Maybritt Kuehn
Technische Universitaet Darmstadt, Germany
Title: Band alignment in organic light emitting diodes – on the track of thickness dependent onset voltage shifts
Biography
Biography: Maybritt Kuehn
Abstract
Organic light emitting diodes play an important role in our daily life, e.g. as displays in smart phones. Nevertheless these modern multilayer devices oft en show unexpected eff ects during operation. One of these phenomena - the thickness dependent onset voltage shift - is topic of this contribution. Th e investigations concentrate on two OLEDs that only differ in the emission layer but show an entirely diff erent current-voltage behaviour. If the emission layer consists of the triplet host TH-A a shift in onset voltage in case of emission layer thickness variation can be observed. Using TH-B in the emission layer, an isomer to TH-A, the onset voltage remains unchanged. In a previous publication, we could show that an electric interface fi eld is responsible for the thickness dependent onset voltage shift . Th e interface fi eld is already present in the currentless case. Th is presentation now deals with the origin of such an interface fi eld. Th erefore the energetic alignment at the internal interfaces in the two diff erent devices is measured by performing in-situ step by step interface experiments using photoelectron spectroscopy. In case of the device showing no onset voltage shift a fl at band situation is measured, while in case of the other device (where there is the onset voltage shift ) the formation of space charge regions is detected. A further stack modifi cation proofs that the band bending at the hole injecting interface into the emission layer is responsible for the onset voltage shift .