Studies into the underlying mechanisms of organic magnetoresistance

Abstract

The organic magnetoresistance effect has been observed to alter the current and efficiency of organic light emitting diodes where no magnetic materials are present. As yet, no consensus has been reached in the literature regarding the origin of the magnetic field effect, though several models have been proposed that feature different charge species playing the key role, including polaron pairs, triplet excitons, and bipolarons. Each model relies on some mechanism of spin mixing, commonly presumed to be hyper fine interactions between polarons and hydrogen nuclei in the active layer. Through measurements of the relative change in current and effciency with magnetic field, this thesis utilises slight alterations to the device properties, namely deuteration of the active layer, and the addition of a doped sensing layer, to address the assumption about the importance of the hyper fine interaction and to attempt to di erentiate between the di erent models for organic magnetoresistance. It is found that the hyper fine interaction is not the only spin mixing mechanism occurring in the active layer, and that triplets play a pivotal role in the organic magnetoresistance effect, potentially defining the profile of the organic magnetoresistance response in doped devices through their population profile.