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Improved Brightness and Efficiency in ZnS:Mn Electroluminescent
Sputter Deposited Thin Films by Codoping with KCl

J.S. Lewis,* M.R. Davidson,# and P.H. Holloway*#
*-Department of Materials Science and Engineering
University of Florida, Gainesville, FL 32611-6400

The mechanisms leading to improved brightness, efficiency, and stability of alternating-current thin-film electroluminescent (ACTFEL) ZnS:Mn phosphors have been studied. Ex-situ co-doping of the sputter deposited ZnS:Mn active layer with K and Cl results in a 53% improvement in brightness, a 62 % improvement in efficiency, and better 100-hour accelerated aging stability. These improvements result from a 75% increase in excitation efficiency for conduction electrons, combined with a small decrease in both light outcoupling and non-radiative recombination efficiencies in the electroluminescent films. Electrical properties data show a reduced amount of static space charge in the do-doped films, resulting in a larger average field, increased excitation efficiency, and increased charge multiplication.  The reduced space charge is attributed to the addition of charge compensating zinc vacancy-chlorine complexes and isolated chlorine point defects, which are acceptor and donor defects, respectively, and the reduction of zinc vacancy deep hole traps.  It is postulated that higher average fields result in so much higher injected charge from electron multiplication or donor ionization that EL excitation is limited by the phosphor resistance rather than  capacitance or density of states.  The possibility of using these mechanisms to increase the efficiency of generic ACTFEL phosphors will be discussed.

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