Electric field and conduction current in ac thin-film electroluminescent display devices

Abstract

Current flow through the phosphor layer of ac thin film electroluminescent display devices has been analyzed. It is shown that the traditional capacitive bridge technique for measuring the time dependence of phosphor conduction current [iLP(t)] can yield inaccurate results, especially when the multiplication factor of [(CI + Cp)/CI] is not used, and the phosphor layer has bulk space charge; CI and Cp are insulator and phosphor layer capacitances, respectively. A new method for the measurement of iLP(t) has been developed. It is more accurate and is applicable even when the bulk charge in the phosphor layer is not negligible. The new method was applied to the study of ZnS:Mn ac thin-film electroluminescent (ACTFEL) devices excited by voltage pulses of three different slew rates. iLP(t) measured by the new method was compared against the bridge current (ibridge). Electric field and displacement current in the phosphor were calculated. Transferred charge computed by integrating iLP and ibridge over half cycle was compared against the polarization charge obtained from the Q–V curve of the device.