Tailoring Interlayered Spacers of Two-Dimensional Cesium-Based Perovskite Ferroelectrics toward Exceptional Ferro-Pyro-Phototronic Effects.

Ferro-pyro-phototronic (FPP) effect is a triple coupling of ferroelectricity, light-induced pyroelectricity, and photo-excitation, which holds a bright promise for next-generation modern optoelectronic devices. However, except for few oxides (e.g., BaTiO3 ), new FPP-active candidates remain extremely scarce due to the knowledge lacking on the underlying role of three coupling components. By tailoring the interlayered spacers, the authors present a series of 2D cesium-based perovskite ferroelectrics, (A')2 CsPb2 Br7 (where A'-site cation is organic spacer), showing remarkable FPP-active properties. As expected, the dynamic ordering and reorientation of spacers along with atomic displacement of Cs+ in the perovskite cavity lead to their ferroelectric polarizations. Particularly, exceptional FPP properties are created through this cooperation; the most FPP-active candidate (n-hexylammonium)2 CsPb2 Br7 endows a giant contrast up to 1500% for photopyroelectric current to photovoltaic signal. This figure-of-merit is far beyond most inorganic oxide counterparts, such as ≈110% for BaTiO3 . Further, the electric switching and controlling of FPP directions confirm a crucial role of ferroelectric polarization to this coupling effect. To the authors' best knowledge, this is the first study on an FPP-active candidate of 2D hybrid perovskites, which affords a new avenue to design ferroelectrics with targeted physical properties and forward their potentials to smart optoelectronic device application.