  Magneto-electrical measurements were performed on diodes and bulk
heterojunction solar cells (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 disappearance of the MR. Our results clearly
show that MR is a direct consequence of e-h pair formation. We also found that
the MR line shape follows a power law-dependence of B0.5 at higher fields.
