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

Abstract

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.

Introduction

CuSbS₂ is a direct bandgap material, which can be adjusted between 1.4 and 1.6 eV (Medina-Montes et al., 2018; Pal et al., 2020), and its optical absorption coefficient is greater than 10⁵ cm⁻¹ (Vinayakumar et al., 2019). Its grain growth temperature is within 300°C–450°C (Yang et al., 2014; Riha et al., 2017), which is lower than those of Cu (In, Ga) Se₂ and Cu₂ZnSnS₄. It is an excellent alternative material for flexible photovoltaic devices due to its low grain growth temperature. Various techniques, such as spray pyrolysis (Ramos Aquino et al., 2016; Wan et al., 2019; Zhou et al., 2021), ink (Banu et al., 2016; Banu et al., 2019), chemical bath deposition (Macías et al., 2017; Loranca-Ramos et al., 2018), sputtering (de Souza Lucas et al., 2016; Kang et al., 2018), evaporation (Rabhi et al., 2009; Wan et al., 2016), and electrodeposition (Septina et al., 2014; García et al., 2020), have been employed to fabricate CuSbS₂ thin films. Wan et al., (2016) fabricated stoichiometric CuSbS₂ thin films and Mo/CuSbS₂/CdS/ZnO/ZnO: Al/Ag structure solar cells via a two-stage co-evaporation, achieving a 1.9% efficiency with a 520-mV open-circuit voltage. Macías et al., (2017) fabricated CuSbS₂ thin films via a chemical bath method and treated the films at 380°C, achieving an encouraging efficiency of 0.6%. Zhang et al.(2019) fabricated Mo/CuSbS₂/CdS/ZnO/Al structure cells via a one-step sputtering method and obtained a photovoltaic conversion efficiency of 2.55% with a 622-mV open-circuit voltage. The best efficiency of 3.22% was obtained by Banu et al., (2016).

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 CuSbS₂ thin films via electrodeposition has been favored by researchers. The highest efficiency of CuSbS₂ 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 CuSbS₂ thin-film 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 Sb₂S₃, Cu₁₂Sb₄S₁₃, and Cu₃SbS₄, easily form during annealing (Kang et al., 2018; Pal et al., 2020).

Zhang et al. (2016) have demonstrated the crystallinity of CuSbS₂ thin films fabricated by electrodepositing Mo/Cu/Sb metal layers followed by sulfurizing in 20% H₂S + 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 CuSbS₂ 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 CuSbS₂. In addition, the influences of temperature on structures, compositions, phases, and morphologies of CuSbS₂ thin films were systematically studied. Ultimately, 0.90% efficient CuSbS₂ solar cells were fabricated with a 13.58-mA/cm² current density.

Materials and Methods

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 SbCl₃ and 2.2-M HCl, whereas a Cu solution was prepared using 0.8-M CuSO₄ and 0.76-M H₂SO₄. 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/cm²; 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/cm², 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/cm². 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

CuSbS₂ 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 CuSbS₂ thin films were removed from the furnace.

Preparation of Devices

CuSbS₂ thin-film solar cells were prepared with the traditional structure of Mo/CuSbS₂/CdS/i-ZnO/ZnO: Al/Ni/Al. A CdS buffer layer of 50 nm was prepared via the chemical water bath method on CuSbS₂ 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 cm².

Characterization

The structure of the CuSbS₂ 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 CuSbS₂ solar cells were measured under a standard AM_1.5 spectrum using a solar simulator; the illumination intensity was 1000 WM⁻².

Results and Discussion

Characteristics of the Sb/Cu Metal Precursors

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).

FIGURE 1

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/cm²; 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).

FIGURE 2

Figures 2D–F show the FE-SEM images of Sb deposited with different pulse current densities: 31.25, 62.5, and 125 mA/cm². 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/cm². 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/cm², a uniform and compact Sb film was obtained. However, an uneven surface appeared when the pulse current density increased to 125 mA/cm², 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/cm².

The Sb film deposited with a pulse current density of 62.5 mA/cm² 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/cm² and a pulse frequency of 10,000 Hz. Figure 3 shows a smooth, compact Cu layer without any dendritic clusters being obtained.

FIGURE 3

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 CuSbS₂ absorbers.

FIGURE 4

Analysis of the Composition, Phase, and Morphology of the CuSbS₂ Thin Film

Sb₂S₃ 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 CuSbS₂ absorbers. Figure 5A shows the XRD patterns of the CuSbS₂ thin films annealed at 300, 320, 340, 360, 380, and 400°C for 30 min. The sample sulfurized at 300°C detected the CuSbS₂ chalcostibite compound (JCPDS No. 44-1417) and weak diffraction peaks of Sb₂S₃ (JCPDS No. 42-1393) and Cu_2−xS. The CuSbS₂ 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 Sb₂S₃ at approximately 300°C. When the temperature was sufficiently high (300°C), the binary sulfides of Cu and Sb reacted to form the CuSbS₂ ternary compound. The specific reaction equations are as follows (Colombara et al., 2012):With the increase in temperature, the intensity of the Sb₂S₃ secondary phase decreased and the crystallinity of CuSbS₂ thin films improved. In the temperature range of 320°C–340°C, the films mainly comprised CuSbS₂ and Sb₂S₃ phases. However, the diffraction peak intensity of CuSbS₂ gradually increased with the temperature increase, whereas the intensity of the Sb₂S₃ peak gradually decreased. At 360°C, the typical bimodal structure of the CuSbS₂ phase was observed in the range of 28°–31°. In addition, the CuSbS₂ thin film showed the (301) preferred orientation. No other obvious secondary phases were observed, indicating that the film was relatively pure. Sb₂S₃ 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 CuSbS₂ began to decrease; the preferential orientation of the CuSbS₂ 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 CuSbS₂ phase transformed into the Cu₃SbS₄ phase (JCPDS No. 35-0581). CuSbS₂ tended to decompose into Cu₃SbS₄ and Sb₂S₃. The diffraction peak of Sb₂S₃ was not detected, because Sb₂S₃ evaporated due to its low melting point. The decomposition reaction was as follows:

FIGURE 5

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 CuSbS₂ 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−xS 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 Sb₂S₃ secondary phase (Efthimiopoulos et al., 2016). No Cu_2−xS phase was detected in the films annealed in the temperature range of 320°C–340°C, indicating that Cu_2−xS and Sb₂S₃ had reacted completely to form CuSbS₂. At 360°C, there was a prominent peak at 332 cm⁻¹, which corresponded to CuSbS₂. 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 Cu₃SbS₄ 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 Sb₂S₃. 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 Sb₂S₃ and did not fully participate in the reaction to generate CuSbS₂. When the temperature increased to 360°C, CuS and Sb₂S₃ fully reacted to produce CuSbS₂, 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 Cu₃SbS₄.

Figure 6 shows the FE-SEM images of the films sulfurized at different temperatures. The morphologies of the CuSbS₂ 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, micron-sized rod-like grains were formed and the crystallization degree and compactness of CuSbS₂ 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 CuSbS₂. 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 CuSbS₂ phase—where a large amount of Sb loss destroyed the completeness of the film. A relatively pure CuSbS₂ thin film could be obtained in the temperature range of 360°C–380°C.

FIGURE 6

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 CuSbS₂ thin films was good and columnar large grains were formed. The CuSbS₂ 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 CuSbS₂ thin films annealed at 360°C was high and more suitable to prepare CuSbS₂ devices.

FIGURE 7

XPS Analysis of the CuSbS₂ Thin Films

XPS was employed to measure the chemical states of elements in the CuSbS₂ thin films sulfurized at 360°C (Figure 8). From Figure 8A, the main elements of CuSbS₂ thin films were Cu, Sb, and S, indicating no additional doping element in the CuSbS₂ thin film. As shown in 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 CuSbS₂ (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 Sb³⁺ (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 S²⁻ in CuSbS₂ (van Embden et al., 2020). The results agreed with the chemical states of standard CuSbS₂, which proved the pure phase CuSbS₂ thin films to be prepared at the annealing temperature of 360°C.

FIGURE 8

The CuSbS₂ thin films annealed at 360°C and 380°C were selected to prepare solar cells. The J–V curves of the CuSbS₂ 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 CuSbS₂ 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/cm², and a filling factor (FF) of 31.77%. However, the photovoltaic conversion efficiency of the device, based on the CuSbS₂ 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/cm². The CuSbS₂ thin films annealed at 360°C were more compact than those annealed at 380°C (Figures 6D,E). The voids in the CuSbS₂ thin films could influence carrier transportation, which would reduce the current density. Combined with the XRD analysis, the crystal-preferred orientation of the CuSbS₂ thin films changed from (301) to (410) with an increase in annealing temperature. Based on the analysis of the Cu (In, Ga) Se₂-preferred orientation (Kim et al., 2018), the changed-preferred orientation of CuSbS₂ might induce different defect densities, which would influence the open-circuit voltage. In addition, the band structure diagram of CuSbS₂ thin film solar cells is shown in Supplementary Figure S2.

FIGURE 9

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/cm², 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 CuSbS₂ absorbers. The influences of annealing temperature on the composition, phase, and morphology of CuSbS₂ thin films were studied. A compact and relatively pure CuSbS₂ thin film was fabricated at 360°C. Finally, a 0.90% efficiency of CuSbS₂ thin-film solar cell was obtained with an open-circuit voltage of 0.21 V, a short circuit current density of 13.58 mA/cm², and an FF of 31.77%.

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