Quantification of hole-trap concentration in degraded polymer light-emitting diodes using impedance spectroscopy

The degradation of polymer light-emitting diodes (PLEDs) under current stress is governed by the formation of hole traps. The presence of traps is reflected in the low-frequency response of PLEDs by a negative contribution to the capacitance that originates from trap-assisted recombination. Since the relaxation time scales with the (inverse) concentration of traps, impedance spectroscopy measurements allow for a quantitative determination of the amount of traps formed during degradation. We demonstrate that the obtained hole trap concentration is in agreement with the amount found by numerically modeling the increase in the PLED driving voltage. Impedance spectroscopy measurements are therefore useful as an in-situ characterization tool during PLED degradation, providing information on trap formation without numerical device modeling.