Energy band alignment at Cu₂O/ZnO heterojunctions characterized by in situ x-ray photoelectron spectroscopy

With the increasing interest in Cu₂O-based devices for photovoltaic applications, the energy band alignment at the Cu₂O/ZnO heterojunction has received more and more attention. In this work, a high-quality Cu₂O/ZnO heterojunction is fabricated on a c-Al₂O₃ substrate by laser-molecular beam epitaxy, and the energy band alignment is determined by x-ray photoelectron spectroscopy. The valence band of ZnO is found to be 1.97 eV below that of Cu₂O. A type-Ⅱ band alignment exists at the Cu₂O/ZnO heterojunction with a resulting conduction band offset of 0.77 eV, which is especially favorable for enhancing the efficiency of Cu₂O/ZnO solar cells.