Alloying Small‐Cation into Low‐Dimensional Perovskite Motif Gives Multiaxial Ferroelectrics with Enhanced Photo‐Pyroelectricity

Photoactive ferroelectrics have shown huge potentials for assembling high-performance optoelectronic devices. Bismuth-based halides are emerging as a vigor ferroelectric subclass due to unique structural diversity and flexibility, while studies on their photo-ferroelectricity coupling remain extremely scarce. Here, we have alloyed organic mixed-cations into Bismuth-based prototype to design a series of new multiaxial ferroelectrics with strong photo-pyroelectricity, as notably exemplified by (BZA)₂(DMA)BiBr₆ (1, where BZA is benzylammonium; DMA is dimethylammonium). The dynamic motions of small DMA⁺ cation afford the driving force to trigger symmetry breaking of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:mover><mml:mn>3</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mi>m</mml:mi> <mml:mi>F</mml:mi> <mml:mi>m</mml:mi></mml:mrow> <mml:annotation>$\bar 3m{\rm F}m $</mml:annotation></mml:semantics> </mml:math> , thus giving its multiaxial ferroelectricity with 6 equivalent polarization directions. Strikingly, 1 exhibits photo-pyroelectricity in a broad ultraviolet-to-NIR spectral range (266-980 nm), which couples with ferroelectric bulk photovoltaics to create giant photocurrent enhancement ratio of ∼4800%. This merit far surpasses those of known ferroelectric materials. These results shed light on the design of new photoactive ferroelectric candidates for smart photoelectronic device applications.