Investigations of organic electroluminescent diodes by impedance spectroscopy, photo-impedance spectroscopy and modulated photovoltage spectroscopy

• Published in: Synthetic metals Vol. 76; no. 1; pp. 71 - 75
• Main Authors: Harrison, M.G., Grüner, J., Spencer, G.C.W.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Lausanne Elsevier B.V 1996; Amsterdam Elsevier Science; New York, NY
• Subjects: Application fields; Applied sciences; Diodes; Electroluminescence; Electronics; Exact sciences and technology; Optoelectronic devices; Polymer industry, paints, wood; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Spectroscopy; Technology of polymers; Spectroscopy; Diodes; Electroluminescence; Phenylene derivative polymer; Phenylene(vinylene) polymer; Excitation spectrum; Electrical conductor; Experimental study; Electrical characteristic; Light emitting diode
• DOI: 10.1016/0379-6779(95)03422-G
• ISSN: 0379-6779

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 Å, 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 π-π * 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.