We investigate the magnetic field effect on the current of a diode based on a copolymer, poly{[4,4-bis(2-ethylhexyl)-cyclopenta-(2,1-b;3,4-b′)dithiophen]-2,6-diyl-alt-(2,1,3-benzo-thiadiazole)24,7-diyl} (PCPDTBT), which is intensively used in bulk heterojunction solar cells. Magnetoconductance measurements are carried out for applied voltages corresponding to the trap-charge-limited current regime. An effective mobility is introduced to take into account the presence of traps. The magnetoconductance is positive and monotonic in the full range of the applied voltages. The stochastic Liouville equation is a useful tool to interpret the magnetic field effect on the current. The latter equation is adapted in the framework of the electron–hole pair model to reproduce the line shape of magnetoconductance. The recombination rates, deduced from the fitting of the experimental data, are kS ≈ 109 s–1 and kT ≈ 105 s–1 for singlet and triplet e–h pairs, respectively.