**4. Huge electrocaloric effect in LB ferroelectric polymer thin films**

Recently, the huge electrocaloric effect (ECE) resulting from changes in the entropy and temperature of a material under an applied electric field has attracted the attention of re‐ searchers to ferroelectric materials [11,12]. The ECE occurs in both ferroelectric and paraelectric phases and is found to be larger in the paraelectric phase just above the ferroelectric–para‐ electric phase transition [13].

In a working cycle-based ECE, the working material contacts the load and absorbs entropy from it. Then the material is isolated from the load and an electric field is applied. With an increase in the electric field, the polarization and temperature of the working material increase under adiabatic condition. The material is then placed in thermal contact with the heat sink and transfers the entropy absorbed from the load to the heat sink. Then the material is isolated from the heat sink. As the applied field is reduced, the temperature of the material decreases under 350MVm<sup>−</sup><sup>1</sup>

**Films**

back to the temperature of the cooling load. Thus, the larger the ECE of the working material, the better the efficiency of cooling. the material decreases back to the temperature of the cooling load. Thus, the larger the ECE of the working material, the better the efficiency of cooling.

the material is isolated from the heat sink. As the applied field is reduced, the temperature of

We present a detailed investigation of the ECE of P(VDF-TrFE) and P(VDF-TrFE-CFE) films grown by the LB technique on technologically desirable aluminized flexible polyimide substrates. Our results clearly show a large ECE, as the variations of adiabatic temperature ∆*T* of P(VDF-TrFE) films (at 390 K under 300 MVm−1) and P(VDF-TrFE-CFE) films (at 350 K under 350MVm−1) are 21 K (shown in Fig. 5). We present a detailed investigation of the ECE of P(VDF-TrFE) and P(VDF-TrFE-CFE) films grown by the LB technique on technologically desirable aluminized flexible polyimide substrates. Our results clearly show a large ECE, as the variations of adiabatic temperature *T* of P(VDF-TrFE) films (at 390 K under 300 MVm<sup>−</sup><sup>1</sup> ) and P(VDF-TrFE-CFE) films (at 350 K

) are 21 K (shown in Fig. 5).

Figure 5. Polarizations from *P*-*E* loops and adiabatic temperature change *T* versus temperature in different electric fields for (a) P(VDF-TrFE) films and(b) P(VDF-TrFE-CFE) **Figure 5.** Polarizations from *P*-*E* loops and adiabatic temperature change ∆*T* versus temperature in different electric fields for (a) P(VDF-TrFE) films and(b) P(VDF-TrFE-CFE) films

films Especially for P(VDF-TrFE-CFE) terpolymer films, the peak of *T* is close to room temperature. Considering their practical availability, ferroelectric PVDF-based polymers are potentially applicable for refrigeration by ECE. Professor J. F. Scott has pointed out that Especially for P(VDF-TrFE-CFE) terpolymer films, the peak of ∆*T* is close to room temperature. Considering their practical availability, ferroelectric PVDF-based polymers are potentially applicable for refrigeration by ECE. Professor J. F. Scott has pointed out that PVDF-based polymers are the most promising commercial material due to their large temperature cooling per volt and to their scalability in size and shape [14].

PVDF-based polymers are the most promising commercial material due to their large

### temperature cooling per volt and to their scalability in size and shape [14]. **<H1>Ferroelectric-Like Phase Transition in P(VDF-TrFE-CFE) Terpolymer Ultrathin 5. Ferroelectric-like phase transition in P(VDF-TrFE-CFE) terpolymer ultrathin films**

P(VDF-TrFE-CFE) terpolymer has attracted considerable attention for its fruitful properties and related potential application. Investigations have shown that the all-trans (*TTTT*)polar conformations have been converted into shorter all-trans conformations or polar nanodomains by the induced defect modifications [15]. Further studies verify that there coexist various molecular conformations in the terpolymer, such as trans-gauche (*TGTG)*, and less polar *TTTG* conformations, and the *TTTG* is the dominant one responsible for the relaxor nature. Klein et al. found that the crystallization conditions can also impact the P(VDF-TrFE-CFE) terpolymer has attracted considerable attention for its fruitful properties and related potential application. Investigations have shown that the all-trans (*TTTT*) polar conformations have been converted into shorter all-trans conformations or polar nanodomains by the induced defect modifications [15]. Further studies verify that there coexist various molecular conformations in the terpolymer, such as trans-gauche (*TGTG)*, and less polar *TTTG* conformations, and the *TTTG* is the dominant one responsible for the relaxor nature. Klein et al. found that the crystallization conditions can also impact the microstructures and the ferroelectric properties [16]. We have observed that the electric fields can change the length of the conformation.

The temperature dependence of the complex permittivity for the terpolymer films measured at 1 kHz in the heating process is shown in Fig. 6. For the film with 10 nm, the temperature of permittivity maximum *Tm* is at 308 K, which is consistent with the bulk terpolymer or thick films. Note that *Tm* increases with the decreasing thickness, and it is up to 360 K for the film with one transferred layer, which is even higher than the Curie temperature *T*c of the copoly‐ mers with the same composition.

back to the temperature of the cooling load. Thus, the larger the ECE of the working material,

of the working material, the better the efficiency of cooling.

*T* of P(VDF-TrFE) films (at 390 K under 300 MVm<sup>−</sup><sup>1</sup>

fields for (a) P(VDF-TrFE) films and(b) P(VDF-TrFE-CFE) films

per volt and to their scalability in size and shape [14].

) are 21 K (shown in Fig. 5).

the material is isolated from the heat sink. As the applied field is reduced, the temperature of the material decreases back to the temperature of the cooling load. Thus, the larger the ECE

We present a detailed investigation of the ECE of P(VDF-TrFE) and P(VDF-TrFE-CFE) films grown by the LB technique on technologically desirable aluminized flexible polyimide substrates. Our results clearly show a large ECE, as the variations of adiabatic temperature

Figure 5. Polarizations from *P*-*E* loops and adiabatic temperature change *T* versus temperature in different electric fields for (a) P(VDF-TrFE) films and(b) P(VDF-TrFE-CFE)

**Figure 5.** Polarizations from *P*-*E* loops and adiabatic temperature change ∆*T* versus temperature in different electric

Especially for P(VDF-TrFE-CFE) terpolymer films, the peak of ∆*T* is close to room temperature. Considering their practical availability, ferroelectric PVDF-based polymers are potentially applicable for refrigeration by ECE. Professor J. F. Scott has pointed out that PVDF-based polymers are the most promising commercial material due to their large temperature cooling

Especially for P(VDF-TrFE-CFE) terpolymer films, the peak of *T* is close to room temperature. Considering their practical availability, ferroelectric PVDF-based polymers are potentially applicable for refrigeration by ECE. Professor J. F. Scott has pointed out that PVDF-based polymers are the most promising commercial material due to their large

**<H1>Ferroelectric-Like Phase Transition in P(VDF-TrFE-CFE) Terpolymer Ultrathin** 

P(VDF-TrFE-CFE) terpolymer has attracted considerable attention for its fruitful properties and related potential application. Investigations have shown that the all-trans (*TTTT*)polar conformations have been converted into shorter all-trans conformations or polar nanodomains by the induced defect modifications [15]. Further studies verify that there coexist various molecular conformations in the terpolymer, such as trans-gauche (*TGTG)*, and less polar *TTTG* conformations, and the *TTTG* is the dominant one responsible for the relaxor nature. Klein et al. found that the crystallization conditions can also impact the

temperature cooling per volt and to their scalability in size and shape [14].

**5. Ferroelectric-like phase transition in P(VDF-TrFE-CFE) terpolymer**

P(VDF-TrFE-CFE) terpolymer has attracted considerable attention for its fruitful properties and related potential application. Investigations have shown that the all-trans (*TTTT*) polar conformations have been converted into shorter all-trans conformations or polar nanodomains by the induced defect modifications [15]. Further studies verify that there coexist various molecular conformations in the terpolymer, such as trans-gauche (*TGTG)*, and less polar *TTTG* conformations, and the *TTTG* is the dominant one responsible for the relaxor nature. Klein et al. found that the crystallization conditions can also impact the microstructures and the ferroelectric properties [16]. We have observed that the electric fields can change the

) and P(VDF-TrFE-CFE) films (at 350 K

We present a detailed investigation of the ECE of P(VDF-TrFE) and P(VDF-TrFE-CFE) films grown by the LB technique on technologically desirable aluminized flexible polyimide substrates. Our results clearly show a large ECE, as the variations of adiabatic temperature ∆*T* of P(VDF-TrFE) films (at 390 K under 300 MVm−1) and P(VDF-TrFE-CFE) films (at 350 K under

the better the efficiency of cooling.

156 Ferroelectric Materials – Synthesis and Characterization

350MVm−1) are 21 K (shown in Fig. 5).

under 350MVm<sup>−</sup><sup>1</sup>

films

**Films**

**ultrathin films**

length of the conformation.

**Figure 6.** The dielectric constant of different thickness terpolymer films as a function of temperature of at 1 k Hz

The coercive voltage is 1.2 V, which is also in accord with the polarization versus electric field (*P*-*E)* loop and *C*-*V* curve. In addition, the temperature dependence of the *C*-*V* curves for the terpolymer film with one monolayer is measured and shown in Fig. 7. The butterfly-shape feature disappears as the temperature is higher than 360 K, indicating a transition from ferroelectric-like to paraelectric phase. This is in accord with the temperature dependence of the capacitance.

A hypothesis can be deduced from the above results that all-trans-like molecular conforma‐ tions form in the ultrathin terpolymer films. As mentioned earlier, the dominant conformation in P(VDF-TrFE-CFE) terpolymer is the less polar *TTTG* conformation. It has been reported that the polar phase or ferroelectric-like phase can be induced by controlling the kinetic factors of the annealing process of terpolymer because the crystal structure is determined by both thermodynamics and kinetics. As it is known, the large PVDF dipoles can induce mirror charges on conducting substrates, thus forming an extrinsic vertical electric field on the substrate [17]. Here, it is considered that the terpolymer dipoles are also large enough to induce mirror charges on the conducting Al electrodes, which then can lead to a vertical electric field. Under such high vertical electric field, the less polar conformations are converted into higher polar conformations.

**Figure 7.** The temperature dependence of *C*-*V* curve at 10 kHz for one transferred layer terpolymer film. (a) 300 K, (b) 340 K, (c) 360 K, and (d) 375 K

The ferroelectric-like phase transition is observed in the P(VDF-TrFE-CFE) terpolymer films as the thickness is lower than 3 nm. The ferroelectric-like features are considered to result from the induced electric field due to the mirror charges in the electrodes.
