Determination of Charge-Carrier Mobility in Organic Light-Emitting Diodes by Impedance Spectroscopy in Presence of Localized States

The determination of charge-carrier mobility in organic light-emitting diodes (OLEDs) in the presence of localized states by impedance spectroscopy (IS) measurements is carried out. The influences of localized-state distributions on charge-carrier mobility determination by IS measurements are examined on the basis of a theory for single-injection space-charge-limited current. It is shown that the drift mobilities of OLEDs can be determined by IS measurements even in the presence of localized states. It is also shown that distributed localized states cause low frequency behavior of capacitance (an increase in capacitance with decreasing frequency). Measurements of the characteristic temperatures of conduction- and valence-band tail states are demonstrated in polyfluorene-based LUMATION TM (Sumation TM ) Green 1300 Series light-emitting polymers.