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) $\left[{\mathrm{SbI}}_5\right]$ ($\mathrm{M}{\mathrm{V}}^{2+}=\mathrm{N},{\mathrm{N}}^{\prime }$-dimethyl-4,4′-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 ∼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) $\left[{\mathrm{SbI}}_5\right]$ 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.

Figure 1

(a) and (b) Crystallographic structures of (MV) $\left[{\mathrm{SbI}}_5\right]$ along the one-dimensional (1D) chain ($a$) and $c$ directions, respectively. (c) Origin of spontaneous polarization in (MV) $\left[{\mathrm{SbI}}_5\right]$. (d) The absorption spectrum of (MV) $\left[{\mathrm{SbI}}_5\right]$. (e) P-E hysteresis loop and I-V curve of (MV) $\left[{\mathrm{SbI}}_5\right]$ measured along the $b$ axis at room temperature. (f) Energy band structure. (g) Partial density of states (PDOS) of (MV) $\left[{\mathrm{SbI}}_5\right]$. (h) Density of states (DOS); inset: zoomed-in DOS.

Figure 2

(a) Photovoltaic response of (MV) $\left[{\mathrm{SbI}}_5\right]$. Both directions of $V_{\mathrm{oc}}$ and $I_{\mathrm{sc}}$ can be reversed by switching the spontaneous polarization. Schematics of tunable photocurrent directions in the (b) positive and (c) negative polarization states. (d) Current-time response acquired with the illumination ON and OFF. (e) Photovoltaic response of (MV) $\left[{\mathrm{SbI}}_5\right]$ at different illumination intensities. (f) $V_{\mathrm{oc}}$ and $I_{\mathrm{sc}}$ as a function of illumination intensity.

Figure 3

(a) I-V characteristics of (MV) $\left[{\mathrm{SbI}}_5\right]$ upon illumination with light of different wavelengths. (b) Magnified view of the I-V characteristics of (MV) $\left[{\mathrm{SbI}}_5\right]$ under infrared light irradiation. (c) I-$t$ response acquired with light ON and OFF. (d) Normalized photocurrent under light of different wavelengths.

Figure 4

(a), (d), and (g) Three pathways for electron excitation from the valence band to the conduction band; arrows represent the different transitions considered here. The insets show the charge density for the corresponding conduction bands and valence bands at E (−0.5 0.5 0.5) $k$ point and Y (−0.5 0 0) $k$ point. (b), (e), and (h) Charge density differences between the conduction band and the valence band for the three electron excitation pathways; the dashed lines represent cross-sections along the I-Sb-I direction of the same $\left[{\mathrm{SbI}}_5\right]$ inorganic chain. Yellow symbolizes the accumulation of electrons, while blue signifies the depletion of electrons. (c), (f), and (i) respectively depict the two-dimensional charge density differences along the cross-sections in (b), (e), and (h).