The fabrication of soda lime glass/fluorine-doped tin oxide (SLG/FTO) transparent substrates represents an important advance in the fabrication of tandem solar cells. In addition, possible passivation using Al2O3, which can be deposited using cheap systems, can open up the development of Cu2ZnSnS4 (CZTS) solar cells. In this work, we report the simultaneous use of FTO/Mo (20 nm) as a transparent substrate and Al2O3 as apassivation layer at the p–n interface in CZTS solar cells. The best devices show efficiencies of 7.7% (Voc = 677 mV) and 7.3% (Voc = 700 mV). Complete analysis, in terms of morphology, structure and electrical properties, is discussed.