Optical Materials 105 (2020) 109917 Available online 25 April 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved. Effect of pH treatment on the structural and optical properties of Sn 6 Sb 10 S 21 thin flms facilely synthesized using a spin coating method Warunee Kumrueng a , Khotchanan Sawanthai a , Auttasit Tubtimtae b, * , Witawat Ponhan c a Department of Chemistry, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaeng Saen Campus, Nakhon Pathom, 73140, Thailand b Department of Physics, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaeng Saen Campus, Nakhon Pathom, 73140, Thailand c Department of Physics, Faculty of Science and Technology, Sakon Nakhon Rajabhat University, Sakon Nakhon, 47000, Thailand A R T I C L E INFO Keywords: Tin antimony sulfde Thin flm pH treatment Structural and optical properties Spin coating method ABSTRACT This study aimed to examine the effect of pH value for the Sn–Sb–S solution on the growth of tin antimony sulfde (TAS) thin flms facilely synthesized and deposited on fuorine-doped tin oxide or borosilicate slide glass sub- strates using a spin coating method. Field-emission scanning electron microscopy (FE-SEM) micrographs confrmed that some aggregated NPs, the distribution of residuals and larger-sized clusters or non-uniform surfaces of thin flms on the substrate surface were signifcantly affected by pH alteration. XRD analysis revealed that the most intense peak of TAS thin flms exhibited a monoclinic Sn 6 Sb 10 S 21 crystal structure with an orthorhombic Sn 5 Sb 2 S 9 phase. However, the presence of a binary Sb 2 S 3 phase was also observed; its peak in- tensity gradually reduced with increasing pH value. In addition, the investigation revealed that change in the structural and optical parameters under the alteration of the pH treatment in a solution had a signifcant impact on the formation, nucleation and growth of the clusters with increasing residuals and non-uniformity of thin flm. It could be concluded from the overall results of our demonstration that TAS thin flms with adjusted pH treatment can be attributed to the growth of TAS thin flms for use as window layers in solar cell devices and may have potential for use in photodetector, optical communication, and optical dispersion devices in the future. 1. Introduction Currently, the trends of development for photovoltaic devices including semiconductor thin flms that are non-toxic, environmentally friendly, and abundant have attracted much attention. Previously, thin flm photovoltaics such as cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) or Cu (In,Ga) Se 2 based solar cells exhibited power conversion effciency reaching 22.1 and 22.9%, respectively [1, 2], which led to progress in updating photovoltaic technology. However, both materials contain In and Ga, which are toxic and rare materials [3–5]. This is a critical issue for further improvement of CIGS-based solar cells. In this situation, material containing sulfdes and more earth-abundant materials (Cu, Zn, Sn, Sb, and S) could be used for less toxicity than material containing selenides. Based on our knowledge, sulfosalt material belongs to a class of complex sulfdes with the general formula of A m B n X p , where A means metallic elements like Sn and Pb, B represents formally trivalent, semi-metallic elements such as As, Sb and Bi, and X indicates either S or Se [6]. This kind of material has various advantageous properties such as low cost, non-toxicity, and high potential for use in many technological applications that can be used in solid-state and photovoltaic devices [7–10] as well as thermoelectric energy conversion [11]. Sulfosalts also have direct band gaps in the range of 1.78–2.5 eV, which are very close to the theoretical value for power conversion of light into electrical energy for photovoltaic cells [12,13]. This criterion is preferable as an absorber layer in solar cell devices [14]. Alternatively, a p-type Cu 2 ZnSnS 4 (CZTS) thin flm was synthesized as a new absorber for solar cell applications. The CZTS has a suitable band gap ~1.4–1.5 eV, which is close to the optimal bandgap required for a single junction solar cell and high absorption coeffcient (�10 4 cm 1 ) [15,16]. For example, a combination of Zn 1x Sn x O (ZTO) and a short annealing time of CZTS with a Cd-free buffer layer results in effciency of 9.7% [17]. A Cd-free CZTS solar cell exhibits energy con- version effciency of 10.2%, resulting from the application of an aluminum oxide (Al 2 O 3 ) passivation layer [18] and a certifed 11% ef- fciency CZTS solar cell with a high open-circuit voltage of 730 mV, has been obtained through directly inducing Cd atoms to occupy Zn or Cu lattice sites by heat treatment to reduce heterojunction recombination [19]. Moreover, some groups of researchers have synthesized ternary * Corresponding author. E-mail address: tubtimtae@gmail.com (A. Tubtimtae). Contents lists available at ScienceDirect Optical Materials journal homepage: http://www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2020.109917 Received 17 March 2020; Accepted 13 April 2020