First-Principles Understanding of Mono- and Dual-Emissions in AZnOS:Bi³⁺ (A = Ba, Ca) Phosphors

The AZnOS:Bi³⁺ (A = Ba, Ca) phosphors exhibit mono- and dual-emission phenomena based on the different choices of cation, making them an ideal prototype for dual-emission mechanism studies of Bi³⁺ ions. Here, first-principles calculations were performed to investigate the site occupancy, defect levels, and luminescence properties of the AZnOS:Bi³⁺ systems. The formation energy calculations show that the bismuth dopants are mainly in the trivalent charge state, with the Bi³⁺ ions preferring the Ca sites in CaZnOS but the Zn sites in BaZnOS. Such cation-selective occupancy mainly results in the difference between the mono- and dual-emission phenomena in the two hosts. The excitation and emission energies predicted by calculations are in good agreement with the measurements. Our calculations show that the lowest excited state ³P_0,1 provides the dominant emission in both CaZnOS:Bi³⁺ and BaZnOS:Bi³⁺ phosphors. In light of the experimental and theoretical results, the metastable excited state of Bi²⁺ + h_VBM (hole at the valence band maximum) is the origin of the additional emission bands in BaZnOS:Bi³⁺. These results provide the basis of emission band tuning and novel material design for Bi³⁺-doped phosphors.