Sign change in the organic magnetoresistance of tris(8-hydroxyquinolinato)aluminum upon annealing

We report the sign change in the magnetoresistance of a tris(8-hydroxyquinolinato)aluminum film with the morphological change from amorphous to crystalline state upon annealing. The negative component of the magnetoresistance followed power law behavior, whereas the positive one showed non-Lorentzian function behavior. The decreasing absolute values of the negative component with increased annealing temperature may be understood by both intersystem-crossing-based mechanism and quenching of triplet excitons. The increasing values of the positive component with increased annealing temperature may be explained by the increase in the hopping probability of charge carriers with increased crystallinity of the film. (C) 2012 American Institute of Physics. [doi:10.1063/1.3684873]