Roles of Non-switchable Domains and Internal Bias in Electrocaloric and Pyroelectric effects

TL;DR

This study investigates how non-switchable ferroelectric polarization and internal bias influence electrocaloric and pyroelectric effects in ferroelectric thin films, revealing defect-related domain pinning as a key factor.

Contribution

It provides direct experimental evidence linking non-switchable polarization and internal bias to enhanced pyroelectric and electrocaloric effects, highlighting defect engineering for device optimization.

Findings

Residual polarization causes voltage-axis shifts in hysteresis loops

Non-switchable polarization remains stable after aging, unlike switchable polarization

Internal bias and defect pinning significantly affect pyroelectric and electrocaloric responses

Abstract

Solid-state cooling and energy harvesting via pyroelectric effect (PEE) and electrocaloric effect (ECE) in ferroelectric thin films could be enhanced beyond their intrinsic ferroelectric response by exploiting the recently observed direction-dependent enhancement of the PEE; however, its microscopic origin remains unknown. Herein, we report direct hysteresis measurements of pyrocurrent ($I_{\rm p}$) and ECE-induced temperature change versus bias voltage in 1-$\mu$m-thick Pb(Zr$_{0.65}$Ti$_{0.35}$)O$_3$ capacitors. Both hysteresis loops exhibit pronounced asymmetries along the voltage and response axes. By superimposing direct current voltage offsets, we isolate a residual $I_{\rm p}$-axis shift, revealing a contribution of non-switchable ferroelectric polarization. This non-switchable polarization can be converted into switchable polarization via poling with bipolar triangular pulses, confirming the governing role of defect-induced domain pinning. After 100 pulses, time-dependent aging was observed for pyroelectric and electrocaloric responses, with the switchable contribution markedly decaying and the non-switchable component remaining nearly constant, indicating partial repinning. The change in voltage-axis shift agrees well with the ratio of non-switchable to switchable polarization, demonstrating that voltage shift also arises from pinned domains. These insights clarify the critical role of non-switchable polarization in the PEE and ECE performance, suggesting strategies to optimize the directional response in ferroelectric devices through controlled poling and defect engineering.