Analysis of carrier behavior in [alpha]-NPD/P(VDF-TrFE) double-layer capacitor by using electric-field-induced optical second-harmonic generation

The effect of ferroelectric polymer on carrier behaviors in indium-tin oxide (ITO)/poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)]/N,N'- di-[(1-naphthyl)-N,N'-diph enyl]-(1,1'-biphenyl)-4,4'-diamine ([alpha]-NPD)/Au structure was investigated. The internal electric field in [alpha]-NPD introduced by the dipole moments from ferroelectric P(VDF-TrFE) together with the [alpha]-NPD/P(VDF-TrFE) interface accumulated charges plays an important role in the carrier motion in [alpha]-NPD. In displacement current measurement (DCM), two asymmetric peaks and reduced polarization resulting from a large injection barrier from Au electrode to [alpha]-NPD layer were observed. Coupled with the electric-field-induced optical second-harmonic generation (EFISHG) measurement, the electric field in [alpha]-NPD layer was directly probed and gave a support that only part of the dipoles in P(VDF-TrFE) was polarized. We proposed that the formation of an amorphous insulating layer between amorphous [alpha]-NPD layer and partially crystallized P(VDF-TrFE) layer markedly decreased the voltage directly applied on the P(VDF-TrFE) film and caused a reduced polarization. Our findings here will be helpful in analyzing the carrier behavior in organic electronic devices using ferroelectric layers.