Depth Profile Study of Electroless Deposited Sb₂S₃ Thin Films Using XPS for Photovoltaic Applications

Sb ₂S ₃ has gained tremendous research recently for thin film solar cell absorber material because of their easy synthesis, unique electrical and optical properties. The stoichiometry and composition of electroless Sb ₂S ₃ thin films were analyzed using XPS depth profile studies. The surface layers were found nearly stoichiometric. On the other hand, the inner layer was rich in antimony composition making it more conductive electrically.

• Sb₂S₃
• Depth Profiling
• X-Ray Photoelectron Spectroscopy
• Thin Film
• Electroless

1. Introduction

Photovoltaic technologies implement a long-term, clean and cost-effective solution to fulfill increasing demand of energy by converting solar energy into electricity [1] [2] [3] [4] [5] . It is regrettable that most of this energy is not being used using solar cells because of the environmental pollution and high cost involved in solar cell fabrication [6] . Therefore, the main objective of solar energy research has become to look for stable, efficient, low-cost and environmentally friendly solar cell materials [7] [8] . In this regard, metal chalcogenide solar cells, such as CdTe [9] , Cu(In, Ga)Se₂ [10] , Cu₂ZnSn(S, Se)₄ (CZTSSe) [11] , Sb₂S₃ [12] and Sb₂Se₃ [13] have played important roles in solar energy usages. Due to high absorption coefficiency (α > 10⁴ cm⁻¹), elemental content which is environmentally friendly and suitable band gap (1.70 - 1.90 eV) Sb₂S₃ is a promising material among them [14] [15] [16] [17] .

Antimony trisulfide (Sb₂S₃) has gained particular research attention owing to its high thermoelectric power, suitable valence band position and good photovoltaic properties [18] [19] [20] . This material has been applied in various fields such as thermoelectric cooling devices, optoelectronic devices, switching devices, microwave, visible light-responsive photocatalysis, optical data storage devices and photovoltaic structures [21] - [34] . It is essential to have an idea of stoichiometry of surface and bulk of Sb₂S₃ film as it has a profound impact on cell performance. In the present work, Sb₂S₃ thin films were first synthesized by electroless deposition. Then stoichiometry of Sb₂S₃ thin films was investigated using X-ray photoelectron spectroscopy (XPS) depth profiling.

2. Experimental Details

All the glassware in the experiment has been cleaned by first washing and scrubbing with alconox, followed by a 20 min. sonication in acetone, methanol, and then washed by DI water and isopropanol. Afterwards, the glassware was dried using N₂ gas. An aqueous solution of 650 mg SbCl₃, 25 ml Na₂S₂O₃, 2.5 ml acetone and 72.5 ml water have been used for precursor solution electroless deposition. The precursor solution was continuously stirred for the duration of the experiment to assure a good dispersion of precursor materials in the solution. Substrate temperature was controlled by a hot plate with which a thermocouple was attached. The substrate temperature was maintained within ±1˚C of 10˚C for 4 hours.

Composition of the Sb₂S₃ thin film was studied using XPS. The XPS spectra were obtained by using monochromatic Al Kα radiation (1486.6 eV). through a Kratos AXIS Ultra DLD XPS system at a base pressure of 5 × 10⁻¹⁰ Torr, equipped with an electronic neutralization gun to eliminate the charge effect on the sample surface. The sample was firstly pressed to a 1 × 13 mm disc and fixed to the sample-holder, then it was degassed in the load lock chamber overnight. After that, it was removed to the test chamber for XPS study. All binding energy values were calibrated by using the value of contaminant carbon (C 1s 284.6 eV) as a reference. The sample was then ion sputtered with Ar⁺ at 4000 eV and 15 mA for 1 min and 10 min. The raster area is approximately 6 mm × 6 mm, and the estimated erosion rate for depth profiling study during sputter is 4 nm/min.

XPSPeak software version 4.1 was used to fit all the spectra. The spectra were doconvoluted using a mixture of Lorentzian-Gaussian type peaks and Shirley background was applied in all cases.

3. Results and Discussion

The chemical purity and the composition of Sb₂S₃ thin films were investigated by XPS analysis. The typical XPS survey spectrum of Sb₂S₃ is showed in Figure 1(a). The peaks arising from Sb 4d, 3d, 3p, 3s, Sb Auger, Na 1s, Na Auger, O 2s, O Auger, C 1s, C Auger, S 2p and 2s are clearly seen in the spectrum. No other impurities are observed in the spectrum. Carbon contamination is impossible to avoid in almost all the preparations. All other peaks that arise due to energy loss features on the major peaks are weak and broad. The Sb 3d intensity is very large compared to the Sb 4d intensity, and that is why we have studied just Sb 3d spectra of Sb compounds high resolution spectra of Sb 3d core level and S 2p core level are shown in the Figure 1(b) and Figure 1(c) respectively. The two peaks at 530.2 eV and 539.5 eV can be assigned to the binding energy of Sb 3d_5/2 and 3d_3/2 respectively. The separation of Sb 3d doublet is by 9.3 eV. These binding energy values of Sb 3d are characteristic of antimony in the metal sulfides (Sb₂S₃) [35] . The presence of Oxygen 1s in the surface may be due to our deposition method. It confirmed that Sb₂S₃ could be easily oxidized. The Na Auger peak that is present on the surface that is found in the high resolution spectra of Sb 3d core level is due to precursor solution used for the deposition. The peak of S centered at binding energy of 161.3 eV (Figure 1(c)) corresponds to S in metal sulfides (Sb₂S₃) [36] .

The XPS survey spectrum of Sb₂S₃ thin film after 1 min. Ar⁺ ion sputtering is showed in Figure 2(a). The peaks arising from Sb 4d, 3d, 3p, 3s, Sb Auger, Na 1s, Na Auger, O 2s, O Auger, C 1s, C Auger, S 2p and 2s are clearly seen in the spectrum. Oxygen and Carbon contaminations on the surface were reduced significantly after 1 min. Ar⁺ ion sputtering. High resolution spectra of Sb 3d core level and S 2p core level are shown in the Figure 2(b) and Figure 2(c) respectively. The two peaks at 530.1 eV and 539.4 eV can be assigned to the binding energy of Sb 3d_5/2 and 3d_3/2. The separation of Sb 3d doublet is by 9.3 eV. These binding energy values of Sb 3d are characteristic of antimony in the metal sulfides (Sb₂S₃) [35] . A chemical shift of 0.1 eV was observed in Sb 3d_5/2 after 1 min. Ar⁺ ion sputtering. The Oxygen 1s and Na Auger peak that is found in the high resolution spectra of Sb 3d core level is reduced compared with as-deposited Sb₂S₃ film. After 1 min of Ar⁺ ion sputtering, sulfur can be detected in two states, one with binding energy at 162.0 eV and one at 161.1 eV. These binding energy values of S 2p are characteristic of sulfur in the metal sulfides (Sb₂S₃) [35] . The binding energy differences indicate the small changes of the chemical environment of the Sb and S atoms [37] .

The XPS survey spectrum of Sb₂S₃ thin film after 10 min. Ar⁺ ion sputtering is showed in Figure 3(a). The peaks arising from Sb 4d, 3d, 3p, 3s, Sb Auger, Na 1s, Na Auger, O 2s, O Auger, C 1s, C Auger, S 2p and 2s are clearly seen in the spectrum. Oxygen and Carbon contaminations on the surface were reduced significantly after 10 min. Ar⁺ ion sputtering. High resolution spectra of Sb 3d core level and S 2p core level are shown in the Figure 3(b) and Figure 3(c) respectively. After 10 min of Ar⁺ ion sputtering, we can observe that antimony is in two states, one with binding energy at 530.1 eV corresponding to Sb₂S₃ and one at 528.1 eV corresponding to Sb in metallic state [35] . The two peaks at 530.1 eV and 539.4 eV can be assigned to the binding energy of Sb 3d_5/2 and 3d_3/2. The separation of Sb 3d doublet is by 9.3 eV. A chemical shift of 0.1 eV was observed in Sb 3d_5/2 after 10 min. Ar⁺ ion sputtering relative to as-deposited film. The Oxygen 1s and Na Auger peak that is found in the high resolution spectra of Sb 3d

core level is reduced significantly compared with as-deposited Sb₂S₃ film. Sulfur can be detected in two states as we found with 1 min of Ar⁺ ion sputtering, one with binding energy at 162.0 eV and one at 161.1 eV. These binding energy values of S 2p are characteristic of sulfur in the metal sulfides (Sb₂S₃) [35] . The binding energy differences indicate the small changes of the chemical environment of the Sb and S atoms [37] .

4. Conclusion

The Sb₂S₃ is sensitive to air. The oxide phase of Sb₂S₃ affects the cell performance. It is important to analyze the purity of the as-deposited film because oxygen impurity may create recombination centers resulting in deterioration of the cell performance. To this point of view, we examined the purity of Sb₂S₃ using powerful depth-profiling X-ray photoelectron spectroscopy (XPS). XPS depth profile analysis in this report reveals that composition of Sb₂S₃ thin films close to surface is almost stoichiometric. As sputter time is increased, the peak intensity of O(1s) becomes lower because of lower oxygen content at deeper surface. The Sb 3d core level binding energy is decreased by 0.1 eV during Ar⁺ ion sputtering. Sulfur can be detected in two states as we found during Ar⁺ ion sputtering. The binding energy differences indicate the small changes of the chemical environment of the Sb and S atoms. After 10 min. of Ar⁺ ion sputtering, we can observe that antimony is in two states. One as antimony sulfide (Sb₂S₃) and other as Sb in metallic state.

Acknowledgements

The work was supported by the Advanced Support Program for Innovative Research Excellence-(ASPIRE-I), grant number 15530-E404 and Support to Promote Advancement of Research and Creativity (SPARC), grant number 15530-E413 of the University of South Carolina, Columbia, USA.

Conflicts of Interest

The author declares no conflicts of interest regarding the publication of this paper.

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