Strain-Reduced Inversion Symmetry in Ultrathin SnP₂Se₆ Crystals for Giant Bulk Piezophotovoltaic Generation

With the potential to surpass the Shockley-Queisser (S-Q) limitation for solar energy conversion, the bulk photovoltaic (BPV) effect, which is induced by the broken inversion symmetry of the lattice, presents prospects for future light-harvesting technologies. However, the development of BPV is largely limited by the low solar spectrum conversion efficiency of existing noncentrosymmetric materials with wide band gaps. This study reports that the strain-induced reduction of inversion symmetry can enhance the second-order nonlinear susceptibility (χ⁽²⁾) of SnP₂Se₆ crystals by an order of magnitude, which contributes to an extremely high value of 1.3 × 10^-8 m·V^-1 under 1550 nm excitation, and is high among two-dimensional (2D) crystals. More importantly, owing to the orientation-dependent reduction of lattice symmetry, the BPV generation induced by strain, referred to as the bulk piezophotovoltaic effect, is demonstrated in the SnP₂Se₆ crystal with strong in-plane anisotropy. The strain along the Se zigzag direction greatly facilitates the generation of the giant photocurrent covering an extended spectrum ranging from 400 to 1100 nm, resulting in leading-level values of the BPV coefficient among noncentrosymmetric crystals, while the BPV effect is barely modulated along the Se armchair direction even with a large strain of 0.57%. This study highlights the potential of the bulk piezophotovoltaic effects for energy conversion efficiency and offers a promising strategy for the design of next-generation light-harvesting devices.