1：Experimental Design and Method Selection:

An elemental precursor solution method was adopted for preparing Cu2ZnSn1-xInx(S,Se)4 (CZTISSe) thin films on Mo foils by doping In with varied x values (x ? In/(In t Sn)). Solar cells with a structure of Ag/ITO/i-ZnO/CdS/CZTISSe/Mo foil were fabricated and their photovoltaic properties were systematically investigated.

2：Sample Selection and Data Sources:

Mo foils were potentiostatically polished in methanol and sulfuric acid mixed solution. The CZTISSe thin films were fabricated on Mo foils by green solution-process.

3：List of Experimental Equipment and Materials:

Cu, Zn, Sn, In, S, and Se powders were used as received without further purification. The scanning electron microscope (SEM) images were collected by a Nova Nano field-emission SEM 450 with an energy dispersive X-ray (EDX) analyzer. The X-ray diffraction (XRD) patterns were obtained by a Rigaku Smartlab. Raman spectra were measured via Renishaw Raman microscope with excitation wavelength of 532 nm and 325 nm, respectively.

4：Experimental Procedures and Operational Workflow:

The CZTSSe and CZTISSe precursor films were prepared by spin-coating the precursor solution on the Mo foils and then selenized in a cylindrical graphite box with Se pellets at 550°C for 15 min. The samples of the films were named as In x, which means film with x concentration (x ? In/(Sn t In) ? 0, 3, 6, 9 and 12%).

5：Data Analysis Methods:

The current density-voltage (J-V) curves were measured by a Keithley 2400 source meter under AM1.5 illumination. The external quantum efficiency (EQE) curves were collected by a Zolix SCS100 QE system under a 150 W xenon light source. The capacitance-voltage (C–V) curves were measured by a Keithley 4200 semiconductor characterization system.