Growth of ZnS thin films obtained by chemical spray pyrolysis: The influence of precursors

https://doi.org/10.1016/j.jcrysgro.2005.07.050Get rights and content

Abstract

Thin films of zinc sulphide (ZnS) were prepared by spray pyrolysis using two mixed aqueous solutions: (a) dehydrated zinc acetate (10−2 M) and thiourea (4×10−2 M) in bidistillated water; (b) zinc chloride (10−2 M) and thiourea (4×10−2 M) in bidistillated water. The structure, surface morphology, chemical composition and optical properties of these films were investigated as a function of initial (Zn:S) ratio in the solution, which varied between (1:1) and (1:6) for both solutions. This parameter was optimized in order to obtain films with a good stoichiometry, crystalline quality and high transmittance. It was found that films prepared from solution A, with a (Zn:S) ratio equal to (1:4) have the largest crystal size, smoothest surface, high optical transmittance and a (Zn/S) ratio in films equal to 0.91, as determined by XPS.

Introduction

Zinc sulphide (ZnS) is an important semiconductor material with an energy band gap of Eg=3.65eV (bulk), which is the highest among all II–VI compound semiconductors [1]. ZnS, in both bulk and thin film forms, is receiving ever-increasing attention owing to its potential uses [2], [3]. ZnS is highly suitable as a window layer in heterojunction photovoltaic solar cells, because the wide band gap decreases the window absorption loses and improves the short circuit current of the cell [4]. In the area of optics, ZnS can be used as a reflector, because of its high refractive index (2.35), and a dielectric filter because of its high transmittance in the visible range [4]. In opto-electronics, it can be used as a light emitting diode in the blue to ultraviolet spectral region due to its wide band gap [5]. However, deposition of a high-quality ZnS thin film over a large area is required if it is to be used effectively in electroluminescent devices and solar cells.

ZnS thin films have been prepared by a variety or techniques, such as chemical bath deposition [6], thermal evaporation [7] and RF reactive sputtering [8]. The technique of spray pyrolysis also offers interesting possibilities for preparing ZnS thin films [3]. Indeed, this technique for the preparation of thin films is very attractive because it is inexpensive, simple and capable of depositing optically smooth, uniform and homogeneous layers. Furthermore, because this simple coating technique involves processing in an ambient atmosphere, it is easy to incorporate it into an industrial production line [9]. With spray pyrolysis, the solution is sprayed directly onto the substrate. A stream of gas (compressed air) is used for atomization of the solution through the nozzle. The main factors in determining the final physical and chemical properties of the films are the initial solution, the nozzle pressure, and the substrate temperature, among other parameters.

In the present study we report on the effect of the initial solution on the chemical composition, surface morphology, optical properties and structure of ZnS thin films for application as an anti-reflective coating in solar cells.

Section snippets

Experimental procedure

Thin films of ZnS were deposited by chemical spray pyrolysis in air. The experimental set-up used for the preparation of pyrolytically sprayed films is described elsewhere [10], [11]. Two different sets of solutions were used in the present work as precursors for the ZnS film: (a) zinc chloride (10−2 M) and thiourea (4×10−2 M) in bidistillated water, solution (A and B) dehydrated zinc acetate (10−2 M) and thiourea (4×10−2 M) in bidistillated water, solution B. The solutions were sprayed at a rate

Chemical composition

The chemical composition of ZnS films prepared by spray pyrolysis was examined by XPS combined with 4 keV Ar+ sputtering. The C1s, O1s, Zn2p3/2, S2p XPS spectra were recorded for all samples; moreover, the Cl2p spectrum for the films obtained from solution A was also recorded. We also obtained the spectra after sputtering with Ar+ (4 keV) for 1, 3 and 5 min; these times approximately corresponded to depths of 3, 9 and 15 nm, respectively.

The films obtained from solution A with a ratio of (1:2)

Conclusions

ZnS thin films were prepared by chemical spray pyrolysis using different types of solutions. The structure, surface morphology, chemical composition and optical properties of these films were found to vary with the (Zn:S) ratio and the Zn precursor employed in the initial solution. XPS analysis revealed that the chemical composition of all the films was ZnS, although they also contained carbon and oxygen on the surface, which could be associated with two photoelectron bands corresponding to C–C

Acknowledgements

The authors are grateful to the European Union and CICYT (Spain) (grant TEC2004-0051) and the Junta de Andalucía trough the research group FQM-192. The bursary holders, M.C. López wish to thank to the MEC (Spain) and the CSIC (Spain).

References (24)

  • B. Elidrissi et al.

    Mater. Chem. Phys.

    (2001)
  • N. Fathy et al.

    Mater. Sci. Eng. B

    (2004)
  • A. Antony et al.

    Mater. Chem. Phys.

    (2005)
  • S.D. Sartale et al.

    Thin Solid Films

    (2005)
  • S.M.A. Durrani et al.

    Thin Solid Films

    (2000)
  • R. Ayouchi et al.

    J. Crystal Growth

    (2003)
  • R. Ayouchi et al.

    Thin Solid Films

    (2003)
  • M. Chen et al.

    Appl. Surf. Sci.

    (2000)
  • S. Lindroos et al.

    Mat. Res. Bull.

    (1997)
  • D.A. Johnston et al.

    Thin Solid Films

    (2002)
  • L. Schonbrodt et al.

    Thin Solid Films

    (1981)
  • A. Ashour et al.

    Thin Solid Films

    (1994)
  • Cited by (83)

    • Synthesis of Bi/Sr doped zinc sulphide by spray pyrolysis technique: Effects of doping tempertures on solar cell application

      2022, Journal of the Indian Chemical Society
      Citation Excerpt :

      pyrolysis technique has proved to be the most simple and friendly of the production methods listed above [26]. In the spray pyrolysis technique, many factors, such as temperature of substrate, solvent, spray distance, and spray rate, affect both the chemical and physical properties of the synthesized thin film [27]. The temperature of a substrate has a very remarkable function during the process of thin film production via the spray pyrolysis route because it affects the dissolution of the precursor that requires heat energy [28–30].

    • Optimization and fabrication of low cost Cu<inf>2</inf>SnS<inf>3</inf>/ZnS thin film heterojunction solar cell using ultrasonic spray pyrolysis

      2022, Optical Materials
      Citation Excerpt :

      The spray method is not only useful for thin film solar cell, even it is used for fabrication of die synthesized, polymer and quantum dot solar cells [20]. Many researchers have used chemical spray pyrolysis (pneumatic) techniques for synthesis of ZnS films [21–24]. Zeng et al. deposited the thin film at very low substrate temperature (∼310 °C) using thioacetamide precursor as a sulphur source and formed the wurtzite structure of ZnS film.

    View all citing articles on Scopus
    View full text