It is demonstrated that the power conversion efficiency (PCE) of organic photovoltaic devices can be increased by inserting an ultrathin film of a ferroelectric co-polymer, poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)), at the metal–organic interface, due to an enhancement of the charge extraction efficiency. Specifically, the effect of P(VDF-TrFE) crystallinity on its function in ferroelectric organic photovoltaic (FE-OPV) devices has been studied by several methods. Highly crystalline and amorphous P(VDF-TrFE) films have been prepared by the Langmuir–Blodgett method and spin-coating from acetone solution, respectively. The polymer solar cell devices with a crystalline P(VDF-TrFE) interfacial layer at the cathode have larger PCE than the structures with amorphous P(VDF-TrFE) and have the unique feature of switchable diode polarity and photovoltaic performance controlled by external applied voltage pulses. The obtained results confirm that the spontaneous polarization of the ferroelectric P(VDF-TrFE) layer is responsible for the enhancement of PCE in FE-OPV devices and that a highly crystalline ferroelectric polymer film is required to observe the enhancement of PCE. Amorphous P(VDF-TrFE) films act as regular dielectric layers with a little poling effect on device PCE. The polarization of P(VDF-TrFE) is shown to be stable, and the photogenerated charges could be collected efficiently by the cathode rather than being compensated.