| The aim of this study was to develop a qualitative understanding of the electronic behavior of a polymer light emitting device by means of electric impedance spectroscopy. The behavior of an organic light emitting device was understood by deriving an analogy to a simple parallel RC circuit, in which the imaginary part of admittance (divided by the angular frequency) represents a capacitance. These capacitance values were plotted against the applied voltage. Several features present in a capacitance-voltage plot of a polymer light-emitting diode (AuPEDOT:PSSPPVBaAl) have been analyzed. Specifically observed were: (1) A voltage independent capacitance at reverse bias, of which the value depends on the presence of PEDOT:PSS (hole transporting layer), and additionally on the resistivity of this layer. (2) An increase (peak) of the capacitance at the built-in-voltage due to the trapping of charge carriers (holes) at defect states. De-trapping can be promoted by UV irradiation. (3) An increase in the capacitance at forward bias owing to a trap-filled space-charge limited current.; Using this technique, polymer light emitting devices made from different research samples of hole transporting layers have been compared and conclusions on the resistivity of that layer has been determined. (Abstract shortened by UMI.)... |
