Giant bulk piezophotovoltaic effect in 3R-MoS₂

Given its innate coupling with wavefunction geometry in solids and its potential to boost the solar energy conversion efficiency, the bulk photovoltaic effect (BPVE) has been of considerable interest in the past decade^1-14. Initially discovered and developed in ferroelectric oxide materials², the BPVE has now been explored in a wide range of emerging materials, such as Weyl semimetals^9,10, van der Waals nanomaterials^11,12,14, oxide superlattices¹⁵, halide perovskites¹⁶, organics¹⁷, bulk Rashba semiconductors¹⁸ and others. However, a feasible experimental approach to optimize the photovoltaic performance is lacking. Here we show that strain-induced polarization can significantly enhance the BPVE in non-centrosymmetric rhombohedral-type MoS₂ multilayer flakes (that is, 3R-MoS₂). This polarization-enhanced BPVE, termed the piezophotovoltaic effect, exhibits distinctive crystallographic orientation dependence, in that the enhancement mainly manifests in the armchair direction of the 3R-MoS₂ lattice while remaining largely intact in the zigzag direction. Moreover, the photocurrent increases by over two orders of magnitude when an in-plane tensile strain of ~0.2% is applied, rivalling that of state-of-the-art materials. This work unravels the potential of strain engineering in boosting the photovoltaic performance, which could potentially promote the exploration of novel photoelectric processes in strained two-dimensional layered materials and their van der Waals heterostructures.