Temperature dependent broadband impedance spectroscopy on poly-(p-phenylene-vinylene) light-emitting diodes

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Abstract
Using temperature dependent impedance spectroscopy in a broad frequency range (10 −1 –10 7 Hz), we have found that the ac behavior of indium-tin oxide (ITO)/poly-(p-phenylene-vinylene) (PPV)/aluminum light-emitting diodes shows several features which cannot be described by the usual simple double RC circuit representing a depleted junction region and an undepleted bulk. Instead, our measurements in combination with a theoretical modeling suggest that the PPV bulk is composed of a highly doped region at the ITO interface and a region with lower doping at a higher distance to the ITO. Moreover, the boundary between these two regions is not sharp but there is a gradual change in dopant concentration. The large frequency range allowed us to identify two distinct processes corresponding to the PPV bulk and a third one to the junction. The bulk relaxation frequencies correspond to the characteristicdielectric relaxation frequencies of charge carriers in the high and low conducting sublayers and are proportional to the respective conductivities. The magnitude and activation energy of the relaxation time correlates well with results obtained from temperature dependent DC conductivity measurements. For ITO substrates we obtain activation energies of 0.4 eV and room temperature conductivity of about 10 −7 and 10 −9 S/cm for the high and low conducting sublayers, respectively. On gold substrates only one bulk process and no junction process with an activation energy of about 0.6 eV and a corresponding conductivity of 3×10 −11 S/cm at room temperature is observed. The Schottky junction has been studied by temperature dependent capacitance–voltage spectroscopy at a low frequency of 0.16 Hz. The obtained acceptor dopant concentration from 1/C 2 plots varies from 1.4×10 17 at room temperature to 6.9×10 16 cm −3 at 200 K. Assuming a density of states between 5×10 20 and 5×10 21 cm −3 for the valence band the temperature dependent acceptor dopant density can be described with an acceptor ionization energy between 0.16 and 0.2 eV.