Electrocaloric effect (ECE) is the temperature and entropy change in a dielectric material as the applied field changes. ECE occurs due to electrical field induced dipole-entropy change in dielectrics, which is an extremely efficient form of energy conversion exhibiting minimum losses, e.g., polarization-electric field coupling approaching 100% efficiency. The past decade has witnessed the discovery and advancement in electrocaloric polymers, which display large electric field induced temperature and entropy changes.
In contrast with a burgeoning literature on large ECE in various ferroelectric materials, there are no EC devices employing these materials, demonstrating a meaningful cooling power. The critical barrier for the transition from high performance EC materials to practical EC devices is the dielectric breakdown. Hence, EC devices have to work under electric fields far below their dielectric breakdown. On the other hand, EC devices to achieve sufficient cooling power require large size EC films, which can further reduce their dielectric breakdown. To address these issues, high performance EC materials should possess a large EC response at fields far below dielectric breakdown. However, the EC response of the state-of-art EC polymer P(VDF-TrFE-CFE) at these practical field range is not high even though it possesses a large ECE at high electric fields (> 100 MV/m).
In this work, inspired by the materials concept of high entropy alloys, in which the presence of a large number of elements increases the entropy of the alloys, we developed a new class of EC polymer, tetrapolymer, which possesses a large dipolar entropy. Moreover, the tetrapolymer exhibits a critical end point behavior at low electric fields, thus leading to a giant EC response at low electric fields, which have the promise for high performance and highly reliable EC coolers.