Asymmetric interface band alignments of Cu2O/ZnO and ZnO/Cu2O heterojunctions

• We fabricated reverse ordered Cu2O–ZnO heterojunctions by magnetron sputtering.
• An obvious asymmetry was observed in the VBOs of Cu2O/ZnO–ZnO/Cu2O by XPS.
• We analyzed the asymmetry by d–p electronic repulsion in the valence band of Cu2O.
• Growth order can affect crystal structure of the top layer of the heterojunction.

X-ray photoelectron spectroscopy was used to investigate the valence-band offsets (VBOs) of the Cu2O–ZnO heterojunctions fabricated by magnetron sputtering. A significant forward–backward asymmetry was observed in the band alignments of Cu2O/ZnO–ZnO/Cu2O heterojunctions. The valence band offsets of 2.91 eV and 2.52 eV were achieved in Cu2O/ZnO and ZnO/Cu2O heterojunctions respectively. The asymmetry was attributed to the lattice distortion of the Cu2O at the interface of Cu2O/ZnO heterojunction. The compressed Cu2O lattices lead to the upward shift of the top of the valence-band of Cu2O, and thus the measured VBO of Cu2O/ZnO is larger than that of ZnO/Cu2O. Considering the band alignments, the ZnO/Cu2O structure is expected to have more advantage in photovoltaic application than Cu2O/ZnO structure.