  A ferroelectric field effect in the bulk heterojunction was found when an
external electric field (EEF) was applied on the active layer of polymer solar
cells (PSCs) during the annealing process of the active layer spin-coated with
poly (3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM).
For one direction field, the short circuit current density of PSCs was improved
from 7.2 to 8.0 mA/cm2, the power conversion efficiency increased from 2.4 to
2.8%, and the incident photon-to-current conversion efficiency increased from
42 to 49% corresponding to the different EEF magnitude. For an opposite
direction field, the applied EEF brought a minus effect on the performance
mentioned above. EEF treatment can orientate molecular ordering of the polymer,
and change the morphology of the active layer. The authors suggest a
explanation that the ferroelectric field has been built in the active layer,
and therefore it plays a key role in PSCs system. A needle-like surface
morphology of the active film was also discussed.
