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Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI₃). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI₃ is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI₃ (unlike MAPbBr₃) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material’s relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity’s hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI₃, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material’s noncentrosymmetry. We note that the material’s ferroelectric nature, can, but need not be important in a PV cell at room temperature.

More than a decade of research on the electrocaloric (EC) effect has resulted in EC materials and EC multilayer chips that satisfy a minimum EC temperature change of 5 K required for caloric heat pumps 1 – 3 . However, these EC temperature changes are generated through the application of high electric fields 4 – 8 (close to their dielectric breakdown strengths), which result in rapid degradation and fatigue of EC performance. Here we report a class of EC polymer that exhibits an EC entropy change of 37.5 J kg −1  K −1 and a temperature change of 7.5 K under 50 MV m −1 , a 275% enhancement over the state-of-the-art EC polymers under the same field strength. We show that converting a small number of the chlorofluoroethylene groups in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer into covalent double bonds markedly increases the number of the polar entities and enhances the polar–nonpolar interfacial areas of the polymer. The polar phases in the polymer adopt a loosely correlated, high-entropy state with a low energy barrier for electric-field-induced switching. The polymer maintains performance for more than one million cycles at the low fields necessary for practical EC cooling applications, suggesting that this strategy may yield materials suitable for use in caloric heat pumps. A study reports and characterizes a high-entropy electrocaloric polymer that switches under low fields, and discusses its potential suitability for use in caloric heat pumps.

Pyroelectricity describes the generation of electricity by temporal temperature change in polar materials 1 – 3 . When free-standing pyroelectric materials approach the 2D crystalline limit, how pyroelectricity behaves remained largely unknown. Here, using three model pyroelectric materials whose bonding characters along the out-of-plane direction vary from van der Waals (In 2 Se 3 ), quasi-van der Waals (CsBiNb 2 O 7 ) to ionic/covalent (ZnO), we experimentally show the dimensionality effect on pyroelectricity and the relation between lattice dynamics and pyroelectricity. We find that, for all three materials, when the thickness of free-standing sheets becomes small, their pyroelectric coefficients increase rapidly. We show that the material with chemical bonds along the out-of-plane direction exhibits the greatest dimensionality effect. Experimental observations evidence the possible influence of changed phonon dynamics in crystals with reduced thickness on their pyroelectricity. Our findings should stimulate fundamental study on pyroelectricity in ultra-thin materials and inspire technological development for potential pyroelectric applications in thermal imaging and energy harvesting. By examining three model pyroelectric materials with different bonding characters along the out-of-plane direction, it is shown that their pyroelectric coefficients increase rapidly when the thickness of free-standing sheets becomes small.

Interfaces in heterostructures have been a key point of interest in condensed-matter physics for decades owing to a plethora of distinctive phenomena—such as rectification 1 , the photovoltaic effect 2 , the quantum Hall effect 3 and high-temperature superconductivity 4 —and their critical roles in present-day technical devices. However, the symmetry modulation at interfaces and the resultant effects have been largely overlooked. Here we show that a built-in electric field that originates from band bending at heterostructure interfaces induces polar symmetry therein that results in emergent functionalities, including piezoelectricity and pyroelectricity, even though the component materials are centrosymmetric. We study classic interfaces—namely, Schottky junctions—formed by noble metal and centrosymmetric semiconductors, including niobium-doped strontium titanium oxide crystals, niobium-doped titanium dioxide crystals, niobium-doped barium strontium titanium oxide ceramics, and silicon. The built-in electric field in the depletion region induces polar structures in the semiconductors and generates substantial piezoelectric and pyroelectric effects. In particular, the pyroelectric coefficient and figure of merit of the interface are over one order of magnitude larger than those of conventional bulk polar materials. Our study enriches the functionalities of heterostructure interfaces, offering a distinctive approach to realizing energy transduction beyond the conventional limitation imposed by intrinsic symmetry. A built-in electric field at the interface of metals and centrosymmetric semiconductors is shown to induce polar structures in the semiconductors and generate substantial piezoelectric and pyroelectric effects.

Heat pumps based on magnetocaloric and electrocaloric working bodies—in which entropic phase transitions are driven by changes of magnetic and electric field, respectively—use displaceable fluids to establish relatively large temperature spans between loads to be cooled and heat sinks 1 , 2 . However, the performance of prototypes is limited because practical magnetocaloric working bodies driven by permanent magnets 3 – 5 and electrocaloric working bodies driven by voltage 6 – 16 display temperature changes of less than 3 kelvin. Here we show that high-quality multilayer capacitors of PbSc 0.5 Ta 0.5 O 3 display large electrocaloric effects over a wide range of starting temperatures when the first-order ferroelectric phase transition is driven supercritically (as verified by Landau theory) above the Curie temperature of 290 kelvin by electric fields of 29.0 volts per micrometre. Changes of temperature in the large central area of the capacitor peak at 5.5 kelvin near room temperature and exceed 3 kelvin for starting temperatures that span 176 kelvin (complete thermalization would reduce these values from 5.5 to 3.3 kelvin and from 176 to 73 kelvin). If magnetocaloric working bodies were to be replaced with multilayer capacitors of PbSc 0.5 Ta 0.5 O 3 , then the established design principles behind magnetocaloric heat pumps could be repurposed for better performance without bulky and expensive permanent magnets. High-quality multilayer capacitors of a perovskite oxide show that large electric-field-driven caloric effects could improve solid-state refrigeration technology and challenge today’s standard (based on magnetocaloric effects in gadolinium).

Lithium niobate (LN) has experienced significant developments during past decades due to its versatile properties, especially its large electro-optic (EO) coefficient. For example, bulk LN-based modulators with high speeds and a superior linearity are widely used in typical fiber-optic communication systems. However, with ever-increasing demands for signal transmission capacity, the high power and large size of bulk LN-based devices pose great challenges, especially when one of its counterparts, integrated silicon photonics, has experienced dramatic developments in recent decades. Not long ago, high-quality thin-film LN on insulator (LNOI) became commercially available, which has paved the way for integrated LN photonics and opened a hot research area of LN photonics devices. LNOI allows a large refractive index contrast, thus light can be confined within a more compact structure. Together with other properties of LN, such as nonlinear/acousto-optic/pyroelectric effects, various kinds of high-performance integrated LN devices can be demonstrated. A comprehensive summary of advances in LN photonics is provided. As LN photonics has experienced several decades of development, our review includes some of the typical bulk LN devices as well as recently developed thin film LN devices. In this way, readers may be inspired by a complete picture of the evolution of this technology. We first introduce the basic material properties of LN and several key processing technologies for fabricating photonics devices. After that, various kinds of functional devices based on different effects are summarized. Finally, we give a short summary and perspective of LN photonics. We hope this review can give readers more insight into recent advances in LN photonics and contribute to the further development of LN related research.

Broadband spectral photoresponse has shown bright prospects for various optoelectronic devices, while fulfilling high photoactivity beyond the material bandgap is a great challenge. Here, we present a molecular pyroelectric, N -isopropylbenzylaminium trifluoroacetate ( N -IBATFA), of which the broadband photo-pyroelectric effects allow for self-driven wide spectral photodetection. As a simple organic binary salt, N -IBATFA possesses a large polarization (~9.5 μC cm −2 ), high pyroelectric coefficient (~6.9 μC cm −2 K −1 ) and figures-of-merits ( F V  = 187.9 × 10 −2 cm 2 μC −1 ; F D  = 881.5 × 10 −5  Pa −0.5 ) comparable to the state-of-art pyroelectric materials. Particularly, such intriguing attributes endow broadband photo-pyroelectric effect, namely, transient currents covering ultraviolet (UV, 266 nm) to near-infrared (NIR, 1950 nm) spectral regime, which breaks the restriction of its optical absorption and thus allows wide UV-NIR spectral photodetection. Our finding highlights the potential of molecular system as high-performance candidates toward self-powered wide spectral photodetection. Broadband spectral photoresponse has potential for optoelectronic devices, but obtaining high photoactivity beyond the material bandgap is challenging. Here, the authors report the development of a molecular pyroelectric material with broadband photopyroelectric effects.

Poly(vinylidene fluoride) (PVDF) and its copolymers are key polymers, displaying properties such as flexibility and electroactive responses, including piezoelectricity, pyroelectricity, and ferroelectricity. In the past several years, they have been applied in numerous applications, such as memory, transducers, actuators, and energy harvesting and have shown thriving prospects in the ongoing research and commercialization process. The crystalline polymorphs of PVDF can present nonpolar α, ε phase and polar β, γ, and δ phases with different processing methods. The copolymers, such as poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), can crystallize directly into a phase analogous to the β phase of PVDF. Since the β phase shows the highest dipole moment among polar phases, many reproducible and efficient methods producing β-phase PVDF and its copolymer have been proposed. In this review, PVDF and its copolymer films prepared by spin-coating and Langmuir–Blodgett (LB) method are introduced, and relevant characterization techniques are highlighted. Finally, the development of memory, artificial synapses, and medical applications based on PVDF and its copolymers is elaborated.

Thermal losses in photoelectric devices limit their energy conversion efficiency, and cyclic input of energy coupled with pyroelectricity can overcome this limit. Here, incorporating a pyroelectric absorber into a photovoltaic heterostructure device enables efficient electricity generation by leveraging spontaneous polarization based on pulsed light-induced thermal changes. The proposed pyroelectric-photovoltaic device outperforms traditional photovoltaic devices by 2.5 times due to the long-range electric field that occurs under pulse illumination. Optimization of parameters such as pulse frequency, scan speed, and illumination wavelength enhances power harvesting, as demonstrated by a power conversion efficiency of 11.9% and an incident-photon-to-current conversion efficiency of 200% under optimized conditions. This breakthrough enables reconfigurable electrostatic devices and presents an opportunity to accelerate technology that surpasses conventional limits in energy generation. The full potential of photoelectric devices can possibly be maximized through pyroelectricity for power generation beyond thermodynamic limit. Here, authors report photovoltaic heterostructure device with pyroelectric absorber, achieving 2.5 times more output power due to long-range electric field.

Surface pyroelectricity and piezoelectricity induced by water incorporation during growth in α-glycine were investigated. Using the periodic temperature change technique, we have determined the thickness (~280 µm) of the near surface layer (NSL) and its pyroelectric coefficient (160 pC/(K × cm2) at 23 °C) independently. The thickness of NSL remains nearly constant till 60 °C and the pyroelectric effect vanishes abruptly by 70 °C. The piezoelectric effect, 0.1 pm/V at 23 °C measured with an interferometer, followed the same temperature dependence as the pyroelectric effect. Abrupt disappearance of both effects at 70 °C is irreversible and suggests that water incorporation to α-glycine forms a well defined near surface phase, which is different form α-glycine because it is polar but it too close to α-glycine to be distinguished by X-ray diffraction (XRD). The secondary pyroelectric effect was found to be <14% of the total, which is unexpectedly small for a material with a large thermal expansion coefficient. This implies that water incorporation infers minimal distortions in the host lattice. This finding suggests a path for the control of the piezoelectric and pyroelectric effects of the crystals using stereospecific incorporation of the guest molecules.