Switchable\nPhotoelectric Response in High-Temperature Leadless Molecular Ferroelectric\n[C<sub>4</sub>N<sub>2</sub>H<sub>14</sub>][BiI<sub>5</sub>]

Lead-free molecular ferroelectrics\nhave garnered considerable\nattention for their promising potential, but such species with narrow\nband gap and sensitive photoelectric response are yet inadequate.\nHerein, we demonstrated the bulk ferroelectric photovoltaic effect\nin a novel lead-free molecular ferroelectric [C<sub>4</sub>N<sub>2</sub>H<sub>14</sub>][BiI<sub>5</sub>] with a Curie temperature (<i>T</i><sub>c</sub>) of 366 K and a narrow band gap (<i>E</i><sub>g</sub>) of 1.92 eV. The transformation of the crystal structure\nfrom the polar space group <i>P</i>2<sub>1</sub> to the\nnonpolar space group <i>P</i>2<sub>1</sub>/<i>m</i> was elucidated using single-crystal X-ray diffraction. Room-temperature\n(RT) hysteresis loop reveals the intrinsic ferroelectricity of [C<sub>4</sub>N<sub>2</sub>H<sub>14</sub>][BiI<sub>5</sub>] with a relative\nsmall coercive field (<i>E</i><sub>c</sub> ∼ 0.27\nkV/cm), saturation polarization (<i>P</i><sub>s</sub> ∼\n1.87 μC/cm<sup>2</sup>), and remanent polarization (<i>P</i><sub>r</sub> ∼ 1.61 μC/cm<sup>2</sup>). [C<sub>4</sub>N<sub>2</sub>H<sub>14</sub>][BiI<sub>5</sub>]-based solar\ndevice exhibits significant PV effects with a steady-state photocurrent\n(<i>J</i><sub>sc</sub>) of 3.54 μA/cm<sup>2</sup> and\na photovoltage (<i>V</i><sub>oc</sub>) of 0.34 V under AM\n1.5 G illumination, which can be significantly improved by adjusting\nthe ferroelectric polarization, reaching a maximum <i>J</i><sub>sc</sub> of 140 μA/cm<sup>2</sup> and <i>V</i><sub>oc</sub> of 0.51 V. This work offers a promising avenue for\nlead-free molecular ferroelectric materials in the field of optoelectronic\ndevices.