  The electron and hole drift mobilities of organic semiconductor layers,
localized tail state distributions, and bimolecular recombination constants in
working polymer light-emitting diodes (PLEDs) are determined simultaneously
using impedance spectroscopy (IS). The organic light-emitting layers of these
PLEDs are composed of poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT).
Electron and hole transit time effects are observed in the
capacitance-frequency characteristics of the PLEDs and their drift mobilities
are determined over wide temperature and electric field ranges. The drift
mobilities exhibit thermally activated behavior and the localized tail state
distributions from the conduction band and valence band mobility edges are then
determined from analysis of the electric field dependences of the activation
energies. The bimolecular recombination constants are determined from the
inductive response of the impedance-frequency characteristics. The IS technique
is also applicable to degradation analysis of the PLEDs; changes in the
mobility balance, the localized tail state distributions, and the bimolecular
recombination constant caused by aging are all shown.
