Enhancing crystalline/optical quality, and photoluminescence properties of the Na and Sn substituted ZnS thin films for optoelectronic and solar cell applications; a comparative study

Highlights

• The first comparative study on the green synthesized Na⁺/Sn²⁺substituted ZnS thin films.

• Improved crystalline quality in Zn_1-xSn_xS (ZnSnS) films compared with Zn_1-xNa_xS (ZnNaS).

• Comparatively, the ZnNaS thin films exihibited the lower surface roughness than those of ZnSnS films. .

• Amongst all, the highest optical film quality is observed for the ZnSnS thin films.

Abstract

Zinc sulphide (ZnS) is one of the most important semiconductor for the optoelectronic devices. It is generally used as a buffer layer in highly efficient Cu(In,Ga)Se₂ solar cells due to its environmentally friendly characteristics. Here, we studied the effects of Na⁺/Sn²⁺ substituent using a solution based method for improving characteristics and functions of Zn_1-xNa_xS/Zn_1-xSn_xS (x = 0–15%) thin films for such applications. The formation of Zn²⁺-S^2-, Zn²⁺-Sn²⁺-S^2-, and Zn²⁺-Na⁺-S^2- bonding as well as the existence of Zn²⁺/Zn, Sn²⁺/Sn, S²⁻/S, Na⁺/Na, ions/elements have been examined by the FT-IR and XPS/EDX analysis. The produced films have mixed cubic and hexagonal polycrystalline structure with highly preferred orientation along (111) plane of predominant cubic phase. Comparatively, XRD, SEM, and AFM analysis clearly showed that the crystalline quality of the Zn_1-xSn_xS thin films is higher than those of Zn_1-xNa_xS thin films. The surface morphology of both type films is dense and homogeneous. The surface roughness of Zn_1-xSn_xS films has higher value as compared to the Zn_1-xNa_xS thin films. The optical quality of Zn_1-xSn_xS films was better than those Zn_1-xNa_xS films confirmed by UV–Vis analysis. Amongst of all the synthesized materials, 1% Sn substituted ZnS film sample has the best film crystallization and optical quality. The observed results suggested that it has a promising potential for the optoelectronic and solar cell applications.

Introduction

Nano-structured semiconductors are the curial materials that have huge potential for the development of science and technology. Recent progresses in nano-scaled technology could be expected to improve several of new and sophisticated materials/devices, which possibly can be applied in several fields of technology including nanodevices and nanosystems [[1], [2], [3]]. At nanoscale level, the microstructure, morphology, optical, magnetic, and electrical characteristics tunes incredibly because of the quantum size effect and increased surface to volume ratio [4]. Therefore, nanosized wide band gap semiconductors are the most promising materials for the production of multifunctional devices. They have extraordinary importance for scientific, environmental and industrial concerns and also their variety coating applications due to environment-friendly [5].

Among the wide band gap IV-VI compound semiconductors, ZnS is a metal chalcogenide and it has gained the most extensive research interest due to its great potential in different application fields of photo catalysis, light emitting diodes, flat panel displays, sensors, electroluminescent phosphors, optoelectronics, photovoltaic, and antimicrobial activities [[6], [7], [8], [9], [10], [11]]. It also has crystalline structures including a cubic phase (zinc blende) at room temperature and a hexagonal phase (wurtzite) at higher temperatures [11]. Additionally, the band gap of it can be tuned according to the type, nature, and the concentration of dopant ions [12]. Moreover, it has relatively superior characteristics such as wide direct band gap (3.68 eV), low toxicity, cheapness, earth abundance, chemical stability, n-type conductivity, and high refractive index (~2.3) [[13], [14], [15]].

Various physical and chemical methods including chemical bath deposition (CBD), sol-gel (SG), spray pyrolysis, thermal evaporation, atomic layer deposition, metal-organic vapor phase epitaxy, magnetron sputtering and successive ionic layer adsorption and reaction have been extensively used to produce the nanostructured ZnS thin films [[14], [15], [16], [17], [18], [19], [20], [21]]. Among all, the SG technique has recently gained great interest due to its highly efficient film quality with a high homogeneity, perfect bonding, easy thickness control of films, low equipment cost, smooth film surface, excellent packing density, the possibility of large film area coating and easy controllability [12,17].

Prospective employment in solar cell and optoelectronic devices require best mastering of the components controlling the nanostructure, morphology, optical, and photoluminescence properties of the ZnS films. Doping of different metals (Mn, Cu, Al, Co, Fe, K, Cr, Nd, Ce, La) within various semiconductor nanostructures can lead to new change to modify their physical (structural, optical, magnetic etc.) and chemical (toxicity, stability, reactivity, oxidation state, etc.) properties [[22], [23], [24], [25], [26], [27], [28], [29], [30], [31]]. There are a relatively less/few papers and no reports on tin (Sn)/sodium (Na) substituted ZnS nanostructured thin films. Nevertheless, to tune the mentioned properties of the ZnS films, Na and Sn have been considered as good candidates since their different electronic shell and the similar physical and chemical properties as well as the ionic size of the Na⁺¹ (0.95 Å), Sn⁺² (0.99 Å) and Zn⁺² (0.74 Å) [[32], [33], [34], [35]]. Recently, Sn substituted ZnO/ZnS thin films have been get a great interest due to the increasing the different-type solar cell efficiency [36]. For example, the structural, optical and magnetic properties of the Sn doped ZnS powders produced by solid state reaction have investigated by Kumar et al. [32]. Wanjala et al. have also deposited ZnS:Sn thin films on the glass holders by the CBD technique for solar cell applications [33]. More recently, Sn_xZn_1-xS thin films have been deposited on to a glass substrate by the electron beam evaporation methods to evaluate it's room temperature ferromagnetism [34].

To our best knowledge, the Na substituted ZnS thin films and also the SG deposited Zn_1-xSn_xS thin films have not been reported by any film preparation technique yet. Thus, in this study we investigate and compare their nanostructure, morphological, optical and photoluminescence characteristics of the ZnS nanostructured thin films effected by the substitution levels of the Na and Sn. As far as we aware this is the first report studying the nanostructure, morphological, optical and photoluminescence properties of the first SG derived Zn_1-xSn_xS and Zn_1-xNa_xS (0≤x ≤ 15) thin films, which are highly promising materials for the luminescence, optoelectronic, photocatalytic and photovoltaic applications [9,33,36].