Recently, it was discovered that the current through an organic semiconductor, sandwiched between two non-magnetic electrodes, can be changed significantly (up to 25%) by applying a small (a few millitesla) magnetic field. At present, the microscopic mechanisms underlying this so-called organic magnetoresistance (OMAR) are intensively being debated. One of the mechanisms which can successfully describe the magnetic field effects on the current in pristine organic semiconductor devices uses the reactions of triplet excitons and polarons. Here, we present a proof of concept study in which we tune these interactions in the device by deliberately doping our devices with fullerene, creating additional charge transfer states (CTS). By engineering devices with different energetic alignments of the CTS and triplet exciton, we can influence the triplet exciton density in the device. We correlate pronounced changes in the magnetic field effect magnitude and lineshape to the energy of the CTS with respect to the triplet exciton. 2014 Elsevier B.V. All rights reserved.