Controlled growth of Cu ₂ZnSnS ₄ (CZTS) thin films for heterojunction solar-cell applications

Cu ₂ZnSnS ₄ (CZTS) thin films (absorber layers) were successfully synthesized on glass substrates by using a RF magnetron sputtering system. The films were rapidly thermally annealed in a nitrogen atmosphere for 20 minutes to improve the crytallinity. The formation of kesterite structures (JCPDS-26-0575) in the film was confirmed using X-ray diffraction (XRD) measurements. The improved crytallinity of the CZTS with a (112) orientation was observed with increasing annealing temperature. The band gaps of all the as-deposited and annealed films were found to be in the range from 1.97 to 1.55 eV. The films' stoichiometry and morphologies were investigated using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) measurements. The sample annealed at 500 °C showed a uniform granular structure with an elemental composition near stoichiometric CZTS. Next, a buffer layer of ZnS for a heterojuntion solar cell was fabricated using a chemical bath deposition (CBD) technique. The films adhered well, were optically transparent and had band gap energy of 3.6 eV.