The role of electron – hole pair formation in organic magnetoresistance Sayani Majumdar, 1,2 * Himadri S. Majumdar, 1 Harri Aarnio, 1 Dirk Vanderzande 3 , Reino Laiho 2 and Ronald Österbacka 1 1 Center for Functional Materials and Department of Physics, Åbo Akademi University, Turku, Finland. 2 Wihuri Physical Laboratory, University of Turku, Turku, Finland. 3 Chemistry Division, Institute of Material Research, Universiteit Hasselt, B3590 Diepenbeek, Belgium. * Corresponding author, e-mail: sayani.majumdar@utu.fi Abstract 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 B 0.5 at higher fields. Key Words: Organic semiconductor, Magnetoresistance, Bulk heterojunction solar cells, electron-hole recombination.