram of CuSbS 2 thin film solar cells is shown in Supplementary  Figure S2.

CONCLUSION
In this study, Sb/Cu metal layers were prepared on Mo substrates via the pulse current electrodeposition method. By adjusting the pulse parameters (T on /T off of 1:3, a pulse current density of 62.5 mA/cm 2 , and a pulse current frequency of 10,000 Hz), a compact and uniform Sb layer was prepared on the Mo substrate. With this uniform Sb underlayer, dense, uniform, and smooth Cu layers without dendric clusters were realized. A two-step sulfurization process was employed to fabricate CuSbS 2 absorbers. The influences of annealing temperature on the composition, phase, and morphology of CuSbS 2 thin films were studied. A compact and relatively pure CuSbS 2 thin film was fabricated at 360°C. Finally, a 0.90% efficiency of CuSbS 2 thin-film solar cell was obtained with an open-circuit voltage of 0.21 V, a short circuit current density of 13.58 mA/cm 2 , and an FF of 31.77%.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.