IntroductionA memory device, as one key component of electronics to store information, has attracted enormous interest [1]. To achieve nonvolatile, rewritable, and nondestructive read-out properties of a memory device, many efforts have been exerted to ferroelectric field-effect transistors (FeFETs) in which a ferroelectric (FE) is used as the gate insulator [2]. Among them, organic FE is ideally suited to advance low cost, light weight and flexibility of such devices [3]. However, complex carrier behaviors in FET structure device prevent us realizing efficient FeFET. For example, poly(vinylidene fluoride -trifluoroethylene) [P(VDF-TrFE)] has a large remanent polarization P r , but the surface charge density at the semiconducting channel in the FeFET device was only 35% of P r when P(VDF-TrFE) is used as gate insulator [4]. To find the underlined mechanism, we need to study fundamental carrier behaviors, i.e., carrier injection, accumulation, transport, and polarization reversal. In our previous paper [5], we found 3 peaks generated in I-V (current-voltage) characteristics for P(VDF-TeFE)/pentacene double-layer structure. Later, we observed ferroelectric hysteresis in a IZO/P(VDF-TrFE)/pentacene/Au capacitor structure by electric-field-induced second-harmonic generation (EFISHG) measurements [6]. In more recent study, we demonstrated polarization reversal is fulfilled as a two-stage process, which accounts for the three peaks [7]. In the present contribution, we employ modified I-V measurements, which is more simple and evident to observe the two-stage polarization reversal process. Moreover, we discuss the interaction between polarizations switched at the two stages.
ExperimentA double-layer capacitor structure portrayed in the inset of Fig. 1 was prepared. P(VDF-TrFE) (72:28 in mol) layer with a thickness of 200 nm was spin-coated on the IZO-coated glass substrate, in a manner same in Ref [7]. Afterwards, pentacene layer with a thickness of 200 nm and a subsequent gold electrode were thermally evaporated on the copolymer layer at a vacuum pressure below 10 -3 Pa. The deposition rate of pentacene was regulated to 0.7 Å/s using a quartz crystal microbalance (QCM). The designed effective area for each device was approximately 3 mm 2 . An external voltage V ex applied on IZO side with Au electrode grounded ramped from 0→-V m →V m →0 (V m is the amplitude) with a fixed frequency of 10 mHz using a function generator (NF, WF 1945) together with a power amplifier (NF, BA4825). The I-V characteristics were recorded
