By combining electron paramagnetic resonance, photoluminescence spectroscopic and optical absorption techniques, a strong correlation was detected between green 510nm emissions, the free-carrier concentration, and the density of singly ionized O vacancies in commercial phosphor powders. On the basis of these results, it was demonstrated that free-carrier depletion at the particle surface, and its effect upon the ionization state of the O vacancy, could strongly affect the green emission intensity.

Correlation between Photoluminescence and Oxygen Vacancies in ZnO Phosphors. Vanheusden, K., Seager, C.H., Warren, W.L., Tallant, D.R., Voigt, J.A.: Applied Physics Letters, 1996, 68[3], 403-5