Two-Dimensional Layered Germanium Iodide Perovskite Ferroelectric Semiconductors

The discovery of ferroelectricity in two-dimensional (2D) semiconductors has opened a new and exciting chapter in next-generation electronics and spintronics due to their lattice-dimensionality-induced unique behaviors and fascinating functionalities brought by spontaneous polarization. The emerging layered halide perovskites with 2D lattices provide a great platform for generating reduced symmetry and low-dimensional ferroelectricity. Herein, inspired by the approach of reduced lattice dimensionality, a series of 2D layered germanium iodide perovskite ferroelectric semiconductors A₂CsGe₂I₇ [where A=PA (propylammonium), BA (butylammonium) and AA (amylammonium)] was firstly developed, which demonstrates remarkable semiconducting features including narrow direct band gap (~1.8 eV) and high conductivity over 32.23 nS/cm. Emphatically, these layered germanium iodide perovskites manifest large in-plane ferroelectric polarization over ~10.0 μC/cm², mainly attributed to the large off-centering ion displacement induced by stereo-active lone-pairs of Ge²⁺. More specifically, in contrast to three-dimensional ferroelectric CsGeI₃, the representative 2D layered BA₂CsGe₂I₇ manifests a superior polarization-sensitive bulk photovoltaic effect with a polarization ratio of 1.68 and high short circuit current density up to 81.25 μA/cm², which is superior to those of reported layered halide perovskite ferroelectrics. This work provides an exciting pathway for the development of 2D ferroelectric semiconductors as well as sheds light on their further applications in photoelectronic fields.