Halogen substitution strategy of spacer cations in two-dimensional perovskite ferroelectrics gives giant anomalous photovoltaic effect

Halogen substitution effect, as one of the most feasible chemical strategies for exploiting new ferroelectric materials, holds a promise to optimize spontaneous polarization (P_s) and improve potential phase transition energy barrier. However, it is challenging to rationally regulate the photoferroelectric properties, e.g., ferroelectric photovoltaics. Through the p-site halogen substitution of spacer cations, we have obtained a series of two-dimensional hybrid perovskite ferroelectrics, (4x-benzylammonium)₂(ethylammonium)₂Pb₃Br₁₀ ((4x-BA)₂(EA)₂Pb₃Br₁₀, abbreviated as 4x-BEB, x = F/Cl/Br/I), featuring the typical trilayered perovskite architecture. The p-site halogen-substituted BA⁺ cations enable precise tuning of both the distortion degree of inorganic layer and the electric dipole moment of aromatic spacer. This molecular engineering not only drives a progressive enhancement of P_s but also facilitates Br^- ion migration, which are the driving forces for the ferroelectric anomalous photovoltaic effect (APVE). The member 4I-BEB shows a giant APVE of 38.2 V, exceeding other inorganic oxides and representing the highest value within the family of hybrid semiconductors. This halogen substitution strategy of spacer cations provides an effective way to design APV-active ferroelectric materials.