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Mark Harrison


PPV LEDs - Impedance, photoimpedance and photocurrent spectra

Investigations Of Organic Electroluminescent Diodes By Impedance Spectroscopy, Photo-Impedance Spectroscopy and Modulated Photovoltage Spectroscopy

M. G. Harrison, J. Gruner and G. C. W. Spencer.

Synthetic Metals 76:1-3 (1996) p71-75 .

rectification, cells electroluminescence, diodes, 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.

A much more detailed and expanded paper on this work has now been published in Physical Review B.

If you cannot find any of these papers, drop me a line for a reprint...

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Mark Harrison,       Marburg, May 3, 1998