JiHao Zhang

Imprint effect in ferroelectric poly(vinylidene fluoride-trifluoroethylene) thin films

The imprint effect in ferroelectrics often hinders their performance in practical applications. Large numbers of observations have been reported on the imprint effect in inorganic ferroelectrics, while the imprint effect in ferroelectric polymers has attracted attention just in recent years. Here we reported our recent studies on the imprint effect in ferroelectric poly(vinylidene fluoride-trifluoroethylene) thin films. We measured the shift of ferroelectric switching peaks and the increase in switching time as a function of the waiting time. The influences of imprint pulse profile and short-circuit condition on the imprint effect were also determined. According to the interface screening model, we discussed the origin of the imprint effect in ferroelectric polymers and explained our experimental observations.

Electrical fatigue in ferroelectric P(VDF-TrFE) copolymer films

In recent years, ferroelectric polymer-based nonvolatile memory devices have attracted much attention due to their advantages over inorganic ferroelectrics. Polarization fatigue is a major problem in ferroelectric memories and seldom studies have been reported in this field. Here we will summarize our recent work on electric fatigue in polymers as well as some of work from the other researchers. Our experimental observations indicated that the injection and the trap of space charges acted a crucial role in the origination of polarization fatigue in ferroelectric polymers.

The Strong Dependence of Polarization Fatigue on Poling-Voltage Conditions in Ferroelectric Vinylidene Fluoride and Trifluoroethylene Copolymer Films

Recently, polarization fatigue in ferroelectric polymers has attracted much attention due to their potentials in nonvolatile memory devices, although the mechanism behind is still an open question. In this letter, we report the strong dependence of polarization fatigue on poling voltages, which indicates that the application of a poling electric field slightly below the coercive field can greatly degrade the fatigue endurance. This phenomenon may be directly related to the fatigue mechanism, and its origin is discussed based on the understanding of the injection and redistribution of space charges in ferroelectric films.

Experimental evidence of harmful exciton dissociation at MoO3/CuPc interface in OPV

Organic photovoltaics (OPVs) with three types of double anode buffer layers (DABLs), i.e., 4.5 nm hole-transport material 4,4-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl layer, 1 nm electron-transport material Bphen, and 1 nm typical insulator LiF layer, respectively, deposited onto 10 nm MoO3 layer, were fabricated. All these three DABLs can improve the efficiency of CuPc/C60 based planar heterojunction OPV, especially with about 10% enhancement of short-circuit current (ISC). Based on the external quantum efficiency (EQE) and transient photovoltage (TPV) measurements, a mechanism of depressing harmful exciton dissociation at the MoO3/CuPc interface has been proposed. This harmful dissociation results in exciton loss within the CuPc layer, while a proper ultrathin layer inserted at MoO3/CuPc interface can effectively depress the dissociation and thus improve the total photocurrent.

Symmetric Ferroelectric Switching in Ferroelectric Vinylidene Fluoride and Trifluoroethylene Copolymer Films

We report the observation of asymmetric switching dual peaks in ferroelectric copolymer films. These dual peaks occurs when the poling electric field is just below the coercive field and can be removed by continuous application of high enough switching voltage. Our experimental observations can be explained by the injection and the redistribution of space charges in ferroelectric films.

The role of space charges on polarization fatigue in ferroelectric P(VDF-TrFE) copolymer films

Recently, in our lab we have performed some systemic studies on polarization fatigue in ferroelectric polymers and four possible contributing factors (applied voltages, operation temperature, film crystallinity and electrode structure) are characterized by a homemade sawyer-Tower circuit. All fatigue phenomena observed in these experiments seem to imply the effect of space charges, which are injected from electrodes into ferroelectrics and trapped at the boundaries of crystallites or/and by defects. We believe that the interaction between trapped charges and electric dipoles in ferroelectrics has inhibited ferroelectric switching in polymers and induced the occurrence of polarization fatigue.