Giant In-Plane Bulk Photovoltaic Response in γ-type Indium Selenide from Pressure-Induced Symmetry Breaking.

The bulk photovoltaic effect (BPVE) in polar crystals offers a promising way to surpass the energy-conversion limits of conventional p-n junction devices. In layered materials, BPVE is typically dominated by an out-of-plane component. However, strong in-plane performance is essential for planar photovoltaics, requiring innovative strategies to overcome fundamental material constraints. In this study, an innovative pressure-induced emergence of in-plane BPVE is demonstrated in the planar-nonpolar layered bulk semiconductor γ-type indium selenide. External pressure distorts the lattice and breaks the intrinsic threefold rotational symmetry, triggering in-plane photovoltaic responses that are otherwise symmetry-forbidden at ambient pressure. Furthermore, the in-plane BPVE shows a distinctive crystallographic orientation dependence: the photocurrent is significantly higher along the armchair direction than along the zigzag direction, with a large anisotropy ratio of up to 280 observed at 0.16 GPa. Experimental characterization and first-principles calculations reveal that pressure lifts the symmetry-protected planar nonpolarity, reconstructing the electronic structure and activating a sizable in-plane shift-current response that is forbidden at ambient pressure. More broadly, this work establishes symmetry engineering through pressure as a general strategy for achieving large in-plane bulk photovoltaic responses in layered systems, without relying on material nanostructuring or interfacial design.