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Electron trapping is a well-recognized issue in organic semiconductors, in particular in conjugated polymers, leading to a significant electron mobility reduction in materials with electron affinities smaller than 4 eV. Space-charge limited current measurements in diodes indicate that these traps have similar molecular origin, while calculations show that hydrated molecular oxygen is a plausible molecular candidate, with the tail of the solid-state electron affinity distribution reaching values as high as 4 eV. By decreasing the trap density by mixing conjugated polymers with an insulating polymer matrix, one can fill the traps with charges and hence eliminate their effect on electron mobility. Trap dilution not only improves transport but also reduces trap-assisted recombination, boosting the efficiency of polymer light emitting diodes.