Ligand‐Assisting Controllable Self‐Assembly of Multiaxial Ferroelectrics With Large Bulk Photovoltaics for Solar‐Blind Ultraviolet Photodetection

Multiaxial ferroelectrics that enable strong optical nonlinearity of bulk photovoltaic effect (BPVE) pave a promising pathway to actualize high‐performed photodetection. However, severe fatigue of ferroelectric spontaneous polarization induced by photoexcited carriers leads to low BPVE photocurrents, hindering practical applications. Here, we present a new ligand‐assisting strategy for controllable self‐assembly of multiaxial ferroelectrics in the family of EA n+1 Pb n Cl 3n+1 (where EA is ethylamine, n = 1–3). The thickness of inorganic frameworks is precisely controlled using extra ligand isobutylamine ( i ‐BA) as a chemical scissor. This molecule‐level self‐assembly favors the emergence of a polar lattice and ferroelectricity. Notably, the EA 3 Pb 2 Cl 7 ( n = 2) member exhibits robust multiaxial ferroelectricity with 8 equivalent polarization directions and a large spontaneous polarization of 4.6 µC/cm 2 . The multiaxial ferroelectric properties of EA 3 Pb 2 Cl 7 drive an extremely strong BPVE with giant short‐circuit current densities up to 0.12 mA/cm 2 in the solar‐blind UV spectral region (at 266 nm, 101.9 mW/cm 2 ), nearly two orders of magnitude higher than typical ferroelectric oxides (e.g., BaTiO 3 and BiFeO 3 ). Such BPVE‐directed behaviors further enable solar‐blind UV self‐driven photodetection with superior responsivity and detectivity. Our findings shed light on the rational design of multiaxial ferroelectrics toward smart photoelectric device applications.