Switchable Photoelectric Response in High-Temperature Leadless Molecular Ferroelectric [C₄N₂H₁₄][BiI₅]

Lead-free molecular ferroelectrics have garnered considerable attention for their promising potential, but such species with narrow band gap and sensitive photoelectric response are yet inadequate. Herein, we demonstrated the bulk ferroelectric photovoltaic effect in a novel lead-free molecular ferroelectric [C₄N₂H₁₄][BiI₅] with a Curie temperature (T_c) of 366 K and a narrow band gap (E_g) of 1.92 eV. The transformation of the crystal structure from the polar space group P2₁ to the nonpolar space group P2₁/m was elucidated using single-crystal X-ray diffraction. Room-temperature (RT) hysteresis loop reveals the intrinsic ferroelectricity of [C₄N₂H₁₄][BiI₅] with a relative small coercive field (E_c ∼ 0.27 kV/cm), saturation polarization (P_s ∼ 1.87 μC/cm²), and remanent polarization (P_r ∼ 1.61 μC/cm²). [C₄N₂H₁₄][BiI₅]-based solar device exhibits significant PV effects with a steady-state photocurrent (J_sc) of 3.54 μA/cm² and a photovoltage (V_oc) of 0.34 V under AM 1.5 G illumination, which can be significantly improved by adjusting the ferroelectric polarization, reaching a maximum J_sc of 140 μA/cm² and V_oc of 0.51 V. This work offers a promising avenue for lead-free molecular ferroelectric materials in the field of optoelectronic devices.