1：Experimental Design and Method Selection:

Cu2ZnSnSe4 (CZTSe) and Ag2ZnSnSe4 (AZTSe) thin films were deposited on FTO substrates by a thermal evaporation method. The stoichiometric ratio (2:1:1:4) of metal powders Cu, Ag, Zn, Sn and Se were well ground using an agate mortar and pestle before being placed in a Mo boat for evaporation. The FTO substrates were kept at 15 cm from the Mo boat using a HIND HIVAC (Model 12A4D) coating unit for the deposition of the CZTSe & AZTSe thin films with an operating current of 150 Amp. The FTO substrates were kept at ambient temperature and the pressure was maintained at 10-4 mbar during evaporation. The deposited films were annealed at 300 ?C for 2h in an air atmosphere.

2：Sample Selection and Data Sources:

The prepared FTO/CZTSe and FTO/AZTSe thin films were characterized by X-ray diffraction (XRD, PANalytical X‘ PERT- PRO diffractometer) using Cu Kα radiation (λ=

3：5460?). The diffraction patterns were collected in the range 2θ=10?-80?. An FT-RAMAN spectrometer (BRUKER RFS

Stand-alone FT-Raman Spectrometer) were used and UV-Visible (UV-Vis, UV-2400 PC series UV-Visible spectrometer). For surface morphology and roughness of the prepared films an AFM technique was used (instruments Nanosurf Easyscna 2AFM) with accelerating voltage of 15 keV. The cross-section of the film was recorded using a Scanning electron microscope (SEM, JEOL JSM—5610 LV). The thickness of the films was found to be about 1 μm.

4：List of Experimental Equipment and Materials:

HIND HIVAC (Model 12A4D) coating unit, PANalytical X‘ PERT- PRO diffractometer, BRUKER RFS 27: Stand-alone FT-Raman Spectrometer, UV-2400 PC series UV-Visible spectrometer, Nanosurf Easyscna 2AFM, JEOL JSM—5610 LV SEM.

5：Experimental Procedures and Operational Workflow:

The electrochemical cell was constructed using the standard 3-electrode configuration, in the presence of 0.05 M polysulfide electrolyte (0.5 M NaOH+0.5 m Na2S+0.5 M S) with FTO/CZTSe and FTO/AZTSe films as a working electrode, platinum wire as a counter electrode and Ag/AgCl as a reference electrode. A Mott-Schottky plot was obtained in the range ±1 and frequency=1000 Hz. For the photoelectrochemical cell (PEC) measurement Linear Sweep Voltammetry (LSV) techniques were used. The films were illuminated using a 500 W tungsten filament lamp (intensity 30 mW/cm2). The impedance plot was recorded using AC impedance measurement techniques in the frequency range 1 to 1000 Hz with 0V.

6：05 M polysulfide electrolyte (5 M NaOH+5 m Na2S+5 M S) with FTO/CZTSe and FTO/AZTSe films as a working electrode, platinum wire as a counter electrode and Ag/AgCl as a reference electrode. A Mott-Schottky plot was obtained in the range ±1 and frequency=1000 Hz. For the photoelectrochemical cell (PEC) measurement Linear Sweep Voltammetry (LSV) techniques were used. The films were illuminated using a 500 W tungsten filament lamp (intensity 30 mW/cm2). The impedance plot was recorded using AC impedance measurement techniques in the frequency range 1 to 1000 Hz with 0V. Data Analysis Methods:

5. Data Analysis Methods: The crystallite size, dislocation density, microstrain and the lattice parameters were calculated by using the standard formula. The band-gap values of FTO/CZTSe and FTO/AZTSe films were calculated using Tauc plot. Carrier density was calculated from the slope of the Mott-Schottky plot.