**3.4. Organic diodes**

The structure of ferroelectric device has been shown in **Figure 10a**, in which a PEDOT:PSS layer as a hole-only (electron blocking) transport layer is inserted between the Al top electrode and the PVDF LB nanofilm. The chemical structure of PEDOT:PSS is shown in **Figure 10b**. Ferroelectricity was characterized at different voltage amplitudes (**Figure 5c**), which shows asymmetric displacement-electric field hysteresis loops. The hysteresis loops were characterized by a traditional Sawyer–Tower method. The Pr values depend on the electric field (**Figure 10d**) [70]. The Pr values increase with the increasing electric field, which is accordance with reference report [13]. The dipole moments of PVDF in the films are slightly oriented at a low electric field, and the dipole moments are highly oriented at a high electric field, thereby

The asymmetric hysteresis loops for the capacitor with the PEDOT:PSS layer are different from that of the Al/PVDF LB nanofilms/Al device, which shows symmetric hysteresis loops [11]. Memory devices utilize the hysteresis by associating the positive remanent polarization (+Pr) and negative remanent polarization (−Pr) with a Boolean 1 and 0 [4]. Asadi et al. reported a diode with a Ag/PVDF:P3HT/LiF/Au capacitor, which possesses asymmetric hole

The low-density storage capability and slow switching speed are the major problems for most organic memory devices [71–73]. The performance of these devices can be greatly enhanced by several methods such as forming hybrid organic structures, [74] organic/inorganic

**Figure 10.** (a) Layer structure of the PEDOT:PSS ferroelectric device, (b) The chemical structure of PEDOT:PSS, (c) The hysteresis loops of the device at different applied voltage amplitudes, and (d) Pr values dependence of the electric field

[70]. This research was conducted at Tohoku University of Japan in 2015.

accumulation properties under positive or negative polarization [22].

leading to the increase of Pr.

144 Ferroelectrics and Their Applications

The device structure of FeFETs would complicate the construction of a larger integrated memory technology. In addition, ferroelectric capacitors with relatively simple device structure have a limited scaling capability. Ferroelectric diodes can combine the advantages of FeFETs and capacitors. Therefore, there is an ongoing research activity in a diode structure to realize the resistive switching [4].

The semilogarithmic forward and reverse I–V characteristics of the Au/n-InP and Au/PVDF/n-InP Schottky diodes are shown in **Figure 13**. The I–V was measured using a Keithley source measuring unit 2400. The reverse leakage current of the Au/n-InP diode (6.809 × 10−5 A at −1 V)

**Figure 11.** (a) Schematic diagram of the Pt/PVDF/GR/PVDF/ITO memory device fabricated layer by layer. (b) Typical I–V characteristic curves of the device under different compliance currents varying from 1 to 100 μA [77]. This research was conducted at University of Puerto Rico of USA in 2014.

**Figure 12.** (a) I–V curves of the Hg/PVDF/Au device (b) R–V curves of the Hg/PVDF/Au device [34]. This research was conducted at Pondicherry University of India in 2017.

is higher than that of the Au/PVDF/n-InP diode (2.387 × 10−7 A at −1 V). This proves that the PVDF as an organic interlayer can improve the electrical characteristics of the Schottky diodes. The diode parameters are confirmed from the forward bias I–V curves, which can be described by the thermionic emission theory. The calculated barrier heights based on I–V measurements are 0.57 eV for Au/n-InP and 0.73 eV for Au/PVDF/n-InP. The increased barrier height indicated that the PVDF films can influence the space charge region of n-InP [79]. The Au/PVDF/ Si structure fabricated by Kim et al. showed good ferroelectric properties. The current density and memory window width for the PVDF film were about 10−6 A/cm2 and 1.8 V under a bias voltage of 5 V, respectively [80].

diodes seem to have solved the problems of the MFS structure, but the retention characteristics of MFIS diodes are unreliable for commercialization in spite of considerable advancement

**Figure 14.** (a) Schematic drawings for the MFIS diodes and (b) The equivalent circuit of the MFIS structure [81]. This

**Figure 13.** The forward and reverse bias I–V characteristics of Au/PVDF/n-InP and the Au/n-InP Schottky diodes at room

Preparation and Device Applications of Ferroelectric β-PVDF Films

http://dx.doi.org/10.5772/intechopen.77167

147

temperature [79]. This research was conducted at Sri Venkateswara University of India in 2014.

The metal-insulator-metal diode is composed of the thin layers of an n-type polymer such as PGSt as the dielectric material, and a p-type polymer such as PTMA, PVDF. The retention cycles of the ON and OFF states under open-circuit conditions persisted for more than 10<sup>4</sup>

in data retention time in recent reports [84, 85].

research was conducted at Waseda University of Japan in 2016.

times [86].

The interfacial electrical properties will become very poor due to the diffusion of constituent atoms into the Si substrate, if a ferroelectric film is directly deposited on a Si substrate. In order to solve this problem, a metal-ferroelectric-insulator-semiconductor (MFIS) structure is formed with a buffer layer inserted between Si substrate and the ferroelectric film, as shown in **Figure 14** [81]. The MFIS diodes with low-voltage operation has attracted great interest because they can be used as one-transistor type nonvolatile memories [20, 82, 83]. The MFIS

**Figure 13.** The forward and reverse bias I–V characteristics of Au/PVDF/n-InP and the Au/n-InP Schottky diodes at room temperature [79]. This research was conducted at Sri Venkateswara University of India in 2014.

is higher than that of the Au/PVDF/n-InP diode (2.387 × 10−7 A at −1 V). This proves that the PVDF as an organic interlayer can improve the electrical characteristics of the Schottky diodes. The diode parameters are confirmed from the forward bias I–V curves, which can be described by the thermionic emission theory. The calculated barrier heights based on I–V measurements are 0.57 eV for Au/n-InP and 0.73 eV for Au/PVDF/n-InP. The increased barrier height indicated that the PVDF films can influence the space charge region of n-InP [79]. The Au/PVDF/ Si structure fabricated by Kim et al. showed good ferroelectric properties. The current density

**Figure 12.** (a) I–V curves of the Hg/PVDF/Au device (b) R–V curves of the Hg/PVDF/Au device [34]. This research was

The interfacial electrical properties will become very poor due to the diffusion of constituent atoms into the Si substrate, if a ferroelectric film is directly deposited on a Si substrate. In order to solve this problem, a metal-ferroelectric-insulator-semiconductor (MFIS) structure is formed with a buffer layer inserted between Si substrate and the ferroelectric film, as shown in **Figure 14** [81]. The MFIS diodes with low-voltage operation has attracted great interest because they can be used as one-transistor type nonvolatile memories [20, 82, 83]. The MFIS

and 1.8 V under a bias

and memory window width for the PVDF film were about 10−6 A/cm2

voltage of 5 V, respectively [80].

conducted at Pondicherry University of India in 2017.

146 Ferroelectrics and Their Applications

**Figure 14.** (a) Schematic drawings for the MFIS diodes and (b) The equivalent circuit of the MFIS structure [81]. This research was conducted at Waseda University of Japan in 2016.

diodes seem to have solved the problems of the MFS structure, but the retention characteristics of MFIS diodes are unreliable for commercialization in spite of considerable advancement in data retention time in recent reports [84, 85].

The metal-insulator-metal diode is composed of the thin layers of an n-type polymer such as PGSt as the dielectric material, and a p-type polymer such as PTMA, PVDF. The retention cycles of the ON and OFF states under open-circuit conditions persisted for more than 10<sup>4</sup> times [86].
