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. Layered crystals could enable efficient planar solar devices, but their bulk photovoltaic response is usually weak in-plane. Lin et al. used pressure to break in-plane symmetry in indium selenide, activating a strong, direction-dependent photocurrent.