**9. Electronic transport property in ferroelectric polymer films**

The leakage current mechanism of ferroelectric copolymer of P(VDF-TrFE) prepared by LB was investigated in the temperature range from 100 K to 350 K. The electron as the dominant injected carrier was observed in the ferroelectric copolymer films. The transport mechanisms in copolymer strongly depend on the temperature and applied voltage. From 100 K to 200 K, Schottky emission dominates the conduction. With the increase of temperature, the Frenkel– Poole emission instead of the Schottky emission conducts the carrier transport. When the temperature gets to 260 K, the leakage current becomes independent of temperature, and the space charge limited current conduction was observed [23].

from the self-polarization of the ultrathin polymer film [22]. It was due to the preferred alignment of the dipoles on the Al substrates. This process can be applied for designing stable

**Figure 12.** Pyroelectric voltage of the device under infrared radiation (a) before poling, (b) after poling at -1 V, (c) after

The fresh unpolarized device shows an appreciable pyroelectric voltage response, suggesting a preferential polarization. Upon polarizing the device at -1 V, the voltage response increases by a factor of 2, compared with the fresh device. Upon polarizing at +1 V, the voltage response decreases, in comparison with the fresh device. The applied 1 V is higher than the coercive electric field of the P(VDF-TrFE) ultrathin film with only 1 ML, reported in our previous investigation. Thus, the pyroelectric voltage responses under different polarizing directions should exhibit a 180° phase difference, but no such phase difference is observed in Fig. 12. This may be due to the back switching of domains, after removal of the positive poling voltage. It also suggests that the preferential polarization of the fresh device is aligned from the bottom electrode to the surface of the P(VDF-TrFE) film. The unpolarized detector exhibited a

This was considered to result from the self-polarization of the ultrathin P(VDF-TrFE) polymer film, due to the preferred alignment of the dipoles on the Al substrates. This result can be used

The leakage current mechanism of ferroelectric copolymer of P(VDF-TrFE) prepared by LB was investigated in the temperature range from 100 K to 350 K. The electron as the dominant

to fabricate fast-response room temperature infrared detectors.

**9. Electronic transport property in ferroelectric polymer films**

fast-response infrared detectors.

162 Ferroelectric Materials – Synthesis and Characterization

poling at +1 V, and (d) 12 h after poling at -1 V

preferential voltage response.

The P(VDF-TrFE) film shows a saturated hysteresis loop with a remanent polarization (Pr) of ~6.8 μC/cm2 and saturation polarization (Ps) of ~11 μC/cm2 , respectively, which indicates good ferroelectricity. It was previously reported that the electron affinity of β-P(VDF-TrFE) was about 4 eV, based on a density functional theory study [24]. The work function value of Au and Al metals are 5.1 eV and 4.1 eV [25], respectively.

The conduction through the lowest unoccupied molecular level (LUMO) of P(VDF-TrFE) and the leakage current is controlled either by the interface energy barrier that exists between the Fermi level of the metal and the LUMO level of the polymer or by the bulk-controlled mechanics such as Frenkel–Poole emission and space-charge-limited current (SCLC) conduc‐ tion [26,27]. The schematic diagram of the band structure of the P(VDF-TrFE) and the work functions of Au and Al are presented in the figure. The temperature dependence of the I-V behaviors from 100 K to 350 K was measured, and the temperature dependence of the current density under voltage 5 V (about 70 MV/m) is presented in Fig. 13. It can be seen that the current density increased with the temperature, increasing from 100 K to 260 K, but it is nearly independent of temperature as the temperature is higher than 260 K.

**Figure 13.** Temperature dependence of the current for P(VDF–TrFE) film under 5 V. (b) ln(I/T2 ) vs. 1/T plots at different voltages. The direction of the arrow indicates a reducing temperature for increased voltage

The electric conduction of the ferroelectric P(VDF-TrFE) copolymer films has been compre‐ hensively investigated. It is found that the electrons are the dominant injected carriers in the P(VDF-TrFE) films, and the charge injection occurs either at the polymer/electrode interface or in the bulk polymer films. Various transport mechanisms are observed in the P(VDF-TrFE) films, which are influenced by both temperature and applied voltage.

Schottky emission and Frenkel–Poole emission are found to be the dominant transport mechanism in the temperature range from 100 K to 200 K and the range from 200 K to 260 K, respectively. Space-charge-limited current conduction is the main transport mechanism as the temperature is higher than 260 K.
