Innovative photovoltaic (PV) technologies with excellent power conversion efficiency (PCE) and inexpensive mass production costs are required to expand solar energy utilization [1] . Huge research interest is developed in thin-film photovoltaic (TFPV) technologies due to the advantages of scalable flexibility, lesser material usage and greater power generation [2] [3] [4] . Important successes have been achieved in the representative cadmium telluride (CdTe) [5] , copper indium gallium selenide (CIGS) [5] [6] , cadmium telluride (CdTe) [7] and perovskites [8] [9] among various types of thin-film solar cells. However, the main disadvantages of these solar cells are the toxicity of cadmium (Cd), the scarcity of tellurium(Te) and indium (In) and the problem of achieving a stoichiometric ratio. As a result, different novel earth-abundant materials, such as CuSbSe₂ [10] , Cu₂SnS₃ [11] , Cu₂ZnSnSe₄ [12] , SnSe [13] , CuSbS₂ [14] , Sb₂S₃ [15] and Sb₂Se₃ [16] have been suggested as a substitute for the low cost and eco-friendly of thin film solar cells.

Among those, antimony selenide (Sb₂Se₃) has emerged as a promising candidate for next-generation solar absorber material. It is a compound semiconductor belonging to the group V-VI with a suitable band gap of 1.1 - 1.2 eV and finds widespread applications in optoelectronic, thermoelectric photoconducting targets, infrared spectroscopy and television camera [17] [18] . It was used in the 1950s [19] in mineral anti-monselite [20] . It exhibits good photovoltaic and thermoelectric properties which allow possible usage for thermophotovoltaic, thermoelectric [21] [22] [23] [24] and solar cells [25] . Sb₂Se₃ also finds usage in batteries, photodetectors, and memory gadgets [26] . Tremendous research has been focused on Sb₂Se₃ for hybrid solar cell fabrication as light absorber materials [21] [27] due to its low cost, narrow band gap, non-toxic and comparatively earth-abundant [28] . In the present work, electroless deposition of Sb₂Se₃ thin films was done and examination of stoichiometry of Sb₂Se₃ thin films using X-ray photoelectron spectroscopy (XPS) depth profiling was carried out.

All the glassware in our experiment has been cleaned by first washing and scrubbing with Alconox, followed by a 20 min. sonication in acetone, and methanol, and then washed with isopropanol and DI water. Afterward, the glassware was dried using N₂ gas. An aqueous solution of 0.227 g Sb₂Se₃, 0.259 g Na₂SeO₃, 3 ml hydrazine hydrate and 50 ml water have been used for precursor solution electroless deposition. The substrate temperature was controlled by a hot plate with which a thermocouple was attached. The substrate temperature was maintained within ±1˚C of 40˚C for 50 min.

The composition of the Sb₂Se₃ 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, and 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.

XPSPeak software version 4.1 was used to obtain all the spectra. The spectra were deconvoluted using a mixture of Lorentzian-Gaussian type peaks.

The chemical purity and the composition of Sb₂Se₃ thin films were investigated by XPS analysis. The typical XPS survey spectrum Sb₂Se₃ is shown in Figure 1(a). The peaks arising from Sb 3p, 3d, Sb Auger, Na 1s, Na Auger, C 1s, Se 3p and 3d are clearly seen in the spectrum. Carbon contamination is present in almost all the preparations. All other peaks that arise due to energy loss features on the major peaks are weak and broad. The Se 3d intensity is very large compared to the Se 3p intensity, and that is why we have reported just the Se 3d spectra of Se compounds. High-resolution spectra of the Sb 3d core level and Se 3d core level are shown in Figure 1(b) and Figure 1(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 respectively. The separation of Sb 3d doublet is by 9.3 eV. These binding energy values of Sb 3d are characteristic of antimony tri-selenide [18] . No oxygen peak was observed in this hydrazine-processed Sb₂Se₃ thin film in the high-resolution spectra of the Sb 3d core level. The binding energy value of 54.96 eV of Se 3d is characteristic of antimony tri-selenide [18] .

The XPS survey spectrum of Sb₂Se₃ thin film after 1 min. Ar⁺ ion sputtering is shown in Figure 2(a). The peaks arising from Sb 3p, 3d, Sb Auger, Na 1s, Na Auger, C 1s, Se 3p and 3d are clearly seen in the spectrum. Carbon contaminations on the surface were reduced significantly after 1 min. Ar⁺ ion sputtering. High resolution spectra of Sb 3d core level and Se 3d 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 respectively. The separation of Sb 3d double is by 9.3 eV. These binding energy values of Sb 3d are characteristic of antimony tri-selenide [18] . No oxygen peak was observed in this hydrazine-processed Sb₂Se₃ thin film in the high-resolution spectra of the Sb 3d core level. The binding energy value of 54.96 eV of Se 3d is characteristic of antimony tri-selenide [18] . No chemical shift was observed in Sb 3d and Se 3d core levels after 1 min. of Ar⁺ ion sputtering.

The XPS survey spectrum of Sb₂Se₃ thin film after 10 min. Ar⁺ ion sputtering is shown in Figure 3(a). The peaks arising from Sb 3p, 3d, Sb Auger, Na 1s, C 1s, Se 3p and 3d are clearly seen in the spectrum. Carbon contaminations and the peak corresponding to Na 1s core level were reduced to a low level after 10 min. Ar⁺ ion sputtering. High-resolution spectra of the Sb 3d core level and Se 3d core level are shown in Figure 3(b) and Figure 3(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 double is by 9.3 eV. These binding energy values of Sb 3d are characteristic of antimony tri-selenide [18] . No oxygen peak was observed in this hydrazine-processed Sb₂Se₃ thin film in the high-resolution spectra of the Sb 3d core level. The binding energy value of 54.96 eV of the Se 3d is characteristic of antimony tri-selenide [18] . The binding energy value of the Sb 3d core level is increased by 0.1 eV and no chemical shift was observed in the Se 3d core level after 10 min. of Ar⁺ ion sputtering.

The device performance deteriorates due to the presence of recombination centers created by oxygen impurity [18] . Oxygen can be present in Sb₂Se₃ thin films as it is air-sensitive. So X-ray photoelectron spectroscopy (XPS) was used to investigate the composition of Sb₂Se₃ thin film. Almost stoichiometric composition of Sb₂Se₃ thin films close to the surface is observed using the XPS depth profile result in this work. The peak intensity of C(1s) becomes smaller at the deeper surface as sputter time is increased. No oxygen peak was observed in this hydrazine-processed Sb₂Se₃ thin film in the high-resolution spectra of the Sb 3d core level which is good for device performance. The peak corresponding to Na 1s core level decreased significantly with Ar⁺ ion sputtering time. The Sb 3d core level binding energy is increased by 0.1 eV after 10 min Ar⁺ ion sputtering. No chemical shift was observed in the Se 3d core level during Ar⁺ ion sputtering.

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.

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

Chowdhury, T.A. (2023) XPS Study of Electroless Deposited Sb₂Se₃ Thin Films for Solar Cell Absorber Material. Energy and Power Engineering, 15, 363-371. https://doi.org/10.4236/epe.2023.1511021