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'-hexyloxy) para-phenylene vinylene) (MEH-PPV), using indium-tin oxide (ITO) or semi-transparent gold as hole-injecting anodes and aluminium or calcium as electron-injecting cathodes. We measured impedance spectra of the devices, both in the dark and under illumination. We observed no evidence for depletion layers. Some devices show a thin highly resistive barrier layer, of width about 100 Angstrom, independent of bias. From measurements of the modulation in the open-circuit photovoltage across the device, in response to chopped monochromatic light incident on the ITO electrode, we resolved two distinct spectral features close to the pi-pi* energy of the polymers: a narrow response in the low-energy tail of the absorption spectrum and a much broader response which follows the absorption spectrum. Studies of bias dependence and frequency dependence also indicate that the two spectral features have different physical origins.