Fabrication of Cd<inf>1−x</inf>Zn<inf>x</inf>S buffer layer deposited by Chemical Bath Deposition for photovoltaic applications

Abstract

Cd 1−x Zn x S films suitable for solar cell photovoltaic applications have been deposited by chemical bath deposition (CBD) on to indium tin oxide (ITO) coated glass substrates. An aqueous solution containing cadmium sulfide, zinc sulfide and thiourea were used as sources of Cd+2, Zn+2, and S−2, respectively. Triethenolamine was used as complexing agent to control the Cd+2 and Zn+2 ions concentrations and ammonia to adjust the pH of the solution. The temperature of the bath was kept at 70 °C. The as deposited films are well adherent, homogeneous and free from pinholes. The incorporation of Zn in CdS was found to be dependent on the annealing temperature. The structure of the CdZnS thin films, as observed by X-ray diffraction, was polycrystalline with hexagonal structure. Several diffraction peaks corresponding to crystallographic planes (100), (002), (102), (110) (103) and (004) were observed. According to energy dispersive spectroscopy (EDS) the films are non-stoichiometric due to a deficit of sulfur, which becomes more important as the Zn content increases. The absorption edge shifts towards the lower wavelength region and hence the band gap of the films increases as the Zn content increases. The values of the absorption edge are found to vary from (Eg ∼ 2.42 eV) for the CdS film and (Eg ∼ 3.30 eV) for the ZnS film. Cd 1−x Zn x S thin films can be useful as buffer and window layers in Cu(In,Ga)Se 2 and Cadmium telluride (CdTe) thin films solar cells due to ability to tune the band gap through the Zn/Cd ratio present in the chemical bath.