Electrocaloric effect driven by electric fields displays great potential in realizing highly efficient solid-state refrigeration. Nevertheless, most known electrocaloric materials exhibit relatively poor cooling performance near room temperature, which hinders their further applications. The emerging family of hybrid perovskite ferroelectrics, which exhibits superior structural diversity, large heat exchange and broad property tenability, offers an ideal platform. Herein, we report an exceptionally large electrocaloric effect near room temperature in a designed hybrid perovskite ferroelectric [(CH₃)₂CHCH₂NH₃]₂PbCl₄, which exhibits a sharp first-order phase transition at 302 K, superior spontaneous polarization (>4.8 μC/cm²) and relatively small coercive field (<15 kV/cm). Strikingly, a large isothermal entropy change ΔS of 25.64 J/kg/K and adiabatic temperature change ΔT of 11.06 K under a small electric field ΔE of 29.7 kV/cm at room temperature are achieved, with giant electrocaloric strengths of isothermal ΔS/ΔE of 0.86 J·cm/kg/K/kV and adiabatic ΔT/ΔE of 370 mK·cm/kV, which is larger than those of traditional ferroelectrics. This work presents a general approach to the design of hybrid perovskite ferroelectrics, as well as provides a family of candidate materials with potentially prominent electrocaloric performance for room temperature solid-state refrigeration.

The intense research activities on the hybrid organic–inorganic perovskites (HOIPs) have led to the greatly improved light absorbers for solar cells with high power conversion efficiency (PCE). However, it is still challenging to find an alternative lead-free perovskite to replace the organohalide lead perovskites to achieve high PCE. This is because both previous experimental and theoretical investigations have shown that the Pb²⁺ cations play a dominating role in contributing the desirable frontier electronic bands of the HOIPs for light absorbing. Recent advances in the chemical synthesis of three-dimensional (3D) metal-free perovskites, by replacing Pb²⁺ with NH₄⁺, have markedly enriched the family of multifunctionalized perovskites (Ye et al., Science 2018, 361, 151−155). These metal-free perovskites possess the chemical formula of A(NH₄)X₃, where A is divalent organic cations and X denotes halogen atoms. Without involving transition-metal cations, the metal-free A(NH₄)X₃ perovskites can entail notably different frontier electronic band features from those of the organohalide lead perovskites. Indeed, the valence and conduction bands of A(NH₄)X₃ perovskites are mainly attributed by the halogen atoms and the divalent A²⁺ organic cations, respectively. Importantly, a linear relationship between the bandgaps of A(NH₄)X₃ perovskites and the lowest unoccupied molecular orbital energies of the A²⁺ cations is identified, suggesting that bandgaps can be tailored via molecular design, especially through a chemical modification of the A²⁺ cations. Our comprehensive computational study and molecular design predict a metal-free perovskite, namely, 6-ammonio-1-methyl-5-nitropyrimidin-1-ium-(NH₄)I₃, with a desirable bandgap of ∼1.74 eV and good optical absorption property, both being important requirements for photovoltaic applications. Moreover, the application of strain can further fine-tune the bandgap of this metal-free perovskite. Our proposed design principle not only offers chemical insights into the structure–property relationship of the multifunctional metal-free perovskites but also can facilitate the discovery of highly efficient alternative, lead-free perovskites for potential photovoltaic or optoelectronic applications.

Circular polarized light-dependent anomalous bulk photovoltaic effect - a steady anomalous photovoltaic current can be manipulated by changing the light helicity, is an increasingly interesting topic in contexts ranging from physics to chemistry. Herein, circular polarized light-dependent anomalous bulk photovoltaic effect is presented in achiral hybrid perovskites, (4-AMP)BiI₅ (ABI, 4-AMP is 4-(aminomethyl)piperidinium), breaking conventional realization that it can only happen in chiral species. Achiral hybrid perovskite ABI crystallizes in chiroptical-active asymmetric point group m (C_s), showing an anomalous bulk photovoltaic effect with giant photovoltage of 25 V, as well as strong circular polarized light - sensitive properties. Significantly, conspicuous circular polarized light-dependent anomalous bulk photovoltaic effect is reflected in the large degree of dependence of anomalous bulk photovoltaic effect on left-and right-CPL helicity, which is associated with left and right-handed screw optical axes of ABI. Such degree of dependence is demonstrated by a large asymmetry factor of 0.24, which almost falls around the highest value of hybrid perovskites. These unprecedented results may provide a perspective to develop opto-spintronic functionalities in hybrid perovskites.

Rational design of ferroelectric/piezoelectric materials is still a big challenge due to our incomplete understanding of the underlying phase-transition mechanisms. Herein, by using first-principles calculations, three prototypes of one-dimensional (1D) hybrid organic–inorganic perovskites (HOIPs) as benchmarks are investigated, from which an approach to measure their ferroelectricity/piezoelectric-related key parameters is developed. Specifically, the ferroelectric transition temperature can be assessed by the computed energies and polarizations, and the piezoelectricity can be evaluated by the change of lattice parameters β during the phase transition. Based on this computational approach, we have examined a series of organic cations to design new 1D HOIPs via conscious chemical modification. Among them, seven potential candidates with excellent ferroelectricity or piezoelectricity are identified, especially for two of them, trimethyl(2,2,2-trifluoroethyl)ammonium trichloromanganese(II) (TMTFE-MnCl₃) and diethylmethyl(2-fluoroethyl)ammonium trichloromanganese(II) (DEMFE-MnCl₃). We predict that the TMTFE-MnCl₃ and DEMFE-MnCl₃ crystals outperform all previously reported 1D ferroelectric HOIPs in terms of high spontaneous polarization, high phase transition temperature, and strong piezoelectricity. The newly proposed design strategy for 1D HOIPs with room-temperature ferroelectricity or strong piezoelectricity can be validated for broader applications if both outstanding ferroelectric/piezoelectric perovskites can be synthesized and confirmed in the laboratory.