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

Cu
₂
ZnSnS
₄
(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 H
₂
S flow at 540°C. Aqueous solution containing CuCl, ZnCl
₂
, SnCl
₄
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 >;10
⁴
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 10
¹⁸
-10
²⁰
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/cm
²
under AM 1.5 illumination. Details of CZTS thin film fabrication, processing, and characterization results are presented.