By dimensional reduction of the 3D motif of Cs2AgBiBr6, a lead‐free 2D hybrid double perovskite, (i‐PA)2CsAgBiBr7 (1, i‐PA=isopentylammonium), was successfully designed. It adopts a quantum‐confined bilayered structure with alternating organic and inorganic sheets. Strikingly, the unique 2D architecture endows it highly anisotropic nature of physical properties, including electric conductivity and optical absorption (the ratio αb/αc=1.9 at 405 nm). Such anisotropy attributes result in the strong polarization‐sensitive responses with large dichroic ratios up to 1.35, being comparable to some 2D inorganic materials. This is the first study on the hybrid double perovskites with strong polarization sensitivity. A crystal device of 1 also exhibits rapid response speed (ca. 200 μs) and excellent stabilities. The family of 2D hybrid double perovskites are promising optoelectronic candidates, and this work paves a new pathway for exploring new green polarization‐sensitive materials.

In terms of strong light-polarization coupling, ferroelectric materials with bulk photovoltaic effects afford a promising avenue for optoelectronic devices. However, due to severe polarization deterioration caused by leakage current of photoexcited carriers, most of ferroelectrics are merely capable of absorbing 8–20% of visible-light spectra. Ferroelectrics with the narrow bandgap (<2.0 eV) are still scarce, hindering their practical applications. Here, we present a lead-iodide hybrid biaxial ferroelectric, (isopentylammonium)2(ethylammonium)2Pb3I10, which shows large spontaneous polarization (~5.2 μC/cm2) and a narrow direct bandgap (~1.80 eV). Particularly, the symmetry breaking of 4/mmmFmm2 species results in its biaxial attributes, which has four equivalent polar directions. Accordingly, exceptional in-plane photovoltaic effects are exploited along the crystallographic [001] and [010] axes directions inside the crystallographic bc-plane. The coupling between ferroelectricity and photovoltaic effects endows great possibility toward self-driven photodetection. This study sheds light on future optoelectronic device applications.

Multiphoton absorption (MPA) has been utilized for important technological applications. High‐order multiphoton harvesting (e.g., five‐photon absorption, 5PA) exhibits unique properties that could benefit biophotonics. Within this field, perovskite oxide ferroelectrics (e.g., BaTiO3) enable low‐order optical nonlinearities of 2PA/3PA processes. However, it is challenging to obtain efficient, high‐order 5PA effects. Herein, for the first time, giant and broadband MPA properties are presented in the 2D hybrid perovskite ferroelectric (IA)2(MA)2Pb3Br10 (1; IA = isoamylammonium and MA = methylammonium), where multiphoton‐excited optical nonlinearities related to different MPA mechanisms over a broadband range of 550–2400 nm are observed. Strikingly, its 5PA absorption cross‐section (σ5) reaches up to 1.2 × 10−132 cm10 s4 photon−4 (at 2400 nm), almost 10 orders larger than some state‐of‐the‐art organic molecules and a record‐high value among all known ferroelectrics. This unprecedented 5PA effect results from the quantum‐confined motif of inorganic trilayer sheets (wells) and organic cations (barriers) in 1. Moreover, its large ferroelectric polarization of 5 µC cm−2 could promote modulation of MPA effects under external electric fields. As far as it is known, this is the first report on giant, broadband high‐order MPA properties in ferroelectrics, which provides potential, novel electric‐ordered materials for next‐generation biophotonic applications.