Fabrication and characterization of ZNS buffer layer for thin film solar cell using chemical bath deposition (CBD) method

Abstract

Zinc sulfide thin films were prepared by chemical bath deposition technique using zinc sulfate (ZnSO 4 .7H 2 O) and thiourea [SC(NH 2 ) 2 ] as sources of Zn 2+ and S ions, and ammonia (NH 3 ) and hydrazine hydrate (N 2 H ) as complexing agents. ZnS is an important semiconductor compound of excellent physical properties such as wide band-gap energy of 3.7 eV at 300 K, high refractive index (2.35 at 632 nm), transparent to practically all wavelengths of the solar spectrum, non-toxic and environmentally friendly buffer materials that are suitable for industrial production. A very attractive method for producing ZnS thin films, due to the possibility of large-area deposition at low cost is the chemical bath deposition (CBD) method. Chemical bath deposition (CBD) is known to produce solar cell grade films at simpleness, convenience, a low cost and low temperature. The depositions were carried out by varying PH of bath solution from 9.3 to 10.1 and keeping bath temperature fixed at 70 °C for two hours. The thickness of the films was tuned between 72 nm to 319 nm by controlling the ammonia and thiourea concentration. The structural, stoichiometric proportion, morphology and optical properties of the ZnS thin films were investigated as a function of thiourea and ammonia concentrations using X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and UV- visible spectro-photometry measurements. X-ray diffraction analysis of the as-deposited films showed that the films have amorphous structure and poor crystallinity. Energydispersive spectroscopy (EDS) showed that all ZnS thin films exhibited both Zn and S. The UV–visible spectrophotometric measurement showed that, the transmittance of the ZnS thin films vary from 45% to 65%, while the band gap vary from 3.85 eV to 3.89 eV for different ammonia and thiourea concentration.