Engineering Bulk Photovoltaic Effect in 2D Transition Metal Dichalcogenides.

A promising method for producing photocurrents above the Shockley-Queisser limit in non-centrosymmetric materials is the bulk photovoltaic effect (BPVE), especially in low-dimensional materials. 2D transition metal dichalcogenides (2D TMDs) provide a unique platform for BPVE engineering because of their strong light-matter interactions, varied electronic phases, and tunable crystal symmetry. This review systematically examines how 2D TMDs can induce or improve BPVE through phase engineering and structural modification. Phase transitions (such as 2H → 1T/1T' and 2H → 3R) that break inversion symmetry and allow shift currents are discussed. Strain fields, van der Waals (vdWs) heterostructures, twisted bilayers, lattice distortions, and rolled morphologies are considered in the context of structural engineering, all of which modify symmetry and electronic structure to regulate BPVE. Large photocurrents can be driven by spontaneous polarization in piezoelectric and ferroelectric 2D materials, including single- and dual-polarization systems. Attention is also drawn to how the directionality of the polarization field, edge contacts, and depolarization fields affect the photovoltaic response. In this comprehensive review, Design guidelines and new approaches for improving BPVE in 2D TMD systems are presented, with ramifications for energy-harvesting and next-generation optoelectronic devices.