*2.2. Selenization of Cu2Mg0.2Zn0.8Sn(S,Se)4 Films*

For the sake of studying the effect of annealing treatment on the properties of CMZTSSe films, the rapid annealing treatment was implemented for CMZTSSe precursor films at various selenization temperatures and selenization times under the same selenium atmosphere. We used fixed-quality selenium powder (15 mg) to anneal the precursor CMZTSSe films in the selenide annealing furnace, and increased the selenization temperature from 500 to 560 ◦C, while the selenization time was adjusted in the range of 10–20 min to obtain CMZTSSe samples at varied annealing conditions.

## *2.3. Materials Characterization*

The structural characteristics and chemical composition of CMZTSSe were measured using X-ray power diffraction (XRD, Rigaku Corporation, Tokyo, Japan), Raman spectroscopy (Renishaw, London, UK) with a 514 nm laser wavelength, and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Waltham, MA, USA) (Al Kα was used as the X-ray source). Scanning electron microscopy (SEM) (Hitachi S-4800, JEOL Ltd., Tokyo, Japan) was performed to study the surface morphology of CMZTSSe films. The energy-dispersive X-ray spectroscopy (EDS, JEOL Ltd., Tokyo, Japan) system was used to analyze elemental content. The optical and electrical performances of CMZTSSe films were characterized by UV-Vis-NIR spectra (UV-3101PC, Tokyo, Japan) and room temperature Hall measurement, respectively.

## **3. Results and Discussion**
