Low-cost Cu2ZnSnS4 thin films for large-area high-efficiency heterojunction solar cells

Cu₂ZnSnS₄ (CZTS) thin films have been deposited on soda-lime glass (SLG) and Mo-coated SLG substrates using a low-cost spray pyrolysis technique followed by sulfurization under H₂S flow at 540°C. Aqueous solution containing CuCl, ZnCl₂, SnCl₄ and thiourea was used as precursor. Spray deposition was carried out at three different substrate temperatures of 280°C, 360°C and 440°C. Deposition conditions were optimized to obtain the best film properties. Structural, morphological and compositional analysis of the as-deposited and sulfurized CZTS films were carried out by x-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive x-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS). Optical and electrical properties were measured by UV-Vis spectroscopy, van der Pauw and Hall effect measurements. XRD spectra confirmed the formation of kesterite CZTS films. Grown CZTS films showed an absorption coefficient >;10⁴ cm^-1 and the bandgaps were found to lie between 1.42-1.72 eV at room temperature depending on deposition conditions and post deposition sulfurization treatments. All films were found to be of p-type conductivity with an average carrier concentration in the order of 10¹⁸-10²⁰ cm^-3. Optimum quality films were obtained for films fabricated at 360°C and no secondary phases were observed. n-CdS window layer was deposited on p-CZTS films prepared at 360°C substrate temperature to fabricate p-CZTS/n-CdS heterojunction solar cells. The heterojunction exhibited an open-circuit voltage (V_OC) of 290 mV and a short-circuit current density (J_SC) of 3.1 mA/cm² under AM 1.5 illumination. Details of CZTS thin film fabrication, processing, and characterization results are presented.