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 benzylammounium; DMA is dimethylammounium). The dynamic motions of small DMA+ cation afford the driving force to trigger symmetry breaking of 3¯mFm3m Fm , 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.