Energy-band alignment of II-VI/Zn₃P₂ heterojunctions from x-ray photoemission spectroscopy

The energy-band alignments for zb-ZnSe(001)/α-Zn₃P ₂(001), w-CdS(0001)/α-Zn₃P₂(001), and w-ZnO(0001)/α-Zn₃P₂(001) heterojunctions have been determined using high-resolution x-ray photoelectron spectroscopy via the Kraut method. Ab initio hybrid density functional theory calculations of the valence-band density of states were used to determine the energy differences between the core level and valence-band maximum for each of the bulk materials. The ZnSe/Zn₃P₂ heterojunction had a small conduction-band offset, ΔE_C, of -0.03 ± 0.11 eV, demonstrating a nearly ideal energy-band alignment for use in thin-film photovoltaic devices. The CdS/Zn₃P₂ heterojunction was also type-II but had a larger conduction-band offset of ΔE_C = -0.76 ± 0.10 eV. A type-III alignment was observed for the ZnO/Zn₃P₂ heterojunction, with ΔE_C = -1.61 ± 0.16 eV indicating the formation of a tunnel junction at the oxide-phosphide interface. The data also provide insight into the role of the II-VI/Zn₃P₂ band alignment in the reported performance of Zn₃P₂ heterojunction solar cells.