Magnetoelectrical measurements were performed on diodes and bulk heterojunction solar cell blends (BHSCs) to clarify the role of formation of Coulombically bound electron-hole (e-h) pairs on the magnetoresistance (MR) response in organic thin-film devices. BHSCs are suitable model systems because they effectively quench excitons but the probability of forming e-h pairs in them can be tuned over orders of magnitude by the choice of material and solvent in the blend. We have systematically varied the e-h recombination coefficients, which are directly proportional to the probability for the charge carriers to meet in space, and found that a reduced probability of electrons and holes meeting in space lead to the disappearance of the MR. Our results clearly show that MR is a direct consequence of the e-h pair formation. We also found that the MR line shape follows a power-law dependence of $B^{0.5}$ at higher fields.

• Received 21 January 2009

DOI:https://doi.org/10.1103/PhysRevB.79.201202