Performance enhancement of ZnSnP2 solar cells by a Cu3P back buffer layer

Abstract In this study, we investigated reactive diffusion at the interface between a Cu back electrode and a ZnSnP2 absorber, at which ohmic behavior was previously confirmed by appropriate annealing. Cross-sectional observation of the Cu/ZnSnP2 interface revealed that Cu3P was formed along the ZnSnP2 surface after annealing. It was also found that Cu3P was epitaxially formed beside ZnSnP2 with a lattice-mismatch of less than 0.5% by electron diffraction around the Cu3P/ZnSnP2 interface and consideration of atomic arrangements, particularly the phosphorus sublattice, in both phosphides. In addition, photoelectron yield spectroscopy revealed that the work function of Cu3P and ionization potential of ZnSnP2 are both about 5.2 eV, suggesting that the potential barrier against hole transport is small at the Cu3P/ZnSnP2 interface. The formation of Cu3P at the interface leads to higher current density and no roll-over of J-V curves in ZnSnP2 solar cells. Consequently, the highest efficiency of ZnSnP2 solar cells of 3.87% was achieved by intentionally prepared Cu3P using sputtering. It is concluded that Cu3P is a promising back buffer material for ZnSnP2 solar cells due to the alignment with ZnSnP2 from the viewpoints of band and lattice structures.