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

Abstract

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′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD)/Au structure was investigated. The internal electric field in α-NPD introduced by the dipole moments from ferroelectric P(VDF–TrFE) together with the α-NPD/P(VDF–TrFE) interface accumulated charges plays an important role in the carrier motion in α-NPD. In displacement current measurement (DCM), two asymmetric peaks and reduced polarization resulting from a large injection barrier from Au electrode to α-NPD layer were observed. Coupled with the electric-field-induced optical second-harmonic generation (EFISHG) measurement, the electric field in α-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 α-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.