A bulk photovoltaic effect in a zero-dimensional room-temperature molecular ferroelectric [C8N2H22]1.5[Bi2I9]

Abstract

Non-toxic molecular ferroelectrics have attracted significant interest due to their unique flexibility, low costs, and environmental friendliness. However, such materials with narrow bandgaps and ferroelectricity above room temperature (RT) are still scarce. Herein, we present a brand-new lead-free molecular ferroelectric [C₈N₂H₂₂]_1.5[Bi₂I₉] synthesized hydrothermally. [C₈N₂H₂₂]_1.5[Bi₂I₉] features a zero-dimensional (0D) structure with a polar space group of Pc, as confirmed by single-crystal X-ray diffraction and second-harmonic generation (SHG) analyses. The RT hysteresis loop reveals the intrinsic ferroelectricity of [C₈N₂H₂₂]_1.5[Bi₂I₉] with a spontaneous polarization (P_s) of 1.3 μC cm⁻². A visible-light optical bandgap has been confirmed by UV-vis spectroscopy and theoretical calculations. A notable ferroelectric photovoltaic (PV) effect has been revealed in [C₈N₂H₂₂]_1.5[Bi₂I₉]-based photoelectric devices with an open-circuit voltage (V_oc) of 0.39 V and a short-circuit current density (J_sc) of 2.3 μA cm⁻² under AM 1.5G illumination. The PV performance can be significantly enhanced by tuning the ferroelectric polarization, achieving a maximum Voc of 0.47 V and Jsc of about 50 μA cm⁻². This study offers a novel member of the 0D lead-free hybrid organic–inorganic molecular ferroelectric family possessing great promise for optoelectronic devices.