*2.1. Synthesis of Cu2Mg0.2Zn0.8Sn(S,Se)4 Precursor Films*

The CMZTSSe precursor films were synthesized in two steps. The first process was to prepare the CMZTSSe precursor solution by a simple and convenient sol–gel technique. We dissolved Cu(CH3COO)2·H2O (0.8086 g), MgCl2·6H2O (0.1787 g), Zn(CH3COO)2·2H2O (0.4794 g), SnCl2·2H2O (0.5077 g), and thiourea (1.3702 g) into 2-methoxyethanol (10 mL), and stirred for 10–15 min at room temperature. The monoethanolamine (MEA) (0.2 mL) was added to the precursor solutions at the end. During the precursor solution preparation process, in order to obtain high-quality CMZTSSe films, the ratio of Cu/(Mg + Zn + Sn) was 0.82 and the ratio of (Zn + Mg)/Sn was 1.2. In our previous studies, we found that the surface morphology, and optical and electrical properties of Cu2MgxZnxSn(S,Se)4 films were optimal when the proportion of Mg/(Mg + Zn) was 0.2 [17]. Therefore, in the present work, the proportion of Mg/(Mg + Zn) was set to 0.2. Then, in order to dissolve the raw materials completely and obtain the CMZTSSe sol–gel solution at room temperature, we stirred the solution until the precursor solution color became colorless and transparent. The second procedure was to obtain the CMZTSSe precursor films through the spin coating method. We spun the precursor solution of CMZTSSe at 3000 r for 30 s, followed by drying at 300 ◦C for 5 min in air. In order to acquire CMZTSSe films with micrometer thicknesses, the process of the coating and drying was repeated several times.
