INVESTIGATIONS OF ORGANIC ELECTROLUMINESCENT DIODES BY IMPEDANCE SPECTROSCOPY, PHOTO-IMPEDANCE SPECTROSCOPY AND MODULATED PHOTOVOLTAGE SPECTROSCOPY

We have fabricated single-layer organic electroluminescent diodes with poly(para-phenylene vinylene) (PPV) and poly(2-methoxy-5-ethyl(2'-hex yloxy) para-phenylene vinylene) (MEH-PPV), using indium-tin oxide (ITO ) or semi-transparent gold as hole-injecting anodes and aluminium or c alcium as electron-injecting cathodes. We measured impedance spectra o f the devices, both in the dark and under illumination. We observed no evidence for depletion layers. Some devices show a thin highly resist ive barrier layer, of width about 100 Angstrom, independent of bias. F rom measurements of the modulation in the open-circuit photovoltage ac ross the device, in response to chopped monochromatic light incident o n the ITO electrode, we resolved two distinct spectral features close to the pi-pi energy of the polymers: a narrow response in the low-ene rgy tail of the absorption spectrum and a much broader response which follows the absorption spectrum. Studies of bias dependence and freque ncy dependence also indicate that the two spectral features have diffe rent physical origins.