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, the variety of such materials with narrow bandgaps and above room temperature (RT) ferroelectricity is 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. RT hysteresis loop reveals the intrinsic ferroelectricity of [C8N2H22]1.5[Bi2I9] with a spontaneous polarization (Ps) of 1.3 μC/cm². A visible-light optical bandgap has been confirmed by UV-vis spectrum and theoretical calculation. A notable ferroelectric photovoltaic (PV) effect has been revealed in [C8N2H22]1.5[Bi2I9]-based photoelectric device with an open-circuit voltage (Voc) of 0.39 V and a short-circuit current density (Jsc) of 2.3 μA/cm² 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/cm². This study offers a novel member of 0D lead-free hybrid organic-inorganic molecular ferroelectric family possessing great promise for optoelectronic devices.