Spontaneous polarization and bulk photovoltaic effect (BPVE) are two concomitant physical properties in ferroelectric materials. The flipping of ferroelectric order usually accompanies the switching of BPVE in all directions because both of them are reversed under the inversion symmetry. In this study, we report the non-synchronous BPVE in two-dimensional (2D) interlayer-sliding ferroelectric materials featuring unswitchable in-plane BPVE (light-induced photocurrent in the xy plane) and switchable out-of-plane BPVE (light-induced polarization along the z-direction). Symmetry analysis within the abstract bilayer crystal model and first-principles calculations validate these BPVE properties. It is because the positive and negative ferroelectric states caused by interlayer sliding are related by mirror symmetry which cannot flip all the BPVE tensor elements. This finding extends the understanding of the relationship between ferroelectricity and BPVE. On one hand, the switchable out-of-plane BPVE can be used to design switchable photoelectric devices. On the other hand, the in-plane BPVE is robust against the ferroelectric flipping, and the unswitchable character is beneficial to construct larger-scale photoelectric devices.

自发极化和体光伏效应 (BPVE) 是铁电材料的两个伴随物理特性。铁电顺序的翻转通常伴随着BPVE在各个方向上的切换，因为它们在反转对称性下都是反转的。在这项研究中，我们报告了二维 (2D) 层间滑动铁电材料中的非同步 BPVE，该材料具有不可切换的平面内 BPVE（xy平面中的光诱导光电流）和可切换的平面外 BPVE（光-沿z方向的诱导极化-方向）。抽象双层晶体模型中的对称性分析和第一性原理计算验证了这些 BPVE 特性。这是因为层间滑动引起的正负铁电态是镜像对称的，不能翻转所有的BPVE张量元素。这一发现扩展了对铁电性与 BPVE 之间关系的理解。一方面，可切换平面外BPVE可用于设计可切换光电器件。另一方面，面内BPVE对铁电翻转具有鲁棒性，不可切换的特性有利于构建更大规模的光电器件。