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 3P0,1 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+ + hVBM (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.