1：Experimental Design and Method Selection

Electrochemical underpotential co-deposition (UPCD) method was used for the synthesis of Cu3Te2Se2 nanofilms. The method allows for the simultaneous deposition of Cu, Te, and Se from the same solution at a constant potential.

2：Sample Selection and Data Sources

Nanofilms were deposited on indium tin oxide (ITO)-coated polyethylene terephthalate (ITO-PET), ITO-coated glass, and Au plate substrates.

3：List of Experimental Equipment and Materials

CH Instruments (CHI 660C, USA) for cyclic voltammetry and controlled potential electrolysis, X-ray photoelectron spectroscopy (XPS, Thermo-K-Alpha-Monochromated high-performance XPS Spectrometer), X-ray diffractometry (XRD, The PANalytical X'Pert Pro MPD), Environmental scanning electron microscope (FEI Quanta 200 ESEM FEG), Focused ion beam scanning electron microscopy (FIB-SEM, TESCAN, GAIA 3), Raman microscopy (WITEC Alpha300S), UV–Vis–NIR spectrophotometer (Shimadzu UV-3600), Current-voltage characteristic (I-V curve, 6430 Keithley sub-femtoampere remote source meter).

4：Experimental Procedures and Operational Workflow

Cyclic voltammetry and electrolysis experiments were performed to determine the appropriate deposition potentials and grow the nanofilm on the substrates. The deposition was carried out at room temperature (RT) using 0.1 M H2SO4 solution as a supporting electrolyte. The concentrations chosen were 2 mM CuSO4, 2 mM TeO2, 0.2 mM SeO2, and 0.1 M H2SO4 for electrochemical, optical and SEM studies. For XRD, XPS, Raman, and I-V characterization studies, 10 mM CuSO4, 10 mM TeO2, 1 mM SeO2, and 0.1 M H2SO4 were chosen.

5：Data Analysis Methods

The chemical composition was analyzed by XPS, crystalline structure by XRD, morphology by SEM, optical properties by UV-Vis spectroscopy, and electrical properties by I-V studies.