Metal-free chiral molecular ferroelectric photovoltaics

Ferroelectric semiconductors have gained significant attraction for designing photovoltaic devices. However, their wide bandgap results in poor absorption of visible light. Herein, we report a pair of chiral metal-free molecular ferroelectric semiconductors (1R,4R/1S,4S-C5H10NO)I3 (R/S-OABHI; (1R,4R/1S,4S-C5H10NO)+ = 1R,4R/1S,4S-2-oxa-5-azabicyclo[2.2.1]heptonium) with notable ferroelectricity (saturation polarization of ∼6.8 and 6.1 μC/cm2 for R-/S-OABHI, respectively). They demonstrate an appropriate photovoltaic bandgap of ∼ 1.41 and 1.39 eV for R-/S-OABHI, respectively, comparable to MAPbI3 (MA = methylammonium). Experiments and computational simulations reveal that the ferroelectricity of R/S-OABHI originates from their non-centrosymmetric crystal packing in a controllable helical manner, while the I3− contributes to band edges. Meanwhile, photovoltaic effect is firstly observed in the R-OABHI based solar cell with an open-circuit voltage of 0.581 V and a short-circuit current density of 1.734 mA/cm2. Overall, we establish a new approach for realizing metal-free ferroelectric photovoltaics, and it will pave the way for the exploration of multifunctional chiral molecular ferroelectrics.