Citation: Khan, A.U.R.; Ramzan, M.; Iqbal, M.F.; Hafeez, M.; Fadhali, M.M.; Somaily, H.H.; Javid, M.; Mukhtar, M.W.; Saleem, M.F. Effect of the Source-to-Substrate Distance on Structural, Optoelectronic, and Thermoelectric Properties of Zinc Sulfide Thin Films. Materials 2022, 15, 8047. https://doi.org/10.3390/ ma15228047 Received: 11 October 2022 Accepted: 2 November 2022 Published: 14 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Effect of the Source-to-Substrate Distance on Structural, Optoelectronic, and Thermoelectric Properties of Zinc Sulfide Thin Films Asad Ur Rehman Khan 1 , Muhammad Ramzan 1, *, Muhammad Faisal Iqbal 2 , Muhammad Hafeez 3 , Mohammed M. Fadhali 4,5 , Hamoud H. Somaily 6,7 , Muhammad Javid 8 , Muhammad Waqas Mukhtar 1 and Muhammad Farooq Saleem 9, * 1 Institute of Physics, Baghdad-Ul-Jadeed Campus, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan 2 Department of Physics, Riphah International University Faisalabad, Faisalabad 38000, Pakistan 3 Department of Physics, Lahore Garrison University, Lahore 54810, Pakistan 4 Department of Physics, Jazan University, Jazan 82812, Saudi Arabia 5 Department of Physics, Faculty of Science, Ibb University, Ibb 70270, Yemen 6 Research Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia 7 Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61421, Saudi Arabia 8 Institute of Advanced Magnetic Materials, College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310012, China 9 GBA Branch of Aerospace Information Research Institute, Chinese Academy of Sciences, Guangzhou 510700, China * Correspondence: mr.khawar@iub.edu.pk (M.R.); farooq@aircas.ac.cn (M.F.S.) Abstract: Zinc sulfide (ZnS) thin films with variable structural, optical, electrical, and thermoelectric properties were obtained by changing the source-to-substrate (SSD) distance in the physical-vapor- thermal-coating (PVTC) system. The films crystallized into a zinc-blende cubic structure with (111) preferred orientation. The films had a wide 3.54 eV optical band gap. High-quality homogenous thin films were obtained at 60 mm SSD. The sheet resistance and resistivity of the films decreased from 10 11 to 10 10 Ω/Sq. and from 10 6 to 10 5 Ω-cm, when SSD was increased from 20 mm to 60 mm, respectively. The phase and band gap were also verified by first principles that were in agreement with the experimental results. Thermoelectric characteristics were studied by using the semi-classical Boltzmann transport theory. The high quality, wide band gap, and reduced electrical resistance make ZnS a suitable candidate for the window layer in solar cells. Keywords: ZnS thin films; physical-vapor-thermal-coating technique; source-to-substrate distance; solar cells 1. Introduction Serious challenges such as the energy crisis and pollution due to the huge rise in chemical exhaust as well as the burning of fossil fuels, including coal, have motivated scientists and researchers to search for better energy alternatives. Solar energy is a better alternative source of energy that is renewable as well as clean and inexhaustible [1]. To expedite the growth of the optoelectronics industry, especially for solar cells, it is necessary to boost the efficiency of thin-film devices as well as to reduce the production cost. Thin-film technology is one of the bases for high-efficiency, environmentally friendly, and low-cost devices. Several factors such as doping, surface morphology, plasma treatments, and deposition techniques contribute to improving the performance of nanostructured thin films [2]. For optoelectronics applications, thin films of III–V gallium arsenide (GaAs), IV–VI lead sulfide (PbS), and II–VI compounds consisting of cadmium sulfide (CdS), zinc sulfide (ZnS), cadmium telluride (CdTe), zinc telluride (ZnTe), cadmium selenide (CdSe), Materials 2022, 15, 8047. https://doi.org/10.3390/ma15228047 https://www.mdpi.com/journal/materials