Compositionally Graded MoS_2xTe_2(1-x)/MoS₂ van der Waals Heterostructures for Ultrathin Photovoltaic Applications

van der Waals heterojunctions utilizing two-dimensional (2D) transition-metal dichalcogenide (TMD) materials have emerged as focal points in the field of optoelectronic devices, encompassing applications in light-emitting devices, photodetectors, solar cells, and beyond. In this study, we transferred few-atomic-layer films of compositionally graded ternary MoS_2xTe_2(1-x) alloys onto metal-organic chemical vapor deposition-grown molybdenum disulfide (MoS₂) as p- and n-type structures, leading to the creation of a van der Waals vertical heterostructure. The characteristics of the fabricated MoS_2xTe_2(1-x)/MoS₂ vertical-stacked heterojunction were investigated considering the influence of tellurium (Te) incorporation. The systematic variation of parameter x (i.e., 0.8, 0.6, 0.5, 0.3, and 0) allowed for an exploration of the impact of Te incorporation on the photovoltaic performance of these heterojunctions. As a result, the power conversion efficiency was enhanced by approximately 6 orders of magnitude with increasing Te concentration; notably, photoresponsivities as high as ∼6.4 A/W were achieved. These findings emphasize the potential for enhancing ultrathin solar energy conversion in heterojunctions based on 2D TMDs.