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Investigations on the stability of poly(phenylene vinylene)-based organic light-emitting diodes

Pekkola, Oili :
Investigations on the stability of poly(phenylene vinylene)-based organic light-emitting diodes.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2017)

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Item Type: Ph.D. Thesis
Title: Investigations on the stability of poly(phenylene vinylene)-based organic light-emitting diodes
Language: English
Abstract:

The aim of this work was to investigate phenomena that influence the stability of OC3C8-PPV-based OLEDs. The first of the two main topics was the interaction of OC3C8-PPV with oxygen. It was shown that even a residual oxygen concentration in the glovebox is enough to cause changes in OC3C8-PPV. Oxygen was found to diffuse into the polymer during storage in the glovebox, which lead to the creation of electron traps and the subsequent p-doping of the active material. The doping could be observed as an increasing density of equilibrium charges in dark-CELIV measurements with increasing storage time. Additionally, the influence of triplet excitons on the lifetime of OC3C8-PPV -based diodes was investigated. In this work, the concentration of triplet excitons in OC3C8-PPV was increased by incorporating a triplet sensitizer molecule PtOEPK into the polymer. The sensitizer converted part of the OC3C8-PPV singlet excitons to triplets, increasing the triplet-to-singlet ratio. The influence of the increased concentration of triplet excitons on the stability of the devices was then investigated. The increased triplet to singlet ratio lead to a substantial decrease of the t50 lifetime of the diodes. All devices (with and without PtOEPK) showed similar, non-monotonic behavior of luminance and operating voltage during electrical stressing, but the excess triplet excitons were found to affect the time scale of the degradation. The lifetime decreased from 150 h for pristine OC3C8-PPV to 15 h for devices with 1 % PtOEPK. It was found that the devices with an increased concentration of triplet excitons are more prone to hot spot formation than their pristine counterparts. The hot spots act as preferred current paths, decreasing the current flow through the intact part of the device and therefore the luminance. Additionally, the overall temperature of diodes with sensitized polymer layers was ca. 3 K higher than that of pristine diodes, which might lead to small-scale morphological changes.

Alternative Abstract:
Alternative AbstractLanguage
Das Ziel dieser Arbeit war es, Phänomene zu untersuchen, die die Stabilität OC3C8-PPV-basierter organischer Leuchtdioden beeinflussen. Eines der Hauptthemen war die Interaktion des OC3C8-PPV mit Sauerstoff. Es wurde gezeigt, dass die Restsauerstoffkonzentration in der Glovebox ausreicht, um Änderungen im OC3C8-PPV zu bewirken. Während der Lagerung in der Glovebox diffundiert Sauerstoff in das Polymer, was zu einer p-Dotierung des aktiven Materials führt. Die Dotierung konnte als eine mit der Zeit zunehmende Dichte der Ladungsträger in CELIV-Messungen beobachtet werden Zusätzlich wurde der Einfluss von Triplett-Exzitonen auf die Lebensdauer von OC3C8-PPV -basierten Dioden untersucht. In dieser Arbeit wurde die Konzentration von Triplett-Exzitonen in OC3C8-PPV durch die Zugabe eines Triplettkonverters PtOEPK erhöht. PtOEPK konvertiert einen Teil der Singulett-Exzitonen zu Tripletts, wodurch das Triplett-zu-Singulett-Verhältnis erhöht wird. Die erhöhte Konzentration der Tripletts führte zu einer Verringerung der t50-Lebensdauer von 150 h (reines OC3C8-PPV) auf 15 h (1 % PtOEPK). Es wurde festgestellt, dass in Dioden mit einer erhöhten Konzentration der Triplett-Exzitonen mehr bevorzugte Strompfade entstehen. Dadurch sinken die Luminanz und die Betriebsspannung. Zusätzlich war die Betriebstemperatur der Dioden mit PtOEPK etwa 3 K höher als bei dem reinen OC3C8-PPV.German
Place of Publication: Darmstadt
Classification DDC: 500 Naturwissenschaften und Mathematik > 500 Naturwissenschaften
Divisions: 11 Department of Materials and Earth Sciences > Material Science > Electronic Materials
Date Deposited: 17 Jul 2017 10:16
Last Modified: 17 Jul 2017 10:16
URN: urn:nbn:de:tuda-tuprints-62890
Referees: von Seggern, Prof. Dr. Heinz and Rehahn, Prof. Dr. Matthias
Refereed: 25 April 2017
URI: http://tuprints.ulb.tu-darmstadt.de/id/eprint/6289
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