Co-electroplated Cu₂ZnSnS₄ thin-film solar cells: The role of precursor metallic composition

Thin film solar cells with a structure of ZnO/CdS/Cu₂ZnSnS₄ (CZTS)/Mo were fabricated successfully by sulfurization of co-electroplated Cu-Zn-Sn-S precursors with different metallic compositions. The best solar cell performance with an efficiency of 5.5% was achieved from a slightly Zn-rich and Sn-/Cu-poor (i.e. Zn/Sn = 1.16, Cu/(Zn+Sn) = 0.74) precursor. The CZTS absorber layers grown by Zn-rich and Cu-/Sn-poor precursors show a bi-layered structure comprising of a well-crystallized photovoltaic layer and a particulate-like bottom layer with a heavily Sn poor content and ZnS educts. More Zn-rich precursors led to a lower quality CZTS absorber layer with large over-grown ZnS crystallites protruding from the bottom absorber layer into the top film, which directly gave rise to apparent quantum efficiency and a sharply reduced device efficiency. The cell grown by the slightly Sn-rich precursor had the best-developed CZTS absorber with compact and large grains, however, a lower efficiency.