Effect of excited states and applied magnetic fields on the measured hole mobility in an organic semiconductor

The hole mobility in thin films of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine has been measured using the dark injection transient method. These measurements were performed in the presence of a small, variable offset bias in unipolar and ambipolar samples, with and without an applied magnetic field. A reduction in mobility is observed in ambipolar samples at offset values above the turn-on voltage and is consistent with site blocking by triplet excitons. This is directly linked to the presence of electrically generated excited states by measuring the current-voltage-luminescence characteristics of such devices. The application of a similar to 500 mT magnetic field has the effect of increasing the measured mobility; an effect that is enhanced by the presence of excited states in ambipolar samples (from similar to 3.2% below turn-on to similar to 6.5% above), as opposed to the unipolar samples, where it remains constant independent of offset voltage. We thus conclude that the observed mobility enhancement with magnetic field in ambipolar structures is a result of a decrease in the concentration of site-blocking triplet states and provides direct measurement of a microscopic mechanism accounting for the phenomenon of organic magnetoresistance.