1：Experimental Design and Method Selection:

The thin films were deposited on commercial FTO covered glass by the spray pyrolysis deposition technique. The TiO2 layer was deposited using ethanol solution of TiCl4 (

2：05M), at 350oC, using 20 spraying sequences with 60 seconds break between them, followed by annealing at 500oC for 1hour to increase crystallinity. The ZnO layer was obtained using ZnCl2 in a 30% water-70% ethanol solution, using similar deposition and annealing conditions to those employed for the TiO2 layer. The Cu2ZnSnS4 layer was obtained from cationic chlorides and thiourea mixed in a water-ethanol solution (

1) to give an atomic ratio of 2:1:1:

3：Sample Selection and Data Sources:

The samples’ crystallinity was studied by X-Ray Diffraction (XRD) using a Bruker Advanced D8 Discover Diffractometer with locked coupled continuous scan, 2s/step and CuKα1 radiation (

4：5406?), while Raman analysis (LabRAM HR800 Horiba) was used to confirm the CZTS formation. The surface morphology was investigated using Scanning Electron Microscopy (SEM, Hitachi model S-3400N) and Atomic Force Microscopy (AFM, NT-MDT), in semicontact mode, after the deposition and annealing of the CZTS layer. The bandgap energy of the component layers was obtained using the Tauc formula from reflectance and transmittance spectra recorded using a UV–Vis spectrophotometer (Perkin-Elmer Lambda 25UV/Vis), in the 250–2500 nm range. List of Experimental Equipment and Materials:

FTO covered glass (TEC 15/3mm Pilkington), TiCl4, ZnCl2, cationic chlorides, thiourea, ethanol, water.

5：Experimental Procedures and Operational Workflow:

The layers were deposited using spray pyrolysis deposition technique followed by annealing. The samples were characterized using XRD, Raman spectroscopy, SEM, AFM, and UV-Vis spectrophotometry.

6：Data Analysis Methods:

The crystallite sizes were calculated using Scherrer’s formula. The bandgap energy was obtained using the Tauc formula. The fill factor was calculated from the J-V curves.