Research Article Structural, Surface Morphology and Optical Properties of ZnS Films by Chemical Bath Deposition at Various Zn/S Molar Ratios Fei-Peng Yu, 1 Sin-Liang Ou, 1 Pin-Chuan Yao, 2 Bing-Rui Wu, 1 and Dong-Sing Wuu 1,2 1 Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 40227, Taiwan 2 Department of Materials Science and Engineering, Da-Yeh University, Changhua 51591, Taiwan Correspondence should be addressed to Dong-Sing Wuu; dsw@dragon.nchu.edu.tw Received 13 December 2013; Revised 8 March 2014; Accepted 10 March 2014; Published 31 March 2014 Academic Editor: Sheng-Po Chang Copyright © 2014 Fei-Peng Yu et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In this study, ZnS thin ﬁlms were prepared on glass substrates by chemical bath deposition at various Zn/S molar ratios from 1/50 to 1/150. e eﬀects of Zn/S molar ratio in precursor on the characteristics of ZnS ﬁlms were demonstrated by X-ray diﬀraction, scanning electron microscopy, optical transmittance, X-ray photoelectron spectroscopy, and Fourier transform infrared spectrometry. It was found that more voids were formed in the ZnS ﬁlm prepared using the precursor with Zn/S molar ratio of 1/50, and the other ZnS ﬁlms showed the denser structure as the molar ratio was decreased from 1/75 to 1/150. From the analyses of chemical bonding states, the ZnS phase was indeed formed in these ﬁlms. Moreover, the ZnO and Zn(OH) 2 also appeared due to the water absorption on ﬁlm surface during deposition. is would be helpful to the junction in cell device. With changing the Zn/S molar ratio from 1/75 to 1/150, the ZnS ﬁlms demonstrate high transmittance of 75–88% in the visible region, indicating the ﬁlms are potentially useful in photovoltaic applications. 1. Introduction Zinc sulﬁde (ZnS) is a II-VI compound semiconductor with a wide direct band gap (  = 3.6∼3.8 eV). Moreover, ZnS has the high refractive index (2.35 at 632 nm) and high dielectric constant (9 at 1 MHz) [1]. As a result, it can be widely applied in the optoelectronic applications consisting of light emitting diodes with short wavelength, electroluminescent devices, and solar cells. For the photovoltaic applications, ZnS thin ﬁlm is also transparent in all wavelengths of solar spectrum and has high absorption for the wavelength below 520 nm as compared to CdS. Many techniques including sputtering [2], molecular beam epitaxy [3], pulsed laser deposition [4], chemical vapor deposition [5], successive ionic layer adsorp- tion and reaction [6], spray pyrolysis [7], and chemical bath deposition (CBD) [1, 8] have been proposed to fabricate the ZnS thin ﬁlms. Among these methods, CBD is most attractive because it can be employed as the large-area growth without vapor deposition related to physical techniques and free of some inherent problems associated with high temperature fabrication [9]. For the ZnS growth by CBD process, a soluble salt of Zn ion and nonmetallic S source compound dissolved in an aqueous solution is required, reacting by the following equation: Zn 2+ + S 2− → ZnS,  sp = 10 −24.7 [10]. Owing to the low solubility product of Zn 2+ and S 2− , ZnS pre- cipitation will take place rapidly at very low concentration (homogeneous process), which results in the loose structure and poor transmittance in the thin ﬁlm. However, the ZnS thin ﬁlm with good uniformity and high transmittance can be achieved using the complex agents, such as trisodium citrate, ethylenediamine, and nitrilotriacetate (heterogeneous process). With the assistance of complex agents, the metal ions and negative ions were released slowly and then reacted to form the compound. e process is used to avoid the fast precipitation of the compound in the solution [11]. Moreover, Hindawi Publishing Corporation Journal of Nanomaterials Volume 2014, Article ID 594952, 7 pages http://dx.doi.org/10.1155/2014/594952