Optoelectronic Properties of Poly(p-phenylene vinylene) For Application in Light Emitting Devices

Abstract

<p>Organic electroluminescent devices (OLED's) have been intensely researched in recent years. Poly(p-phenylene vinylene) (PPV), due to its ease of processing and potential high quantum yield, is considered a good candidate for future display applications. By synthesizing PPV films and fabricating PPV-based organic ELD's, optoelectronic properties of this conjugated polymer were investigated. The PPV synthesis was modified to improve material quality which affected device stability, lifetime, and luminescent yield.</p> <p>By utilizing transmission electron microscopy, the presence of PPV crystals which lowered the quantum yield was thoroughly investigated. Electron micrographs revealed that aggregates of crystals were embedded in an amorphous matrix. Analysis of diffraction patterns established that NaCI was a major impurity and that its crystals acted as nucleation sites. Steps were accordingly taken to exclude NaCI and prepare a more amorphous material.</p> <p>Impedance spectroscopy was used to measure conductivity and the activation energy. The low activation energy suggested carrier transport via hopping between localized states. Furthermore, the presence of a metal/polymer interface, which limited charge injection into the sample and lowered the efficiency, was identified. By controlling the parameters affecting device/material preparation, this interface was to a great extent improved.</p> <p>Electroluminescent devices with areas of 40 mm² were fabricated. These devices could draw currents of up to 0.2 mA/cm² and emit a yellowish green light, visible under laboratory lighting at various viewing angles. An external efficiency of 0.4 LumlWatt was attained. A conservative internal efficiency of 0.59%, significantly higher than the ones previously reported for single layer organic devices, was calculated. The threshold voltage was reduced to 2-5 volts and stable OLED's with a lifetime of over 160 hours were fabricated.</p> <p>The photovoltaic effect in PPV was explored by employing delayed-collection-field and field-induced fluorescent quenching techniques. A carrier generation efficiency of 42% was obtained. The strong field dependence of the carrier generation efficiency was noticed which supported the formation of bound electron-hole pairs.</p>