TEMPERATURE-DEPENDENT BROAD-BAND IMPEDANCE SPECTROSCOPY ON POLY-(P-PHENYLENE-VINYLENE) LIGHT-EMITTING-DIODES

Using temperature dependent impedance spectroscopy in a broad frequenc y range (10(-1)-10(7) Hz), we have found that the ac behavior of indiu m-tin oxide (ITO)/poly-(p-phenylene-vinylene) (PPV)/aluminum light-emi tting diodes shows several features which cannot be described by the u sual 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 high ly doped region at the ITO interface and a region with lower doping at a higher distance to the ITO. Moreover, the boundary between these tw o regions is not sharp but there is a gradual change in dopant concent ration. The large frequency range allowed us to identify two distinct processes corresponding to the PPV bulk and a third one to the junctio n. The bulk relaxation frequencies correspond to the characteristic di electric relaxation frequencies of charge carriers in the high and low conducting sublayers and are proportional to the respective conductiv ities. The magnitude and activation energy of the relaxation time corr elates well with results obtained from temperature dependent DC conduc tivity 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 acti vation energy of about 0.6 eV and a corresponding conductivity of 3 X 10(-11) S/cm at room temperature is observed. The Schottky junction ha s been studied by temperature dependent capacitance-voltage spectrosco py at a low frequency of 0.16 Hz. The obtained acceptor dopant concent ration from 1/C-2 plots varies from 1.4 X 10(17) at room temperature t o 6.9 X 10(16) cm(-3) at 200 K. Assuming a density of states between 5 X 10(20) and 5 X 10(21) cm(-3) for the valence band the temperature d ependent acceptor dopant density can be described with an. acceptor io nization energy between 0.16 and 0.2 eV. (C) 1998 American Institute o f Physics.