Excitation and deexcitation of ac-driven thin-film ZnS electroluminescent devices

Abstract

Theoretical formulas accounting for the excitation and deexcitation processes of the alternating current-driven thin-film electroluminescent devices have been obtained, which include both the impact excitation and the energy-transfer mechanisms. The empirical equations for the conduction current duration time and the luminescent decay time related to the tunneling emission of electrons at the interface, the capture of holes in traps, and the light emission of luminescent centers lead to the analytical formulas for the transferred charge ΔQ, the luminance L, and other quantities of physical interest as a function of the electric field. The estimates for ΔQ and L in ZnS:Mn and ZnS:TbF3 devices have been made on the basis of Wolff’s distribution function and found to be in good agreement with the experimental data. From the estimated results, it is found that the energy-transfer mechanism depends on various material parameters and drive conditions, and that it plays a role in improvement of the luminance in the low-electric-field region. In the high-electric-field region of interest, the energy transfer from Cu-related sensitizers to luminescent centers in ZnS:Mn and ZnS:TbF3 devices yields an increase of luminance by a factor of about 1.5 and 3, respectively.