Tuning of organic magnetoresistance by reversible modification of the active material

We investigate the mechanism of the recently discovered phenomenon that the organic magnetoresistance effect can be enhanced by electrical device conditioning. In organic light emitting devices based on poly(paraphenylene vinylene) appropriate conditioning can increase the magnetoconductance values from 1% to 25% at 40 mT. While the conditioning procedure increases the magnetoconductance it leads to a reduction of the electro-optical device performance. The favorable effect of conditioning is non-permanent, leading to a slow reduction of the magnetoconductance values once the conditioning procedure has been turned off. We demonstrate that this relaxation process can be accelerated by thermal annealing of the devices. Higher annealing temperatures cause a stronger reduction of the magnetoconductance values as well as a more pronounced improvement of the previously reduced electro-optical performance. We attribute our results to a modification of the charge carrier transport properties in the bulk polymer material during conditioning and annealing. Changes in polymer morphology or in molecular conformation during conditioning are assumed to result in the formation of energetic trap states which are beneficial for the organic magnetoresistance effect. Thermal annealing reverses the material modifications, reduces the number of trap states in the material and thus decreases the magnetoconductance.