Interrelationships between the green 510nm emission, the free-carrier concentration, and the paramagnetic O-vacancy density were studied, in commercial phosphors, by using photoluminescence, optical absorption, and electron paramagnetic resonance techniques. It was found that the green emission intensity was strongly affected by free-carrier depletion at the particle surface; especially in the case of small particles and/or low dopant contents. The data suggested that the singly-ionized O vacancy was responsible for the green emission of ZnO. This emission resulted from the recombination of a photo-generated hole with the singly ionized charge state of this defect.

Mechanisms behind Green Photoluminescence in ZnO Phosphor Powders. Vanheusden, K., Warren, W.L., Seager, C.H., Tallant, D.R., Voigt, J.A., Gnade, B.E.: Journal of Applied Physics, 1996, 79[10], 7983-90