Bulk photovoltaic effect of a hybrid ferroelectric semiconductor

Hybrid ferroelectrics have attracted much attention recently due to their low processing cost and superior piezoelectric responses. However, their photovoltaic properties are less explored. For better performance, ferroelectric semiconductors with small band gaps are desired. Here, we report on an organic-inorganic hybrid material (MV) $[{\mathrm{SbI}}_{5}]$ ($\mathrm{M}{\mathrm{V}}^{2+}=\mathrm{N},{\mathrm{N}}^{\ensuremath{'}}$-dimethyl-4,4\ensuremath{'}-bipyridinium or methylviologen), with a band gap of 1.47 eV, which exhibits ferroelectricity at room temperature. Careful analysis shows that a flat band formed by the MV not only enhances light absorption but also allows for the simultaneous manifestation of small band gap and ferroelectricity in the same material. Under the irradiation of a 445 nm laser, we observed an open circuit voltage of \ensuremath{\sim}5 V, far greater than its band gap. The light polarization-dependent photocurrent confirms that the above-band-gap photovoltage is caused by the bulk photovoltaic effect (BPVE). Further investigations revealed that the contribution of the MV group to the conduction band leads to two distinct electron excitation pathways for (MV) $[{\mathrm{SbI}}_{5}]$ under visible and infrared light illumination, resulting in photocurrents in opposite directions. In this paper, we offer a strategy for designing hybrid ferroelectrics with narrow band gaps and improve our understanding of the BPVE.