Recent progress in molecular ferroelectrics with perovskite structure

Discovered in 1920 by Valasek, a molecular crystal of Rochelle salt (potassium sodium tartrate tetrahydrate) displayed abnormal electrical hysteresis, named as ferroelectric effect. After that, ferroelectrics became an important class of functional materials, due to their unique reversible spontaneous polarization characteristics and other fascinating dielectric, thermoelectric and piezoelectric properties. Although the first reported ferroelectric material is an organic material, the later discovered inorganic materials almost dominated the fields of ferroelectrics for both fundamental studies and practical applications. Among them, inorganic perovskite ceramics received rapid development and showed great application potential in a wide-range of applications such as non-volatile memories, capacitors, sensors, energy convertors, etc., due to their high phase transition temperature and excellent ferroelectric/piezoelectric properties. Comparing to inorganics, molecular materials possess advantages as low acoustic impedance, good flexibility of structure design and composition modification, simple solution synthesis and easy realization of functional properties. Thus, molecular ferroelectrics are expected to be an irreplaceable complement to conventional inorganics, especially in applications like flexible electronics and thin film devices. In the large family of molecular materials, some adapt a similar structure of perovskite with partial substitution of metal ions by organic molecules. These hybrid materials have attracted widespread attention because of their extraordinary optoelectronic and photovoltaic performance. Since last decade, the ferroelectric related properties of molecular perovskite materials were also largely improved. The combination of highly adaptive perovskite structures and variable composition attributes hybrid perovskites them unparalleled potential in rational design of ferroelectricity and piezoelectricity. In this review, we firstly introduced the structural characteristics and dimensionalities of different molecular perovskite ferroelectrics. The next part, we focused on the current design strategy toward rational synthesis of molecular perovskite ferroelectrics, especially on how to tune polarization performance and introducing multiple polar axial properties. In the third part, we briefly introduced the latest progress of molecular piezoelectrics and the role of perovskite structure, molecular design, halogen-bonding and morphotropic phase boundary (MPB) in dramatically improving the piezoelectric coefficient d 33. The fourth part of this review is an application-driven example of tuning band gap in molecular perovskite ferroelectrics, for better optoelectronic and photovoltaic performance. In the last part, a new class of perovskite materials, the metal-free perovskite, is introduced. Without the limitation of metal ions, the metal-free system is believed to have more potential with even better related properties. In summary, the ferroelectric, piezoelectric, and optoelectronic properties of perovskite molecular ferroelectric materials have been greatly developed. This article introduces several design strategies of perovskite molecular ferroelectrics, including “quasi-spherical” theory, H/F substitution and hydrogen bonding introduction method. Meanwhile, we combine comprehensive examples to introduce its recent application and the progress made in other functional characteristics such as band gap regulation and photo electricity. This review provides macroscopic view and general introduction of the evolution and recent progress in molecular perovskite ferroelectrics materials and is suitable for readers in a broad area of research.