Enhanced shift current bulk photovoltaic effect in ferroelectric Rashba semiconductor-GeTe:study from three- to two-dimensional van der Waals layered structures.

The ferroelectric Rashba semiconductors (FERSCs) are endowed with a unique combination of ferroelectricity and the spin degree of freedom, resulting in a long carrier lifetime and impressive bulk photovoltaic (BPV) efficiency that reached 25% in organometal halide perovskites. The BPV efficiency can be further improved by using low-dimensional ferroelectrics however, it is inhibited by the ferroelectric instability in low-dimensional perovskites and toxicity along with phase instability of the lead-halide perovskites. To address these challenges, the-GeTe could be of great importance which is the simplest known lead-free FERSC with an intrinsic layered structure. Therefore, in this work, we investigate the BPV properties of three- to two-dimensional van der Waals structures of-GeTe by calculating the shift current (SHC). We predict that the mono (1.56 Å) and bi-layers (5.44-6.14 Å)-GeTe with the buckled honeycomb structure are dynamically stable and possess the characteristic features of the bulk up to the nanoscale limit. The SHC of ∼70A Vis calculated in bulk α-GeTe which is 20 times larger than that obtained in organometal halides in the visible light. The SHC increases with decreasing the number of layers, reaching a maximum amplitude of ∼300A Vat 2.67 eV in the monolayer which is more than double that obtained in monolayer GeS. We find that the SHC in monolayer α-GeTe can be further enhanced and redshifted by applying a compressive strain; which is correlated with the strong absorption of the-polarized light, stimulated by the more delocalized porbital character of the density of states. Furthermore, in the bilayer structures, the magnitude of the SHC is sensitive to the layers' stacking arrangement and a maximum SHC (∼250A V) can be achieved with an AB-type stacking arrangement. Combining these results with the benefits of being environmental-friendly material makes-GeTe a good candidate for next-generation solar cells application.