Origin of equilibrium charges in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester solar cell devices

► Equilibrium charges are seen in P3HT:PCBM solar cell devices. ► Extracted equilibrium charges are regenerated in the sample by a current backflow. ► The amount of dark charges depends on the contact work function. ► Equilibrium charges are due to band bending caused by Fermi level pinning.