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

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